Merge pull request #292 from mitchmindtree/asio

Add ASIO Host for Windows (rebase and refactor of #221)

.gitignore

@@ -2,4 +2,4 @@
 /Cargo.lock
 .cargo/
 .DS_Store
-recorded.wav
+recorded.wav

Cargo.toml

@@ -8,15 +8,20 @@ documentation = "https://docs.rs/cpal"
 license = "Apache-2.0"
 keywords = ["audio", "sound"]
 
+[features]
+asio = ["asio-sys"] # Only available on Windows. See README for setup instructions.
+
 [dependencies]
 failure = "0.1.5"
 lazy_static = "1.3"
+num-traits = "0.2.6"
 
 [dev-dependencies]
 hound = "3.4"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = { version = "0.3", features = ["audiosessiontypes", "audioclient", "coml2api", "combaseapi", "debug", "devpkey", "handleapi", "ksmedia", "mmdeviceapi", "objbase", "std", "synchapi", "winuser"] }
+asio-sys = { version = "0.1", path = "asio-sys", optional = true }
 
 [target.'cfg(any(target_os = "linux", target_os = "dragonfly", target_os = "freebsd", target_os = "openbsd"))'.dependencies]
 alsa-sys = { version = "0.1", path = "alsa-sys" }

README.md

@@ -13,10 +13,10 @@ This library currently supports the following:
 - Get the current default input and output stream formats for a device.
 - Build and run input and output PCM streams on a chosen device with a given stream format.
 
-Currently supported backends include:
+Currently supported hosts include:
 
 - Linux (via ALSA)
-- Windows
+- Windows (via WASAPI by default, see ASIO instructions below)
 - macOS (via CoreAudio)
 - iOS (via CoreAudio)
 - Emscripten
@@ -24,3 +24,79 @@ Currently supported backends include:
 Note that on Linux, the ALSA development files are required. These are provided
 as part of the `libasound2-dev` package on Debian and Ubuntu distributions and
 `alsa-lib-devel` on Fedora.
+
+## ASIO on Windows
+
+[ASIO](https://en.wikipedia.org/wiki/Audio_Stream_Input/Output) is an audio
+driver protocol by Steinberg. While it is available on multiple operating
+systems, it is most commonly used on Windows to work around limitations of
+WASAPI including access to large numbers of channels and lower-latency audio
+processing.
+
+CPAL allows for using the ASIO SDK as the audio host on Windows instead of
+WASAPI. To do so, follow these steps:
+
+1. **Download the ASIO SDK** `.zip` from [this
+   link](https://www.steinberg.net/en/company/developers.html). The version as
+   of writing this is 2.3.1.
+2. Extract the files and place the directory somewhere you are happy for it to stay
+   (e.g. `~/.asio`).
+3. Assign the full path of the directory (that contains the `readme`, `changes`,
+   `ASIO SDK 2.3` pdf, etc) to the `CPAL_ASIO_DIR` environment variable. This is
+   necessary for the `asio-sys` build script to build and bind to the SDK.
+4. `bindgen`, the library used to generate bindings to the C++ SDK, requires
+   clang. **Download and install LLVM** from
+   [here](http://releases.llvm.org/download.html) under the "Pre-Built Binaries"
+   section. The version as of writing this is 7.0.0.
+5. Add the LLVM `bin` directory to a `LIBCLANG_PATH` environment variable. If
+   you installed LLVM to the default directory, this should work in the command
+   prompt:
+   ```
+   setx LIBCLANG_PATH "C:\Program Files\LLVM\bin"
+   ```
+6. If you don't have any ASIO devices or drivers available, you can [**download
+   and install ASIO4ALL**](http://www.asio4all.org/). Be sure to enable the
+   "offline" feature during installation despite what the installer says about
+   it being useless.
+7. **Loading VCVARS**. `rust-bindgen` uses the C++ tool-chain when generating
+   bindings to the ASIO SDK. As a result, it is necessary to load some
+   environment variables in the command prompt that we use to build our project.
+   On 64-bit machines run:
+   ```
+   "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" amd64
+   ```
+   On 32-bit machines run:
+   ```
+   "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x86
+   ```
+   Note that, depending on your version of Visual Studio, this script might be
+   in a slightly different location.
+8. Select the ASIO host at the start of our program with the following code:
+
+   ```rust
+   let host;
+   #[cfg(target_os = "windows")]
+   {
+       host = cpal::host_from_id(cpal::HostId::Asio).expect("failed to initialise ASIO host");
+   }
+   ```
+
+   If you run into compilations errors produced by `asio-sys` or `bindgen`, make
+   sure that `CPAL_ASIO_DIR` is set correctly and try `cargo clean`.
+9. Make sure to enable the `asio` feature when building CPAL:
+
+   ```
+   cargo build --features "asio"
+   ```
+
+   or if you are using CPAL as a dependency in a downstream project, enable the
+   feature like this:
+
+   ```toml
+   cpal = { version = "*", features = ["asio"] }
+   ```
+
+In the future we would like to work on automating this process to make it
+easier, but we are not familiar enough with the ASIO license to do so yet.
+
+*Updated as of ASIO version 2.3.3.*

asio-sys/.gitignore

@@ -0,0 +1,2 @@
+/target
+Cargo.lock

asio-sys/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "asio-sys"
+version = "0.1.0"
+authors = ["Tom Gowan <tomrgowan@gmail.com>"]
+build = "build.rs"
+
+[target.'cfg(any(target_os = "windows"))'.build-dependencies]
+bindgen = "0.42.0"
+walkdir = "2"
+cc = "1.0.25"
+
+[dependencies]
+lazy_static = "1.0.0"
+num-derive = "0.2"
+num-traits = "0.2"

asio-sys/asio-link/helpers.cpp

@@ -0,0 +1,29 @@
+#include "helpers.hpp"
+#include <stdio.h>
+
+extern "C" ASIOError get_sample_rate(double * rate){
+	return ASIOGetSampleRate(reinterpret_cast<ASIOSampleRate *>(rate));
+}
+
+extern "C" ASIOError set_sample_rate(double rate){
+	return ASIOSetSampleRate(rate);
+}
+
+extern "C" ASIOError can_sample_rate(double rate){
+	return ASIOCanSampleRate(rate);
+}
+
+extern AsioDrivers* asioDrivers;
+bool loadAsioDriver(char *name);
+
+extern "C" bool load_asio_driver(char * name){
+	return loadAsioDriver(name);
+}
+
+extern "C" void remove_current_driver() {
+	asioDrivers->removeCurrentDriver();
+}
+extern "C" long get_driver_names(char **names, long maxDrivers) {
+	AsioDrivers ad;
+	return ad.getDriverNames(names, maxDrivers);
+}

asio-sys/asio-link/helpers.hpp

@@ -0,0 +1,16 @@
+#pragma once
+#include "asiodrivers.h"
+#include "asio.h"
+
+// Helper function to wrap confusing preprocessor
+extern "C" ASIOError get_sample_rate(double * rate);
+
+// Helper function to wrap confusing preprocessor
+extern "C" ASIOError set_sample_rate(double rate);
+
+// Helper function to wrap confusing preprocessor
+extern "C" ASIOError can_sample_rate(double rate);
+
+extern "C" bool load_asio_driver(char * name);
+extern "C" void remove_current_driver();
+extern "C" long get_driver_names(char **names, long maxDrivers);

asio-sys/build.rs

@@ -0,0 +1,216 @@
+extern crate bindgen;
+extern crate cc;
+extern crate walkdir;
+
+use std::env;
+use std::path::PathBuf;
+use walkdir::WalkDir;
+
+const CPAL_ASIO_DIR: &'static str = "CPAL_ASIO_DIR";
+
+const ASIO_HEADER: &'static str = "asio.h";
+const ASIO_SYS_HEADER: &'static str = "asiosys.h";
+const ASIO_DRIVERS_HEADER: &'static str = "asiodrivers.h";
+
+fn main() {
+    // If ASIO directory isn't set silently return early
+    let cpal_asio_dir_var = match env::var(CPAL_ASIO_DIR) {
+        Err(_) => return,
+        Ok(var) => var,
+    };
+
+    // Asio directory
+    let cpal_asio_dir = PathBuf::from(cpal_asio_dir_var);
+
+    // Directory where bindings and library are created
+    let out_dir = PathBuf::from(env::var("OUT_DIR").expect("bad path"));
+
+    // Check if library exists
+    // if it doesn't create it
+    let mut lib_path = out_dir.clone();
+    lib_path.push("libasio.a");
+    if !lib_path.exists() {
+        create_lib(&cpal_asio_dir);
+    }
+
+    // Print out links to needed libraries
+    println!("cargo:rustc-link-lib=dylib=ole32");
+    println!("cargo:rustc-link-lib=dylib=User32");
+    println!("cargo:rustc-link-search={}", out_dir.display());
+    println!("cargo:rustc-link-lib=static=asio");
+    println!("cargo:rustc-cfg=asio");
+
+    // Check if bindings exist
+    // if they dont create them
+    let mut binding_path = out_dir.clone();
+    binding_path.push("asio_bindings.rs");
+    if !binding_path.exists() {
+        create_bindings(&cpal_asio_dir);
+    }
+}
+
+fn create_lib(cpal_asio_dir: &PathBuf) {
+    let mut cpp_paths: Vec<PathBuf> = Vec::new();
+    let mut host_dir = cpal_asio_dir.clone();
+    let mut pc_dir = cpal_asio_dir.clone();
+    let mut common_dir = cpal_asio_dir.clone();
+    host_dir.push("host");
+    common_dir.push("common");
+    pc_dir.push("host/pc");
+
+    // Gathers cpp files from directories
+    let walk_a_dir = |dir_to_walk, paths: &mut Vec<PathBuf>| {
+        for entry in WalkDir::new(&dir_to_walk).max_depth(1) {
+            let entry = match entry {
+                Err(e) => {
+                    println!("error: {}", e);
+                continue
+                },
+                Ok(entry) => entry,
+            };
+            match entry.path().extension().and_then(|s| s.to_str()) {
+                None => continue,
+                Some("cpp") => {
+                    // Skip macos bindings
+                    if entry.path().file_name().unwrap().to_str() == Some("asiodrvr.cpp") {
+                        continue;
+                    }
+                    paths.push(entry.path().to_path_buf())
+                }
+                Some(_) => continue,
+            };
+        }
+    };
+
+    // Get all cpp files for building SDK library
+    walk_a_dir(host_dir, &mut cpp_paths);
+    walk_a_dir(pc_dir, &mut cpp_paths);
+    walk_a_dir(common_dir, &mut cpp_paths);
+
+    // build the asio lib
+    cc::Build::new()
+        .include(format!("{}/{}", cpal_asio_dir.display(), "host"))
+        .include(format!("{}/{}", cpal_asio_dir.display(), "common"))
+        .include(format!("{}/{}", cpal_asio_dir.display(), "host/pc"))
+        .include("asio-link/helpers.hpp")
+        .file("asio-link/helpers.cpp")
+        .files(cpp_paths)
+        .cpp(true)
+        .compile("libasio.a");
+}
+
+fn create_bindings(cpal_asio_dir: &PathBuf) {
+    let mut asio_header = None;
+    let mut asio_sys_header = None;
+    let mut asio_drivers_header = None;
+
+    // Recursively walk given cpal dir to find required headers
+    for entry in WalkDir::new(&cpal_asio_dir) {
+        let entry = match entry {
+            Err(_) => continue,
+            Ok(entry) => entry,
+        };
+        let file_name = match entry.path().file_name().and_then(|s| s.to_str()) {
+            None => continue,
+            Some(file_name) => file_name,
+        };
+
+        match file_name {
+            ASIO_HEADER => asio_header = Some(entry.path().to_path_buf()),
+            ASIO_SYS_HEADER => asio_sys_header = Some(entry.path().to_path_buf()),
+            ASIO_DRIVERS_HEADER => asio_drivers_header = Some(entry.path().to_path_buf()),
+            _ => (),
+        }
+    }
+
+    macro_rules! header_or_panic {
+        ($opt_header:expr, $FILE_NAME:expr) => {
+            match $opt_header.as_ref() {
+                None => {
+                    panic!("Could not find {} in {}: {}", $FILE_NAME, CPAL_ASIO_DIR, cpal_asio_dir.display());
+                },
+                Some(path) => path.to_str().expect("Could not convert path to str"),
+            }
+        };
+    }
+
+    // Only continue if found all headers that we need
+    let asio_header = header_or_panic!(asio_header, ASIO_HEADER);
+    let asio_sys_header = header_or_panic!(asio_sys_header, ASIO_SYS_HEADER);
+    let asio_drivers_header = header_or_panic!(asio_drivers_header, ASIO_DRIVERS_HEADER);
+
+    // The bindgen::Builder is the main entry point
+    // to bindgen, and lets you build up options for
+    // the resulting bindings.
+    let bindings = bindgen::Builder::default()
+        // The input header we would like to generate
+        // bindings for.
+        .header(asio_header)
+        .header(asio_sys_header)
+        .header(asio_drivers_header)
+        .header("asio-link/helpers.hpp")
+        .clang_arg("-x")
+        .clang_arg("c++")
+        .clang_arg("-std=c++14")
+        .clang_arg( format!("-I{}/{}", cpal_asio_dir.display(), "host/pc") )
+        .clang_arg( format!("-I{}/{}", cpal_asio_dir.display(), "host") )
+        .clang_arg( format!("-I{}/{}", cpal_asio_dir.display(), "common") )
+        // Need to whitelist to avoid binding tp c++ std::*
+        .whitelist_type("AsioDrivers")
+        .whitelist_type("AsioDriver")
+        .whitelist_type("ASIOTime")
+        .whitelist_type("ASIOTimeInfo")
+        .whitelist_type("ASIODriverInfo")
+        .whitelist_type("ASIOBufferInfo")
+        .whitelist_type("ASIOCallbacks")
+        .whitelist_type("ASIOSamples")
+        .whitelist_type("ASIOSampleType")
+        .whitelist_type("ASIOSampleRate")
+        .whitelist_type("ASIOChannelInfo")
+        .whitelist_type("AsioTimeInfoFlags")
+        .whitelist_type("ASIOTimeCodeFlags")
+        .whitelist_var("kAsioSelectorSupported")
+        .whitelist_var("kAsioEngineVersion")
+        .whitelist_var("kAsioResetRequest")
+        .whitelist_var("kAsioBufferSizeChange")
+        .whitelist_var("kAsioResyncRequest")
+        .whitelist_var("kAsioLatenciesChanged")
+        .whitelist_var("kAsioSupportsTimeInfo")
+        .whitelist_var("kAsioSupportsTimeCode")
+        .whitelist_var("kAsioMMCCommand")
+        .whitelist_var("kAsioSupportsInputMonitor")
+        .whitelist_var("kAsioSupportsInputGain")
+        .whitelist_var("kAsioSupportsInputMeter")
+        .whitelist_var("kAsioSupportsOutputGain")
+        .whitelist_var("kAsioSupportsOutputMeter")
+        .whitelist_var("kAsioOverload")
+        .whitelist_function("ASIOGetChannels")
+        .whitelist_function("ASIOGetChannelInfo")
+        .whitelist_function("ASIOGetBufferSize")
+        .whitelist_function("ASIOGetSamplePosition")
+        .whitelist_function("get_sample_rate")
+        .whitelist_function("set_sample_rate")
+        .whitelist_function("can_sample_rate")
+        .whitelist_function("ASIOInit")
+        .whitelist_function("ASIOCreateBuffers")
+        .whitelist_function("ASIOStart")
+        .whitelist_function("ASIOStop")
+        .whitelist_function("ASIODisposeBuffers")
+        .whitelist_function("ASIOExit")
+        .whitelist_function("load_asio_driver")
+        .whitelist_function("remove_current_driver")
+        .whitelist_function("get_driver_names")
+        .bitfield_enum("AsioTimeInfoFlags")
+        .bitfield_enum("ASIOTimeCodeFlags")
+        // Finish the builder and generate the bindings.
+        .generate()
+        // Unwrap the Result and panic on failure.
+        .expect("Unable to generate bindings");
+
+    // Write the bindings to the $OUT_DIR/bindings.rs file.
+    let out_path = PathBuf::from(env::var("OUT_DIR").expect("bad path"));
+    //panic!("path: {}", out_path.display());
+    bindings
+        .write_to_file(out_path.join("asio_bindings.rs"))
+        .expect("Couldn't write bindings!");
+}

asio-sys/examples/enumerate.rs

@@ -0,0 +1,56 @@
+/* This example aims to produce the same behaviour
+ * as the enumerate example in cpal
+ * by Tom Gowan
+ */
+
+extern crate asio_sys as sys;
+
+// This is the same data that enumerate
+// is trying to find
+// Basically these are stubbed versions
+//
+// Format that each sample has.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum SampleFormat {
+    // The value 0 corresponds to 0.
+    I16,
+    // The value 0 corresponds to 32768.
+    U16,
+    // The boundaries are (-1.0, 1.0).
+    F32,
+}
+// Number of channels.
+pub type ChannelCount = u16;
+
+// The number of samples processed per second for a single channel of audio.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct SampleRate(pub u32);
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Format {
+    pub channels: ChannelCount,
+    pub sample_rate: SampleRate,
+    pub data_type: SampleFormat,
+}
+
+fn main() {
+    let asio = sys::Asio::new();
+    for name in asio.driver_names() {
+        println!("Driver: {:?}", name);
+        let driver = asio.load_driver(&name).expect("failed to load driver");
+        let channels = driver.channels().expect("failed to retrieve channel counts");
+        let sample_rate = driver.sample_rate().expect("failed to retrieve sample rate");
+        let in_fmt = Format {
+            channels: channels.ins as _,
+            sample_rate: SampleRate(sample_rate as _),
+            data_type: SampleFormat::F32,
+        };
+        let out_fmt = Format {
+            channels: channels.outs as _,
+            sample_rate: SampleRate(sample_rate as _),
+            data_type: SampleFormat::F32,
+        };
+        println!("  Input {:?}", in_fmt);
+        println!("  Output {:?}", out_fmt);
+    }
+}

asio-sys/examples/test.rs

@@ -0,0 +1,11 @@
+extern crate asio_sys as sys;
+
+fn main() {
+    let asio = sys::Asio::new();
+    for driver in asio.driver_names() {
+        println!("Driver: {}", driver);
+        let driver = asio.load_driver(&driver).expect("failed to load drivers");
+        println!("  Channels: {:?}", driver.channels().expect("failed to get channels"));
+        println!("  Sample rate: {:?}", driver.sample_rate().expect("failed to get sample rate"));
+    }
+}

asio-sys/src/bindings/asio_import.rs

@@ -0,0 +1,6 @@
+#![allow(non_upper_case_globals)]
+#![allow(non_camel_case_types)]
+#![allow(non_snake_case)]
+#![allow(dead_code)]
+
+include!(concat!(env!("OUT_DIR"), "/asio_bindings.rs"));

asio-sys/src/bindings/errors.rs

@@ -0,0 +1,124 @@
+use std::error::Error;
+use std::fmt;
+
+/// Errors that might occur during `Asio::load_driver`.
+#[derive(Debug)]
+pub enum LoadDriverError {
+    LoadDriverFailed,
+    DriverAlreadyExists,
+    InitializationFailed(AsioError),
+}
+
+/// General errors returned by ASIO.
+#[derive(Debug)]
+pub enum AsioError {
+    NoDrivers,
+    HardwareMalfunction,
+    InvalidInput,
+    BadMode,
+    HardwareStuck,
+    NoRate,
+    ASE_NoMemory,
+    UnknownError,
+}
+
+#[derive(Debug)]
+pub enum AsioErrorWrapper {
+    ASE_OK = 0,               // This value will be returned whenever the call succeeded
+    ASE_SUCCESS = 0x3f4847a0, // unique success return value for ASIOFuture calls
+    ASE_NotPresent = -1000,   // hardware input or output is not present or available
+    ASE_HWMalfunction,        // hardware is malfunctioning (can be returned by any ASIO function)
+    ASE_InvalidParameter,     // input parameter invalid
+    ASE_InvalidMode,          // hardware is in a bad mode or used in a bad mode
+    ASE_SPNotAdvancing,       // hardware is not running when sample position is inquired
+    ASE_NoClock,              // sample clock or rate cannot be determined or is not present
+    ASE_NoMemory,             // not enough memory for completing the request
+    Invalid,
+}
+
+impl fmt::Display for LoadDriverError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.description())
+    }
+}
+
+impl fmt::Display for AsioError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            AsioError::NoDrivers => {
+                write!(f, "hardware input or output is not present or available")
+            }
+            AsioError::HardwareMalfunction => write!(
+                f,
+                "hardware is malfunctioning (can be returned by any ASIO function)"
+            ),
+            AsioError::InvalidInput => write!(f, "input parameter invalid"),
+            AsioError::BadMode => write!(f, "hardware is in a bad mode or used in a bad mode"),
+            AsioError::HardwareStuck => write!(
+                f,
+                "hardware is not running when sample position is inquired"
+            ),
+            AsioError::NoRate => write!(
+                f,
+                "sample clock or rate cannot be determined or is not present"
+            ),
+            AsioError::ASE_NoMemory => write!(f, "not enough memory for completing the request"),
+            AsioError::UnknownError => write!(f, "Error not in SDK"),
+        }
+    }
+}
+
+impl Error for LoadDriverError {
+    fn description(&self) -> &str {
+        match *self {
+            LoadDriverError::LoadDriverFailed => {
+                "ASIO `loadDriver` function returned `false` indicating failure"
+            }
+            LoadDriverError::InitializationFailed(ref err) => err.description(),
+            LoadDriverError::DriverAlreadyExists => {
+                "ASIO only supports loading one driver at a time"
+            }
+        }
+    }
+}
+
+impl Error for AsioError {
+    fn description(&self) -> &str {
+        match *self {
+            AsioError::NoDrivers => "hardware input or output is not present or available",
+            AsioError::HardwareMalfunction => {
+                "hardware is malfunctioning (can be returned by any ASIO function)"
+            }
+            AsioError::InvalidInput => "input parameter invalid",
+            AsioError::BadMode => "hardware is in a bad mode or used in a bad mode",
+            AsioError::HardwareStuck => "hardware is not running when sample position is inquired",
+            AsioError::NoRate => "sample clock or rate cannot be determined or is not present",
+            AsioError::ASE_NoMemory => "not enough memory for completing the request",
+            AsioError::UnknownError => "Error not in SDK",
+        }
+    }
+}
+
+impl From<AsioError> for LoadDriverError {
+    fn from(err: AsioError) -> Self {
+        LoadDriverError::InitializationFailed(err)
+    }
+}
+
+macro_rules! asio_result {
+    ($e:expr) => {{
+        let res = { $e };
+        match res {
+            r if r == AsioErrorWrapper::ASE_OK as i32 => Ok(()),
+            r if r == AsioErrorWrapper::ASE_SUCCESS as i32 => Ok(()),
+            r if r == AsioErrorWrapper::ASE_NotPresent as i32 => Err(AsioError::NoDrivers),
+            r if r == AsioErrorWrapper::ASE_HWMalfunction as i32 => Err(AsioError::HardwareMalfunction),
+            r if r == AsioErrorWrapper::ASE_InvalidParameter as i32 => Err(AsioError::InvalidInput),
+            r if r == AsioErrorWrapper::ASE_InvalidMode as i32 => Err(AsioError::BadMode),
+            r if r == AsioErrorWrapper::ASE_SPNotAdvancing as i32 => Err(AsioError::HardwareStuck),
+            r if r == AsioErrorWrapper::ASE_NoClock as i32 => Err(AsioError::NoRate),
+            r if r == AsioErrorWrapper::ASE_NoMemory as i32 => Err(AsioError::ASE_NoMemory),
+            _ => Err(AsioError::UnknownError),
+        }
+    }};
+}

asio-sys/src/bindings/mod.rs

@@ -0,0 +1,905 @@
+pub mod asio_import;
+#[macro_use]
+pub mod errors;
+
+use num_traits::FromPrimitive;
+use self::errors::{AsioError, AsioErrorWrapper, LoadDriverError};
+use std::ffi::CStr;
+use std::ffi::CString;
+use std::os::raw::{c_char, c_double, c_long, c_void};
+use std::sync::{Arc, Mutex, Weak};
+
+// Bindings import
+use self::asio_import as ai;
+
+/// A handle to the ASIO API.
+///
+/// There should only be one instance of this type at any point in time.
+#[derive(Debug)]
+pub struct Asio {
+    // Keeps track of whether or not a driver is already loaded.
+    //
+    // This is necessary as ASIO only supports one `Driver` at a time.
+    loaded_driver: Mutex<Weak<DriverInner>>,
+}
+
+/// A handle to a single ASIO driver.
+///
+/// Creating an instance of this type loads and initialises the driver.
+///
+/// Dropping all `Driver` instances will automatically dispose of any resources and de-initialise
+/// the driver.
+#[derive(Clone, Debug)]
+pub struct Driver {
+    inner: Arc<DriverInner>,
+}
+
+// Contains the state associated with a `Driver`.
+//
+// This state may be shared between multiple `Driver` handles representing the same underlying
+// driver. Only when the last `Driver` is dropped will the `Drop` implementation for this type run
+// and the necessary driver resources will be de-allocated and unloaded.
+//
+// The same could be achieved by returning an `Arc<Driver>` from the `Host::load_driver` API,
+// however the `DriverInner` abstraction is required in order to allow for the `Driver::destroy`
+// method to exist safely. By wrapping the `Arc<DriverInner>` in the `Driver` type, we can make
+// sure the user doesn't `try_unwrap` the `Arc` and invalidate the `Asio` instance's weak pointer.
+// This would allow for instantiation of a separate driver before the existing one is destroyed,
+// which is disallowed by ASIO.
+#[derive(Debug)]
+struct DriverInner {
+    state: Mutex<DriverState>,
+    // The unique name associated with this driver.
+    name: String,
+    // Track whether or not the driver has been destroyed.
+    //
+    // This allows for the user to manually destroy the driver and handle any errors if they wish.
+    //
+    // In the case that the driver has been manually destroyed this flag will be set to `true`
+    // indicating to the `drop` implementation that there is nothing to be done.
+    destroyed: bool,
+}
+
+/// All possible states of an ASIO `Driver` instance.
+///
+/// Mapped to the finite state machine in the ASIO SDK docs.
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+pub enum DriverState {
+    Initialized,
+    Prepared,
+    Running,
+}
+
+/// Amount of input and output
+/// channels available.
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+pub struct Channels {
+    pub ins: c_long,
+    pub outs: c_long,
+}
+
+/// Sample rate of the ASIO driver.
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+pub struct SampleRate {
+    pub rate: u32,
+}
+
+/// Holds the pointer to the callbacks that come from cpal
+struct BufferCallback(Box<FnMut(i32) + Send>);
+
+/// Input and Output streams.
+///
+/// There is only ever max one input and one output.
+///
+/// Only one is required.
+pub struct AsioStreams {
+    pub input: Option<AsioStream>,
+    pub output: Option<AsioStream>,
+}
+
+/// A stream to ASIO.
+///
+/// Contains the buffers.
+pub struct AsioStream {
+    /// A Double buffer per channel
+    pub buffer_infos: Vec<AsioBufferInfo>,
+    /// Size of each buffer
+    pub buffer_size: i32,
+}
+
+/// All the possible types from ASIO.
+/// This is a direct copy of the ASIOSampleType
+/// inside ASIO SDK.
+#[derive(Debug, FromPrimitive)]
+#[repr(C)]
+pub enum AsioSampleType {
+    ASIOSTInt16MSB = 0,
+    ASIOSTInt24MSB = 1, // used for 20 bits as well
+    ASIOSTInt32MSB = 2,
+    ASIOSTFloat32MSB = 3, // IEEE 754 32 bit float
+    ASIOSTFloat64MSB = 4, // IEEE 754 64 bit double float
+
+    // these are used for 32 bit data buffer, with different alignment of the data inside
+    // 32 bit PCI bus systems can be more easily used with these
+    ASIOSTInt32MSB16 = 8,  // 32 bit data with 16 bit alignment
+    ASIOSTInt32MSB18 = 9,  // 32 bit data with 18 bit alignment
+    ASIOSTInt32MSB20 = 10, // 32 bit data with 20 bit alignment
+    ASIOSTInt32MSB24 = 11, // 32 bit data with 24 bit alignment
+
+    ASIOSTInt16LSB = 16,
+    ASIOSTInt24LSB = 17, // used for 20 bits as well
+    ASIOSTInt32LSB = 18,
+    ASIOSTFloat32LSB = 19, // IEEE 754 32 bit float, as found on Intel x86 architecture
+    ASIOSTFloat64LSB = 20, // IEEE 754 64 bit double float, as found on Intel x86 architecture
+
+    // these are used for 32 bit data buffer, with different alignment of the data inside
+    // 32 bit PCI bus systems can more easily used with these
+    ASIOSTInt32LSB16 = 24, // 32 bit data with 18 bit alignment
+    ASIOSTInt32LSB18 = 25, // 32 bit data with 18 bit alignment
+    ASIOSTInt32LSB20 = 26, // 32 bit data with 20 bit alignment
+    ASIOSTInt32LSB24 = 27, // 32 bit data with 24 bit alignment
+
+    //	ASIO DSD format.
+    ASIOSTDSDInt8LSB1 = 32, // DSD 1 bit data, 8 samples per byte. First sample in Least significant bit.
+    ASIOSTDSDInt8MSB1 = 33, // DSD 1 bit data, 8 samples per byte. First sample in Most significant bit.
+    ASIOSTDSDInt8NER8 = 40, // DSD 8 bit data, 1 sample per byte. No Endianness required.
+
+    ASIOSTLastEntry,
+}
+
+/// Gives information about buffers
+/// Receives pointers to buffers
+#[derive(Debug, Copy, Clone)]
+#[repr(C)]
+pub struct AsioBufferInfo {
+    /// 0 for output 1 for input
+    pub is_input: c_long,
+    /// Which channel. Starts at 0
+    pub channel_num: c_long,
+    /// Pointer to each half of the double buffer.
+    pub buffers: [*mut c_void; 2],
+}
+
+/// Callbacks that ASIO calls
+#[repr(C)]
+struct AsioCallbacks {
+    buffer_switch: extern "C" fn(double_buffer_index: c_long, direct_process: c_long) -> (),
+    sample_rate_did_change: extern "C" fn(s_rate: c_double) -> (),
+    asio_message:
+        extern "C" fn(selector: c_long, value: c_long, message: *mut (), opt: *mut c_double)
+            -> c_long,
+    buffer_switch_time_info: extern "C" fn(
+        params: *mut ai::ASIOTime,
+        double_buffer_index: c_long,
+        direct_process: c_long,
+    ) -> *mut ai::ASIOTime,
+}
+
+/// A rust-usable version of the `ASIOTime` type that does not contain a binary blob for fields.
+#[repr(C)]
+pub struct AsioTime {
+    /// Must be `0`.
+    pub reserved: [c_long; 4],
+    /// Required.
+    pub time_info: AsioTimeInfo,
+    /// Optional, evaluated if (time_code.flags & ktcValid).
+    pub time_code: AsioTimeCode,
+}
+
+/// A rust-compatible version of the `ASIOTimeInfo` type that does not contain a binary blob for
+/// fields.
+#[repr(C)]
+pub struct AsioTimeInfo {
+    /// Absolute speed (1. = nominal).
+    pub speed: c_double,
+    /// System time related to sample_position, in nanoseconds.
+    ///
+    /// On Windows, must be derived from timeGetTime().
+    pub system_time: ai::ASIOTimeStamp,
+    /// Sample position since `ASIOStart()`.
+    pub sample_position: ai::ASIOSamples,
+    /// Current rate, unsigned.
+    pub sample_rate: AsioSampleRate,
+    /// See `AsioTimeInfoFlags`.
+    pub flags: c_long,
+    /// Must be `0`.
+    pub reserved: [c_char; 12],
+}
+
+/// A rust-compatible version of the `ASIOTimeCode` type that does not use a binary blob for its
+/// fields.
+#[repr(C)]
+pub struct AsioTimeCode {
+    /// Speed relation (fraction of nominal speed) optional.
+    ///
+    /// Set to 0. or 1. if not supported.
+    pub speed: c_double,
+    /// Time in samples unsigned.
+    pub time_code_samples: ai::ASIOSamples,
+    /// See `ASIOTimeCodeFlags`.
+    pub flags: c_long,
+    /// Set to `0`.
+    pub future: [c_char; 64],
+}
+
+/// A rust-compatible version of the `ASIOSampleRate` type that does not use a binary blob for its
+/// fields.
+pub type AsioSampleRate = f64;
+
+// A helper type to simplify retrieval of available buffer sizes.
+#[derive(Default)]
+struct BufferSizes {
+    min: c_long,
+    max: c_long,
+    pref: c_long,
+    grans: c_long,
+}
+
+lazy_static! {
+    /// A global way to access all the callbacks.
+    ///
+    /// This is required because of how ASIO calls the `buffer_switch` function with no data
+    /// parameters.
+    ///
+    /// Options are used so that when a callback is removed we don't change the Vec indices.
+    ///
+    /// The indices are how we match a callback with a stream.
+    static ref BUFFER_CALLBACK: Mutex<Vec<Option<BufferCallback>>> = Mutex::new(Vec::new());
+}
+
+impl Asio {
+    /// Initialise the ASIO API.
+    pub fn new() -> Self {
+        let loaded_driver = Mutex::new(Weak::new());
+        Asio { loaded_driver }
+    }
+
+    /// Returns the name for each available driver.
+    ///
+    /// This is used at the start to allow the user to choose which driver they want.
+    pub fn driver_names(&self) -> Vec<String> {
+        // The most drivers we can take
+        const MAX_DRIVERS: usize = 100;
+        // Max length for divers name
+        const MAX_DRIVER_NAME_LEN: usize = 32;
+
+        // 2D array of driver names set to 0.
+        let mut driver_names: [[c_char; MAX_DRIVER_NAME_LEN]; MAX_DRIVERS] =
+            [[0; MAX_DRIVER_NAME_LEN]; MAX_DRIVERS];
+        // Pointer to each driver name.
+        let mut driver_name_ptrs: [*mut i8; MAX_DRIVERS] = [0 as *mut i8; MAX_DRIVERS];
+        for (ptr, name) in driver_name_ptrs.iter_mut().zip(&mut driver_names[..]) {
+            *ptr = (*name).as_mut_ptr();
+        }
+
+        unsafe {
+            let num_drivers = ai::get_driver_names(driver_name_ptrs.as_mut_ptr(), MAX_DRIVERS as i32);
+            (0 .. num_drivers)
+                .map(|i| driver_name_to_utf8(&driver_names[i as usize]).to_string())
+                .collect()
+        }
+    }
+
+    /// If a driver has already been loaded, this will return that driver.
+    ///
+    /// Returns `None` if no driver is currently loaded.
+    ///
+    /// This can be useful to check before calling `load_driver` as ASIO only supports loading a
+    /// single driver at a time.
+    pub fn loaded_driver(&self) -> Option<Driver> {
+        self.loaded_driver
+            .lock()
+            .expect("failed to acquire loaded driver lock")
+            .upgrade()
+            .map(|inner| Driver { inner })
+    }
+
+    /// Load a driver from the given name.
+    ///
+    /// Driver names compatible with this method can be produced via the `asio.driver_names()`
+    /// method.
+    ///
+    /// NOTE: Despite many requests from users, ASIO only supports loading a single driver at a
+    /// time. Calling this method while a previously loaded `Driver` instance exists will result in
+    /// an error. That said, if this method is called with the name of a driver that has already
+    /// been loaded, that driver will be returned successfully.
+    pub fn load_driver(&self, driver_name: &str) -> Result<Driver, LoadDriverError> {
+        // Check whether or not a driver is already loaded.
+        if let Some(driver) = self.loaded_driver() {
+            if driver.name() == driver_name {
+                return Ok(driver);
+            } else {
+                return Err(LoadDriverError::DriverAlreadyExists);
+            }
+        }
+
+        // Make owned CString to send to load driver
+        let driver_name_cstring = CString::new(driver_name)
+            .expect("failed to create `CString` from driver name");
+        let mut driver_info = ai::ASIODriverInfo {
+            _bindgen_opaque_blob: [0u32; 43],
+        };
+
+        unsafe {
+            // TODO: Check that a driver of the same name does not already exist?
+            match ai::load_asio_driver(driver_name_cstring.as_ptr() as *mut i8) {
+                false => Err(LoadDriverError::LoadDriverFailed),
+                true => {
+                    // Initialize ASIO.
+                    asio_result!(ai::ASIOInit(&mut driver_info))?;
+                    let state = Mutex::new(DriverState::Initialized);
+                    let name = driver_name.to_string();
+                    let destroyed = false;
+                    let inner = Arc::new(DriverInner { name, state, destroyed });
+                    *self.loaded_driver.lock().expect("failed to acquire loaded driver lock") =
+                        Arc::downgrade(&inner);
+                    let driver = Driver { inner };
+                    Ok(driver)
+                }
+            }
+        }
+    }
+}
+
+impl BufferCallback {
+    /// Calls the inner callback.
+    fn run(&mut self, index: i32) {
+        let cb = &mut self.0;
+        cb(index);
+    }
+}
+
+impl Driver {
+    /// The name used to uniquely identify this driver.
+    pub fn name(&self) -> &str {
+        &self.inner.name
+    }
+
+    /// Returns the number of input and output channels available on the driver.
+    pub fn channels(&self) -> Result<Channels, AsioError> {
+        let mut ins: c_long = 0;
+        let mut outs: c_long = 0;
+        unsafe {
+            asio_result!(ai::ASIOGetChannels(&mut ins, &mut outs))?;
+        }
+        let channel = Channels { ins, outs };
+        Ok(channel)
+    }
+
+    /// Get current sample rate of the driver.
+    pub fn sample_rate(&self) -> Result<c_double, AsioError> {
+        let mut rate: c_double = 0.0;
+        unsafe {
+            asio_result!(ai::get_sample_rate(&mut rate))?;
+        }
+        Ok(rate)
+    }
+
+    /// Can the driver accept the given sample rate.
+    pub fn can_sample_rate(&self, sample_rate: c_double) -> Result<bool, AsioError> {
+        unsafe {
+            match asio_result!(ai::can_sample_rate(sample_rate)) {
+                Ok(()) => Ok(true),
+                Err(AsioError::NoRate) => Ok(false),
+                Err(err) => Err(err),
+            }
+        }
+    }
+
+    /// Set the sample rate for the driver.
+    pub fn set_sample_rate(&self, sample_rate: c_double) -> Result<(), AsioError> {
+        unsafe {
+            asio_result!(ai::set_sample_rate(sample_rate))?;
+        }
+        Ok(())
+    }
+
+    /// Get the current data type of the driver's input stream.
+    ///
+    /// This queries a single channel's type assuming all channels have the same sample type.
+    pub fn input_data_type(&self) -> Result<AsioSampleType, AsioError> {
+        stream_data_type(true)
+    }
+
+    /// Get the current data type of the driver's output stream.
+    ///
+    /// This queries a single channel's type assuming all channels have the same sample type.
+    pub fn output_data_type(&self) -> Result<AsioSampleType, AsioError> {
+        stream_data_type(false)
+    }
+
+    /// Ask ASIO to allocate the buffers and give the callback pointers.
+    ///
+    /// This will destroy any already allocated buffers.
+    ///
+    /// The preferred buffer size from ASIO is used.
+    fn create_buffers(&self, buffer_infos: &mut [AsioBufferInfo]) -> Result<c_long, AsioError> {
+        let num_channels = buffer_infos.len();
+
+        // To pass as ai::ASIOCallbacks
+        let mut callbacks = create_asio_callbacks();
+
+        // Retrieve the available buffer sizes.
+        let buffer_sizes = asio_get_buffer_sizes()?;
+        if buffer_sizes.pref <= 0 {
+            panic!(
+                "`ASIOGetBufferSize` produced unusable preferred buffer size of {}",
+                buffer_sizes.pref,
+            );
+        }
+
+        // Ensure the driver is in the `Initialized` state.
+        if let DriverState::Running = self.inner.state() {
+            self.stop()?;
+        }
+        if let DriverState::Prepared = self.inner.state() {
+            self.dispose_buffers()?;
+        }
+
+        unsafe {
+            asio_result!(ai::ASIOCreateBuffers(
+                buffer_infos.as_mut_ptr() as *mut _,
+                num_channels as i32,
+                buffer_sizes.pref,
+                &mut callbacks as *mut _ as *mut _,
+            ))?;
+        }
+
+        self.inner.set_state(DriverState::Prepared);
+        Ok(buffer_sizes.pref)
+    }
+
+    /// Creates the streams.
+    ///
+    /// Both input and output streams need to be created together as a single slice of
+    /// `ASIOBufferInfo`.
+    fn create_streams(
+        &self,
+        mut input_buffer_infos: Vec<AsioBufferInfo>,
+        mut output_buffer_infos: Vec<AsioBufferInfo>,
+    ) -> Result<AsioStreams, AsioError> {
+        let (input, output) = match (input_buffer_infos.is_empty(), output_buffer_infos.is_empty()) {
+            // Both stream exist.
+            (false, false) => {
+                // Create one continuous slice of buffers.
+                let split_point = input_buffer_infos.len();
+                let mut all_buffer_infos = input_buffer_infos;
+                all_buffer_infos.append(&mut output_buffer_infos);
+                // Create the buffers. On success, split the output and input again.
+                let buffer_size = self.create_buffers(&mut all_buffer_infos)?;
+                let output_buffer_infos = all_buffer_infos.split_off(split_point);
+                let input_buffer_infos = all_buffer_infos;
+                let input = Some(AsioStream {
+                    buffer_infos: input_buffer_infos,
+                    buffer_size,
+                });
+                let output = Some(AsioStream {
+                    buffer_infos: output_buffer_infos,
+                    buffer_size,
+                });
+                (input, output)
+            },
+            // Just input
+            (false, true) => {
+                let buffer_size = self.create_buffers(&mut input_buffer_infos)?;
+                let input = Some(AsioStream {
+                    buffer_infos: input_buffer_infos,
+                    buffer_size,
+                });
+                let output = None;
+                (input, output)
+            },
+            // Just output
+            (true, false) => {
+                let buffer_size = self.create_buffers(&mut output_buffer_infos)?;
+                let input = None;
+                let output = Some(AsioStream {
+                    buffer_infos: output_buffer_infos,
+                    buffer_size,
+                });
+                (input, output)
+            },
+            // Impossible
+            (true, true) => unreachable!("Trying to create streams without preparing"),
+        };
+        Ok(AsioStreams { input, output })
+    }
+
+    /// Prepare the input stream.
+    ///
+    /// Because only the latest call to ASIOCreateBuffers is relevant this call will destroy all
+    /// past active buffers and recreate them.
+    ///
+    /// For this reason we take the output stream if it exists.
+    ///
+    /// `num_channels` is the desired number of input channels.
+    ///
+    /// This returns a full AsioStreams with both input and output if output was active.
+    pub fn prepare_input_stream(
+        &self,
+        output: Option<AsioStream>,
+        num_channels: usize,
+    ) -> Result<AsioStreams, AsioError> {
+        let input_buffer_infos = prepare_buffer_infos(true, num_channels);
+        let output_buffer_infos = output
+            .map(|output| output.buffer_infos)
+            .unwrap_or_else(Vec::new);
+        self.create_streams(input_buffer_infos, output_buffer_infos)
+    }
+
+    /// Prepare the output stream.
+    ///
+    /// Because only the latest call to ASIOCreateBuffers is relevant this call will destroy all
+    /// past active buffers and recreate them.
+    ///
+    /// For this reason we take the input stream if it exists.
+    ///
+    /// `num_channels` is the desired number of output channels.
+    ///
+    /// This returns a full AsioStreams with both input and output if input was active.
+    pub fn prepare_output_stream(
+        &self,
+        input: Option<AsioStream>,
+        num_channels: usize,
+    ) -> Result<AsioStreams, AsioError> {
+        let input_buffer_infos = input
+            .map(|input| input.buffer_infos)
+            .unwrap_or_else(Vec::new);
+        let output_buffer_infos = prepare_buffer_infos(false, num_channels);
+        self.create_streams(input_buffer_infos, output_buffer_infos)
+    }
+
+    /// Releases buffers allocations.
+    ///
+    /// This will `stop` the stream if the driver is `Running`.
+    ///
+    /// No-op if no buffers are allocated.
+    pub fn dispose_buffers(&self) -> Result<(), AsioError> {
+        self.inner.dispose_buffers_inner()
+    }
+
+    /// Starts ASIO streams playing.
+    ///
+    /// The driver must be in the `Prepared` state
+    ///
+    /// If called successfully, the driver will be in the `Running` state.
+    ///
+    /// No-op if already `Running`.
+    pub fn start(&self) -> Result<(), AsioError> {
+        if let DriverState::Running = self.inner.state() {
+            return Ok(());
+        }
+        unsafe {
+            asio_result!(ai::ASIOStart())?;
+        }
+        self.inner.set_state(DriverState::Running);
+        Ok(())
+    }
+
+    /// Stops ASIO streams playing.
+    ///
+    /// No-op if the state is not `Running`.
+    ///
+    /// If the state was `Running` and the stream is stopped successfully, the driver will be in
+    /// the `Prepared` state.
+    pub fn stop(&self) -> Result<(), AsioError> {
+        self.inner.stop_inner()
+    }
+
+    /// Adds a callback to the list of active callbacks.
+    ///
+    /// The given function receives the index of the buffer currently ready for processing.
+    pub fn set_callback<F>(&self, callback: F)
+    where
+        F: 'static + FnMut(i32) + Send,
+    {
+        let mut bc = BUFFER_CALLBACK.lock().unwrap();
+        bc.push(Some(BufferCallback(Box::new(callback))));
+    }
+
+    /// Consumes and destroys the `Driver`, stopping the streams if they are running and releasing
+    /// any associated resources.
+    ///
+    /// Returns `Ok(true)` if the driver was successfully destroyed.
+    ///
+    /// Returns `Ok(false)` if the driver was not destroyed because another handle to the driver
+    /// still exists.
+    ///
+    /// Returns `Err` if some switching driver states failed or if ASIO returned an error on exit.
+    pub fn destroy(self) -> Result<bool, AsioError> {
+        let Driver { inner } = self;
+        match Arc::try_unwrap(inner) {
+            Err(_) => Ok(false),
+            Ok(mut inner) => {
+                inner.destroy_inner()?;
+                Ok(true)
+            }
+        }
+    }
+}
+
+impl DriverInner {
+    fn state(&self) -> DriverState {
+        *self.state.lock().expect("failed to lock `DriverState`")
+    }
+
+    fn set_state(&self, state: DriverState) {
+        *self.state.lock().expect("failed to lock `DriverState`") = state;
+    }
+
+    fn stop_inner(&self) -> Result<(), AsioError> {
+        if let DriverState::Running = self.state() {
+            unsafe {
+                asio_result!(ai::ASIOStop())?;
+            }
+            self.set_state(DriverState::Prepared);
+        }
+        Ok(())
+    }
+
+    fn dispose_buffers_inner(&self) -> Result<(), AsioError> {
+        if let DriverState::Initialized = self.state() {
+            return Ok(());
+        }
+        if let DriverState::Running = self.state() {
+            self.stop_inner()?;
+        }
+        unsafe {
+            asio_result!(ai::ASIODisposeBuffers())?;
+        }
+        self.set_state(DriverState::Initialized);
+        Ok(())
+    }
+
+    fn destroy_inner(&mut self) -> Result<(), AsioError> {
+        // Drop back through the driver state machine one state at a time.
+        if let DriverState::Running = self.state() {
+            self.stop_inner()?;
+        }
+        if let DriverState::Prepared = self.state() {
+            self.dispose_buffers_inner()?;
+        }
+        unsafe {
+            asio_result!(ai::ASIOExit())?;
+            ai::remove_current_driver();
+        }
+
+        // Clear any existing stream callbacks.
+        if let Ok(mut bcs) = BUFFER_CALLBACK.lock() {
+            bcs.clear();
+        }
+
+        // Signal that the driver has been destroyed.
+        self.destroyed = true;
+
+        Ok(())
+    }
+}
+
+impl Drop for DriverInner {
+    fn drop(&mut self) {
+        if !self.destroyed {
+            // We probably shouldn't `panic!` in the destructor? We also shouldn't ignore errors
+            // though either.
+            self.destroy_inner().ok();
+        }
+    }
+}
+
+unsafe impl Send for AsioStream {}
+
+/// Used by the input and output stream creation process.
+fn prepare_buffer_infos(is_input: bool, n_channels: usize) -> Vec<AsioBufferInfo> {
+    let is_input = if is_input { 1 } else { 0 };
+    (0..n_channels)
+        .map(|ch| {
+            let channel_num = ch as c_long;
+            // To be filled by ASIOCreateBuffers.
+            let buffers = [std::ptr::null_mut(); 2];
+            AsioBufferInfo { is_input, channel_num, buffers }
+        })
+        .collect()
+}
+
+/// The set of callbacks passed to `ASIOCreateBuffers`.
+fn create_asio_callbacks() -> AsioCallbacks {
+    AsioCallbacks {
+        buffer_switch: buffer_switch,
+        sample_rate_did_change: sample_rate_did_change,
+        asio_message: asio_message,
+        buffer_switch_time_info: buffer_switch_time_info,
+    }
+}
+
+/// Retrieve the minimum, maximum and preferred buffer sizes along with the available
+/// buffer size granularity.
+fn asio_get_buffer_sizes() -> Result<BufferSizes, AsioError> {
+    let mut b = BufferSizes::default();
+    unsafe {
+        let res = ai::ASIOGetBufferSize(&mut b.min, &mut b.max, &mut b.pref, &mut b.grans);
+        asio_result!(res)?;
+    }
+    Ok(b)
+}
+
+/// Retrieve the `ASIOChannelInfo` associated with the channel at the given index on either the
+/// input or output stream (`true` for input).
+fn asio_channel_info(channel: c_long, is_input: bool) -> Result<ai::ASIOChannelInfo, AsioError> {
+    let mut channel_info = ai::ASIOChannelInfo {
+        // Which channel we are querying
+        channel,
+        // Was it input or output
+        isInput: if is_input { 1 } else { 0 },
+        // Was it active
+        isActive: 0,
+        channelGroup: 0,
+        // The sample type
+        type_: 0,
+        name: [0 as c_char; 32],
+    };
+    unsafe {
+        asio_result!(ai::ASIOGetChannelInfo(&mut channel_info))?;
+        Ok(channel_info)
+    }
+}
+
+/// Retrieve the data type of either the input or output stream.
+///
+/// If `is_input` is true, this will be queried on the input stream.
+fn stream_data_type(is_input: bool) -> Result<AsioSampleType, AsioError> {
+    let channel_info = asio_channel_info(0, is_input)?;
+    Ok(FromPrimitive::from_i32(channel_info.type_).expect("unkown `ASIOSampletype` value"))
+}
+
+/// ASIO uses null terminated c strings for driver names.
+///
+/// This converts to utf8.
+fn driver_name_to_utf8(bytes: &[c_char]) -> std::borrow::Cow<str> {
+    unsafe {
+        CStr::from_ptr(bytes.as_ptr()).to_string_lossy()
+    }
+}
+
+/// ASIO uses null terminated c strings for channel names.
+///
+/// This converts to utf8.
+fn _channel_name_to_utf8(bytes: &[c_char]) -> std::borrow::Cow<str> {
+    unsafe {
+        CStr::from_ptr(bytes.as_ptr()).to_string_lossy()
+    }
+}
+
+/// Indicates the stream sample rate has changed.
+///
+/// TODO: Provide some way of allowing CPAL to handle this.
+extern "C" fn sample_rate_did_change(s_rate: c_double) -> () {
+    eprintln!("unhandled sample rate change to {}", s_rate);
+}
+
+/// Message callback for ASIO to notify of certain events.
+extern "C" fn asio_message(
+    selector: c_long,
+    value: c_long,
+    _message: *mut (),
+    _opt: *mut c_double,
+) -> c_long {
+    match selector {
+        ai::kAsioSelectorSupported => {
+            // Indicate what message selectors are supported.
+            match value {
+                | ai::kAsioResetRequest
+                | ai::kAsioEngineVersion
+                | ai::kAsioResyncRequest
+                | ai::kAsioLatenciesChanged
+                // Following added in ASIO 2.0.
+                | ai::kAsioSupportsTimeInfo
+                | ai::kAsioSupportsTimeCode
+                | ai::kAsioSupportsInputMonitor => 1,
+                _ => 0,
+            }
+        }
+
+        ai::kAsioResetRequest => {
+            // Defer the task and perform the reset of the driver during the next "safe" situation
+            // You cannot reset the driver right now, as this code is called from the driver. Reset
+            // the driver is done by completely destruct it. I.e. ASIOStop(), ASIODisposeBuffers(),
+            // Destruction. Afterwards you initialize the driver again.
+            // TODO: Handle this.
+            1
+        }
+
+        ai::kAsioResyncRequest => {
+            // This informs the application, that the driver encountered some non fatal data loss.
+            // It is used for synchronization purposes of different media. Added mainly to work
+            // around the Win16Mutex problems in Windows 95/98 with the Windows Multimedia system,
+            // which could loose data because the Mutex was hold too long by another thread.
+            // However a driver can issue it in other situations, too.
+            // TODO: Handle this.
+            1
+        }
+
+        ai::kAsioLatenciesChanged => {
+            // This will inform the host application that the drivers were latencies changed.
+            // Beware, it this does not mean that the buffer sizes have changed! You might need to
+            // update internal delay data.
+            // TODO: Handle this.
+            1
+        }
+
+        ai::kAsioEngineVersion => {
+            // Return the supported ASIO version of the host application If a host applications
+            // does not implement this selector, ASIO 1.0 is assumed by the driver
+            2
+        }
+
+        ai::kAsioSupportsTimeInfo => {
+            // Informs the driver whether the asioCallbacks.bufferSwitchTimeInfo() callback is
+            // supported. For compatibility with ASIO 1.0 drivers the host application should
+            // always support the "old" bufferSwitch method, too, which we do.
+            1
+        }
+
+        ai::kAsioSupportsTimeCode => {
+            // Informs the driver whether the application is interested in time code info. If an
+            // application does not need to know about time code, the driver has less work to do.
+            // TODO: Provide an option for this?
+            0
+        }
+
+        _ => 0, // Unknown/unhandled message type.
+    }
+}
+
+/// Similar to buffer switch but with time info.
+///
+/// If only `buffer_switch` is called by the driver instead, the `buffer_switch` callback will
+/// create the necessary timing info and call this function.
+///
+/// TODO: Provide some access to `ai::ASIOTime` once CPAL gains support for time stamps.
+extern "C" fn buffer_switch_time_info(
+    time: *mut ai::ASIOTime,
+    double_buffer_index: c_long,
+    _direct_process: c_long,
+) -> *mut ai::ASIOTime {
+    // This lock is probably unavoidable, but locks in the audio stream are not great.
+    let mut bcs = BUFFER_CALLBACK.lock().unwrap();
+    for mut bc in bcs.iter_mut() {
+        if let Some(ref mut bc) = bc {
+            bc.run(double_buffer_index);
+        }
+    }
+    time
+}
+
+/// This is called by ASIO.
+///
+/// Here we run the callback for each stream.
+///
+/// `double_buffer_index` is either `0` or `1`  indicating which buffer to fill.
+extern "C" fn buffer_switch(double_buffer_index: c_long, direct_process: c_long) -> () {
+    // Emulate the time info provided by the `buffer_switch_time_info` callback.
+    // This is an attempt at matching the behaviour in `hostsample.cpp` from the SDK.
+    let mut time = unsafe {
+        let mut time: AsioTime = std::mem::zeroed();
+        let res = ai::ASIOGetSamplePosition(
+            &mut time.time_info.sample_position,
+            &mut time.time_info.system_time,
+        );
+        if let Ok(()) = asio_result!(res) {
+            time.time_info.flags =
+                (ai::AsioTimeInfoFlags::kSystemTimeValid | ai::AsioTimeInfoFlags::kSamplePositionValid).0;
+        }
+        time
+    };
+
+    // Actual processing happens within the `buffer_switch_time_info` callback.
+    let asio_time_ptr = &mut time as *mut AsioTime as *mut ai::ASIOTime;
+    buffer_switch_time_info(asio_time_ptr, double_buffer_index, direct_process);
+}
+
+#[test]
+fn check_type_sizes() {
+    assert_eq!(std::mem::size_of::<AsioSampleRate>(), std::mem::size_of::<ai::ASIOSampleRate>());
+    assert_eq!(std::mem::size_of::<AsioTimeCode>(), std::mem::size_of::<ai::ASIOTimeCode>());
+    assert_eq!(std::mem::size_of::<AsioTime>(), std::mem::size_of::<ai::ASIOTime>());
+}

asio-sys/src/lib.rs

@@ -0,0 +1,18 @@
+#![allow(non_camel_case_types)]
+
+#[allow(unused_imports)]
+#[macro_use]
+extern crate lazy_static;
+
+#[allow(unused_imports)]
+#[macro_use]
+extern crate num_derive;
+#[allow(unused_imports)]
+extern crate num_traits;
+
+#[cfg(asio)]
+pub mod bindings;
+#[cfg(asio)]
+pub use bindings::*;
+#[cfg(asio)]
+pub use bindings::errors::{AsioError, LoadDriverError};

build.rs

@@ -0,0 +1,12 @@
+use std::env;
+
+const CPAL_ASIO_DIR: &'static str = "CPAL_ASIO_DIR";
+
+fn main() {
+    // If ASIO directory isn't set silently return early 
+    // otherwise set the asio config flag
+    match env::var(CPAL_ASIO_DIR) {
+        Err(_) => return,
+        Ok(_) => println!("cargo:rustc-cfg=asio"),
+    };
+}

src/host/asio/device.rs

@@ -0,0 +1,192 @@
+use std;
+pub type SupportedInputFormats = std::vec::IntoIter<SupportedFormat>;
+pub type SupportedOutputFormats = std::vec::IntoIter<SupportedFormat>;
+
+use std::hash::{Hash, Hasher};
+use std::sync::Arc;
+use BackendSpecificError;
+use DefaultFormatError;
+use DeviceNameError;
+use DevicesError;
+use Format;
+use SampleFormat;
+use SampleRate;
+use SupportedFormat;
+use SupportedFormatsError;
+use super::sys;
+
+/// A ASIO Device
+#[derive(Debug)]
+pub struct Device {
+    /// The driver represented by this device.
+    pub driver: Arc<sys::Driver>,
+}
+
+/// All available devices.
+pub struct Devices {
+    asio: Arc<sys::Asio>,
+    drivers: std::vec::IntoIter<String>,
+}
+
+impl PartialEq for Device {
+    fn eq(&self, other: &Self) -> bool {
+        self.driver.name() == other.driver.name()
+    }
+}
+
+impl Eq for Device {}
+
+impl Hash for Device {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.driver.name().hash(state);
+    }
+}
+
+impl Device {
+    pub fn name(&self) -> Result<String, DeviceNameError> {
+        Ok(self.driver.name().to_string())
+    }
+
+    /// Gets the supported input formats.
+    /// TODO currently only supports the default.
+    /// Need to find all possible formats.
+    pub fn supported_input_formats(
+        &self,
+    ) -> Result<SupportedInputFormats, SupportedFormatsError> {
+        // Retrieve the default format for the total supported channels and supported sample
+        // format.
+        let mut f = match self.default_input_format() {
+            Err(_) => return Err(SupportedFormatsError::DeviceNotAvailable),
+            Ok(f) => f,
+        };
+
+        // Collect a format for every combination of supported sample rate and number of channels.
+        let mut supported_formats = vec![];
+        for &rate in ::COMMON_SAMPLE_RATES {
+            if !self.driver.can_sample_rate(rate.0.into()).ok().unwrap_or(false) {
+                continue;
+            }
+            for channels in 1..f.channels + 1 {
+                f.channels = channels;
+                f.sample_rate = rate;
+                supported_formats.push(SupportedFormat::from(f.clone()));
+            }
+        }
+        Ok(supported_formats.into_iter())
+    }
+
+    /// Gets the supported output formats.
+    /// TODO currently only supports the default.
+    /// Need to find all possible formats.
+    pub fn supported_output_formats(
+        &self,
+    ) -> Result<SupportedOutputFormats, SupportedFormatsError> {
+        // Retrieve the default format for the total supported channels and supported sample
+        // format.
+        let mut f = match self.default_output_format() {
+            Err(_) => return Err(SupportedFormatsError::DeviceNotAvailable),
+            Ok(f) => f,
+        };
+
+        // Collect a format for every combination of supported sample rate and number of channels.
+        let mut supported_formats = vec![];
+        for &rate in ::COMMON_SAMPLE_RATES {
+            if !self.driver.can_sample_rate(rate.0.into()).ok().unwrap_or(false) {
+                continue;
+            }
+            for channels in 1..f.channels + 1 {
+                f.channels = channels;
+                f.sample_rate = rate;
+                supported_formats.push(SupportedFormat::from(f.clone()));
+            }
+        }
+        Ok(supported_formats.into_iter())
+    }
+
+    /// Returns the default input format
+    pub fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
+        let channels = self.driver.channels().map_err(default_format_err)?.ins as u16;
+        let sample_rate = SampleRate(self.driver.sample_rate().map_err(default_format_err)? as _);
+        // Map th ASIO sample type to a CPAL sample type
+        let data_type = self.driver.input_data_type().map_err(default_format_err)?;
+        let data_type = convert_data_type(&data_type)
+            .ok_or(DefaultFormatError::StreamTypeNotSupported)?;
+        Ok(Format {
+            channels,
+            sample_rate,
+            data_type,
+        })
+    }
+
+    /// Returns the default output format
+    pub fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
+        let channels = self.driver.channels().map_err(default_format_err)?.outs as u16;
+        let sample_rate = SampleRate(self.driver.sample_rate().map_err(default_format_err)? as _);
+        let data_type = self.driver.output_data_type().map_err(default_format_err)?;
+        let data_type = convert_data_type(&data_type)
+            .ok_or(DefaultFormatError::StreamTypeNotSupported)?;
+        Ok(Format {
+            channels,
+            sample_rate,
+            data_type,
+        })
+    }
+}
+
+impl Devices {
+    pub fn new(asio: Arc<sys::Asio>) -> Result<Self, DevicesError> {
+        let drivers = asio.driver_names().into_iter();
+        Ok(Devices { asio, drivers })
+    }
+}
+
+impl Iterator for Devices {
+    type Item = Device;
+
+    /// Load drivers and return device
+    fn next(&mut self) -> Option<Device> {
+        loop {
+            match self.drivers.next() {
+                Some(name) => match self.asio.load_driver(&name) {
+                    Ok(driver) => return Some(Device { driver: Arc::new(driver) }),
+                    Err(_) => continue,
+                }
+                None => return None,
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        unimplemented!()
+    }
+}
+
+pub(crate) fn convert_data_type(ty: &sys::AsioSampleType) -> Option<SampleFormat> {
+    let fmt = match *ty {
+        sys::AsioSampleType::ASIOSTInt16MSB => SampleFormat::I16,
+        sys::AsioSampleType::ASIOSTInt16LSB => SampleFormat::I16,
+        sys::AsioSampleType::ASIOSTFloat32MSB => SampleFormat::F32,
+        sys::AsioSampleType::ASIOSTFloat32LSB => SampleFormat::F32,
+        // NOTE: While ASIO does not support these formats directly, the stream callback created by
+        // CPAL supports converting back and forth between the following. This is because many ASIO
+        // drivers only support `Int32` formats, while CPAL does not support this format at all. We
+        // allow for this implicit conversion temporarily until CPAL gets support for an `I32`
+        // format.
+        sys::AsioSampleType::ASIOSTInt32MSB => SampleFormat::I16,
+        sys::AsioSampleType::ASIOSTInt32LSB => SampleFormat::I16,
+        _ => return None,
+    };
+    Some(fmt)
+}
+
+fn default_format_err(e: sys::AsioError) -> DefaultFormatError {
+    match e {
+        sys::AsioError::NoDrivers |
+        sys::AsioError::HardwareMalfunction => DefaultFormatError::DeviceNotAvailable,
+        sys::AsioError::NoRate => DefaultFormatError::StreamTypeNotSupported,
+        err => {
+            let description = format!("{}", err);
+            BackendSpecificError { description }.into()
+        }
+    }
+}

src/host/asio/mod.rs

@@ -0,0 +1,136 @@
+extern crate asio_sys as sys;
+
+use {
+    BuildStreamError,
+    DefaultFormatError,
+    DeviceNameError,
+    DevicesError,
+    Format,
+    PauseStreamError,
+    PlayStreamError,
+    StreamDataResult,
+    SupportedFormatsError,
+};
+use traits::{
+    DeviceTrait,
+    EventLoopTrait,
+    HostTrait,
+    StreamIdTrait,
+};
+
+pub use self::device::{Device, Devices, SupportedInputFormats, SupportedOutputFormats};
+pub use self::stream::{EventLoop, StreamId};
+use std::sync::Arc;
+
+mod device;
+mod stream;
+
+/// The host for ASIO.
+#[derive(Debug)]
+pub struct Host {
+    asio: Arc<sys::Asio>,
+}
+
+impl Host {
+    pub fn new() -> Result<Self, crate::HostUnavailable> {
+        let asio = Arc::new(sys::Asio::new());
+        let host = Host { asio };
+        Ok(host)
+    }
+}
+
+impl HostTrait for Host {
+    type Devices = Devices;
+    type Device = Device;
+    type EventLoop = EventLoop;
+
+    fn is_available() -> bool {
+        true
+        //unimplemented!("check how to do this using asio-sys")
+    }
+
+    fn devices(&self) -> Result<Self::Devices, DevicesError> {
+        Devices::new(self.asio.clone())
+    }
+
+    fn default_input_device(&self) -> Option<Self::Device> {
+        // ASIO has no concept of a default device, so just use the first.
+        self.input_devices().ok().and_then(|mut ds| ds.next())
+    }
+
+    fn default_output_device(&self) -> Option<Self::Device> {
+        // ASIO has no concept of a default device, so just use the first.
+        self.output_devices().ok().and_then(|mut ds| ds.next())
+    }
+
+    fn event_loop(&self) -> Self::EventLoop {
+        EventLoop::new()
+    }
+}
+
+impl DeviceTrait for Device {
+    type SupportedInputFormats = SupportedInputFormats;
+    type SupportedOutputFormats = SupportedOutputFormats;
+
+    fn name(&self) -> Result<String, DeviceNameError> {
+        Device::name(self)
+    }
+
+    fn supported_input_formats(&self) -> Result<Self::SupportedInputFormats, SupportedFormatsError> {
+        Device::supported_input_formats(self)
+    }
+
+    fn supported_output_formats(&self) -> Result<Self::SupportedOutputFormats, SupportedFormatsError> {
+        Device::supported_output_formats(self)
+    }
+
+    fn default_input_format(&self) -> Result<Format, DefaultFormatError> {
+        Device::default_input_format(self)
+    }
+
+    fn default_output_format(&self) -> Result<Format, DefaultFormatError> {
+        Device::default_output_format(self)
+    }
+}
+
+impl EventLoopTrait for EventLoop {
+    type Device = Device;
+    type StreamId = StreamId;
+
+    fn build_input_stream(
+        &self,
+        device: &Self::Device,
+        format: &Format,
+    ) -> Result<Self::StreamId, BuildStreamError> {
+        EventLoop::build_input_stream(self, device, format)
+    }
+
+    fn build_output_stream(
+        &self,
+        device: &Self::Device,
+        format: &Format,
+    ) -> Result<Self::StreamId, BuildStreamError> {
+        EventLoop::build_output_stream(self, device, format)
+    }
+
+    fn play_stream(&self, stream: Self::StreamId) -> Result<(), PlayStreamError> {
+        EventLoop::play_stream(self, stream)
+    }
+
+    fn pause_stream(&self, stream: Self::StreamId) -> Result<(), PauseStreamError> {
+        EventLoop::pause_stream(self, stream)
+    }
+
+    fn destroy_stream(&self, stream: Self::StreamId) {
+        EventLoop::destroy_stream(self, stream)
+    }
+
+    fn run<F>(&self, callback: F) -> !
+    where
+        F: FnMut(Self::StreamId, StreamDataResult) + Send,
+    {
+        EventLoop::run(self, callback)
+    }
+}
+
+impl StreamIdTrait for StreamId {}

src/host/asio/stream.rs

@@ -0,0 +1,814 @@
+extern crate asio_sys as sys;
+extern crate num_traits;
+
+use self::num_traits::PrimInt;
+use super::Device;
+use std;
+use std::mem;
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::{Arc, Mutex};
+use std::thread;
+use std::time::Duration;
+use BackendSpecificError;
+use BuildStreamError;
+use Format;
+use PauseStreamError;
+use PlayStreamError;
+use SampleFormat;
+use StreamData;
+use StreamDataResult;
+use UnknownTypeInputBuffer;
+use UnknownTypeOutputBuffer;
+
+/// Sample types whose constant silent value is known.
+trait Silence {
+    const SILENCE: Self;
+}
+
+/// Constraints on the interleaved sample buffer format required by the CPAL API.
+trait InterleavedSample: Clone + Copy + Silence {
+    fn unknown_type_input_buffer(&[Self]) -> UnknownTypeInputBuffer;
+    fn unknown_type_output_buffer(&mut [Self]) -> UnknownTypeOutputBuffer;
+}
+
+/// Constraints on the ASIO sample types.
+trait AsioSample: Clone + Copy + Silence + std::ops::Add<Self, Output = Self> {}
+
+/// Controls all streams
+pub struct EventLoop {
+    /// The input and output ASIO streams
+    asio_streams: Arc<Mutex<sys::AsioStreams>>,
+    /// List of all CPAL streams
+    cpal_streams: Arc<Mutex<Vec<Option<Stream>>>>,
+    /// Total stream count.
+    stream_count: AtomicUsize,
+    /// The CPAL callback that the user gives to fill the buffers.
+    callbacks: Arc<Mutex<Option<&'static mut (FnMut(StreamId, StreamDataResult) + Send)>>>,
+}
+
+/// Id for each stream.
+/// Created depending on the number they are created.
+/// Starting at one! not zero.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub struct StreamId(usize);
+
+/// CPAL stream.
+/// This decouples the many cpal streams
+/// from the single input and single output
+/// ASIO streams.
+/// Each stream can be playing or paused.
+struct Stream {
+    playing: bool,
+    // The driver associated with this stream.
+    driver: Arc<sys::Driver>,
+}
+
+// Used to keep track of whether or not the current current asio stream buffer requires
+// being silencing before summing audio.
+#[derive(Default)]
+struct SilenceAsioBuffer {
+    first: bool,
+    second: bool,
+}
+
+impl EventLoop {
+    pub fn new() -> EventLoop {
+        EventLoop {
+            asio_streams: Arc::new(Mutex::new(sys::AsioStreams {
+                input: None,
+                output: None,
+            })),
+            cpal_streams: Arc::new(Mutex::new(Vec::new())),
+            // This is why the Id's count from one not zero
+            // because at this point there is no streams
+            stream_count: AtomicUsize::new(0),
+            callbacks: Arc::new(Mutex::new(None)),
+        }
+    }
+
+    /// Create a new CPAL Input Stream.
+    ///
+    /// If there is no existing ASIO Input Stream it will be created.
+    ///
+    /// On success, the buffer size of the stream is returned.
+    fn get_or_create_input_stream(
+        &self,
+        driver: &sys::Driver,
+        format: &Format,
+        device: &Device,
+    ) -> Result<usize, BuildStreamError> {
+        match device.default_input_format() {
+            Ok(f) => {
+                let num_asio_channels = f.channels;
+                check_format(driver, format, num_asio_channels)
+            },
+            Err(_) => Err(BuildStreamError::FormatNotSupported),
+        }?;
+        let num_channels = format.channels as usize;
+        let ref mut streams = *self.asio_streams.lock().unwrap();
+        // Either create a stream if thers none or had back the
+        // size of the current one.
+        match streams.input {
+            Some(ref input) => Ok(input.buffer_size as usize),
+            None => {
+                let output = streams.output.take();
+                driver
+                    .prepare_input_stream(output, num_channels)
+                    .map(|new_streams| {
+                        let bs = match new_streams.input {
+                            Some(ref inp) => inp.buffer_size as usize,
+                            None => unreachable!(),
+                        };
+                        *streams = new_streams;
+                        bs
+                    }).map_err(|ref e| {
+                        println!("Error preparing stream: {}", e);
+                        BuildStreamError::DeviceNotAvailable
+                    })
+            }
+        }
+    }
+
+    /// Create a new CPAL Output Stream.
+    ///
+    /// If there is no existing ASIO Output Stream it will be created.
+    ///
+    /// On success, the buffer size of the stream is returned.
+    fn get_or_create_output_stream(
+        &self,
+        driver: &sys::Driver,
+        format: &Format,
+        device: &Device,
+    ) -> Result<usize, BuildStreamError> {
+        match device.default_output_format() {
+            Ok(f) => {
+                let num_asio_channels = f.channels;
+                check_format(driver, format, num_asio_channels)
+            },
+            Err(_) => Err(BuildStreamError::FormatNotSupported),
+        }?;
+        let num_channels = format.channels as usize;
+        let ref mut streams = *self.asio_streams.lock().unwrap();
+        // Either create a stream if there's none or return the size of the current one.
+        match streams.output {
+            Some(ref output) => Ok(output.buffer_size as usize),
+            None => {
+                let input = streams.input.take();
+                driver
+                    .prepare_output_stream(input, num_channels)
+                    .map(|new_streams| {
+                        let bs = match new_streams.output {
+                            Some(ref out) => out.buffer_size as usize,
+                            None => unreachable!(),
+                        };
+                        *streams = new_streams;
+                        bs
+                    }).map_err(|ref e| {
+                        println!("Error preparing stream: {}", e);
+                        BuildStreamError::DeviceNotAvailable
+                    })
+            }
+        }
+    }
+
+    /// Builds a new cpal input stream
+    pub fn build_input_stream(
+        &self,
+        device: &Device,
+        format: &Format,
+    ) -> Result<StreamId, BuildStreamError> {
+        let Device { driver, .. } = device;
+        let stream_type = driver.input_data_type().map_err(build_stream_err)?;
+
+        // Ensure that the desired sample type is supported.
+        let data_type = super::device::convert_data_type(&stream_type)
+            .ok_or(BuildStreamError::FormatNotSupported)?;
+        if format.data_type != data_type {
+            return Err(BuildStreamError::FormatNotSupported);
+        }
+
+        let num_channels = format.channels.clone();
+        let stream_buffer_size = self.get_or_create_input_stream(&driver, format, device)?;
+        let cpal_num_samples = stream_buffer_size * num_channels as usize;
+        let count = self.stream_count.fetch_add(1, Ordering::SeqCst);
+        let asio_streams = self.asio_streams.clone();
+        let cpal_streams = self.cpal_streams.clone();
+        let callbacks = self.callbacks.clone();
+
+        // Create the buffer depending on the size of the data type.
+        let stream_id = StreamId(count);
+        let len_bytes = cpal_num_samples * data_type.sample_size();
+        let mut interleaved = vec![0u8; len_bytes];
+
+        // Set the input callback.
+        // This is most performance critical part of the ASIO bindings.
+        driver.set_callback(move |buffer_index| unsafe {
+            // If not playing return early.
+            // TODO: Don't assume `count` is valid - we should search for the matching `StreamId`.
+            if let Some(s) = cpal_streams.lock().unwrap().get(count) {
+                if let Some(s) = s {
+                    if !s.playing {
+                        return;
+                    }
+                }
+            }
+
+            // Acquire the stream and callback.
+            let stream_lock = asio_streams.lock().unwrap();
+            let ref asio_stream = match stream_lock.input {
+                Some(ref asio_stream) => asio_stream,
+                None => return,
+            };
+            let mut callbacks = callbacks.lock().unwrap();
+            let callback = match callbacks.as_mut() {
+                Some(callback) => callback,
+                None => return,
+            };
+
+            /// 1. Write from the ASIO buffer to the interleaved CPAL buffer.
+            /// 2. Deliver the CPAL buffer to the user callback.
+            unsafe fn process_input_callback<A, B, F, G>(
+                stream_id: StreamId,
+                callback: &mut (dyn FnMut(StreamId, StreamDataResult) + Send),
+                interleaved: &mut [u8],
+                asio_stream: &sys::AsioStream,
+                buffer_index: usize,
+                from_endianness: F,
+                to_cpal_sample: G,
+            )
+            where
+                A: AsioSample,
+                B: InterleavedSample,
+                F: Fn(A) -> A,
+                G: Fn(A) -> B,
+            {
+                // 1. Write the ASIO channels to the CPAL buffer.
+                let interleaved: &mut [B] = cast_slice_mut(interleaved);
+                let n_channels = interleaved.len() / asio_stream.buffer_size as usize;
+                for ch_ix in 0..n_channels {
+                    let asio_channel = asio_channel_slice::<A>(asio_stream, buffer_index, ch_ix);
+                    for (frame, s_asio) in interleaved.chunks_mut(n_channels).zip(asio_channel) {
+                        frame[ch_ix] = to_cpal_sample(from_endianness(*s_asio));
+                    }
+                }
+
+                // 2. Deliver the interleaved buffer to the callback.
+                callback(
+                    stream_id,
+                    Ok(StreamData::Input { buffer: B::unknown_type_input_buffer(interleaved) }),
+                );
+            }
+
+            match (&stream_type, data_type) {
+                (&sys::AsioSampleType::ASIOSTInt16LSB, SampleFormat::I16) => {
+                    process_input_callback::<i16, i16, _, _>(
+                        stream_id,
+                        callback,
+                        &mut interleaved,
+                        asio_stream,
+                        buffer_index as usize,
+                        from_le,
+                        std::convert::identity::<i16>,
+                    );
+                }
+                (&sys::AsioSampleType::ASIOSTInt16MSB, SampleFormat::I16) => {
+                    process_input_callback::<i16, i16, _, _>(
+                        stream_id,
+                        callback,
+                        &mut interleaved,
+                        asio_stream,
+                        buffer_index as usize,
+                        from_be,
+                        std::convert::identity::<i16>,
+                    );
+                }
+
+                // TODO: Handle endianness conversion for floats? We currently use the `PrimInt`
+                // trait for the `to_le` and `to_be` methods, but this does not support floats.
+                (&sys::AsioSampleType::ASIOSTFloat32LSB, SampleFormat::F32) |
+                (&sys::AsioSampleType::ASIOSTFloat32MSB, SampleFormat::F32) => {
+                    process_input_callback::<f32, f32, _, _>(
+                        stream_id,
+                        callback,
+                        &mut interleaved,
+                        asio_stream,
+                        buffer_index as usize,
+                        std::convert::identity::<f32>,
+                        std::convert::identity::<f32>,
+                    );
+                }
+
+                // TODO: Add support for the following sample formats to CPAL and simplify the
+                // `process_output_callback` function above by removing the unnecessary sample
+                // conversion function.
+                (&sys::AsioSampleType::ASIOSTInt32LSB, SampleFormat::I16) => {
+                    process_input_callback::<i32, i16, _, _>(
+                        stream_id,
+                        callback,
+                        &mut interleaved,
+                        asio_stream,
+                        buffer_index as usize,
+                        from_le,
+                        |s| (s >> 16) as i16,
+                    );
+                }
+                (&sys::AsioSampleType::ASIOSTInt32MSB, SampleFormat::I16) => {
+                    process_input_callback::<i32, i16, _, _>(
+                        stream_id,
+                        callback,
+                        &mut interleaved,
+                        asio_stream,
+                        buffer_index as usize,
+                        from_be,
+                        |s| (s >> 16) as i16,
+                    );
+                }
+                // TODO: Handle endianness conversion for floats? We currently use the `PrimInt`
+                // trait for the `to_le` and `to_be` methods, but this does not support floats.
+                (&sys::AsioSampleType::ASIOSTFloat64LSB, SampleFormat::F32) |
+                (&sys::AsioSampleType::ASIOSTFloat64MSB, SampleFormat::F32) => {
+                    process_input_callback::<f64, f32, _, _>(
+                        stream_id,
+                        callback,
+                        &mut interleaved,
+                        asio_stream,
+                        buffer_index as usize,
+                        std::convert::identity::<f64>,
+                        |s| s as f32,
+                    );
+                }
+
+                unsupported_format_pair => {
+                    unreachable!("`build_input_stream` should have returned with unsupported \
+                                 format {:?}", unsupported_format_pair)
+                }
+            }
+        });
+
+        // Create stream and set to paused
+        self.cpal_streams
+            .lock()
+            .unwrap()
+            .push(Some(Stream { driver: driver.clone(), playing: false }));
+
+        Ok(StreamId(count))
+    }
+
+    /// Create the an output cpal stream.
+    pub fn build_output_stream(
+        &self,
+        device: &Device,
+        format: &Format,
+    ) -> Result<StreamId, BuildStreamError> {
+        let Device { driver, .. } = device;
+        let stream_type = driver.output_data_type().map_err(build_stream_err)?;
+
+        // Ensure that the desired sample type is supported.
+        let data_type = super::device::convert_data_type(&stream_type)
+            .ok_or(BuildStreamError::FormatNotSupported)?;
+        if format.data_type != data_type {
+            return Err(BuildStreamError::FormatNotSupported);
+        }
+
+        let num_channels = format.channels.clone();
+        let stream_buffer_size = self.get_or_create_output_stream(&driver, format, device)?;
+        let cpal_num_samples = stream_buffer_size * num_channels as usize;
+        let count = self.stream_count.fetch_add(1, Ordering::SeqCst);
+        let asio_streams = self.asio_streams.clone();
+        let cpal_streams = self.cpal_streams.clone();
+        let callbacks = self.callbacks.clone();
+
+        // Create buffers depending on data type.
+        let stream_id = StreamId(count);
+        let len_bytes = cpal_num_samples * data_type.sample_size();
+        let mut interleaved = vec![0u8; len_bytes];
+        let mut silence_asio_buffer = SilenceAsioBuffer::default();
+
+        driver.set_callback(move |buffer_index| unsafe {
+            // If not playing, return early.
+            // TODO: Don't assume `count` is valid - we should search for the matching `StreamId`.
+            if let Some(s) = cpal_streams.lock().unwrap().get(count) {
+                if let Some(s) = s {
+                    if !s.playing {
+                        return ();
+                    }
+                }
+            }
+
+            // Acquire the stream and callback.
+            let stream_lock = asio_streams.lock().unwrap();
+            let ref asio_stream = match stream_lock.output {
+                Some(ref asio_stream) => asio_stream,
+                None => return,
+            };
+            let mut callbacks = callbacks.lock().unwrap();
+            let callback = callbacks.as_mut();
+
+            // Silence the ASIO buffer that is about to be used.
+            //
+            // This checks if any other callbacks have already silenced the buffer associated with
+            // the current `buffer_index`.
+            //
+            // If not, we will silence it and set the opposite buffer half to unsilenced.
+            let silence = match buffer_index {
+                0 if !silence_asio_buffer.first => {
+                    silence_asio_buffer.first = true;
+                    silence_asio_buffer.second = false;
+                    true
+                }
+                0 => false,
+                1 if !silence_asio_buffer.second => {
+                    silence_asio_buffer.second = true;
+                    silence_asio_buffer.first = false;
+                    true
+                }
+                1 => false,
+                _ => unreachable!("ASIO uses a double-buffer so there should only be 2"),
+            };
+
+            /// 1. Render the given callback to the given buffer of interleaved samples.
+            /// 2. If required, silence the ASIO buffer.
+            /// 3. Finally, write the interleaved data to the non-interleaved ASIO buffer,
+            ///    performing endianness conversions as necessary.
+            unsafe fn process_output_callback<A, B, F, G>(
+                stream_id: StreamId,
+                callback: Option<&mut &mut (dyn FnMut(StreamId, StreamDataResult) + Send)>,
+                interleaved: &mut [u8],
+                silence_asio_buffer: bool,
+                asio_stream: &sys::AsioStream,
+                buffer_index: usize,
+                to_asio_sample: F,
+                to_endianness: G,
+            )
+            where
+                A: InterleavedSample,
+                B: AsioSample,
+                F: Fn(A) -> B,
+                G: Fn(B) -> B,
+            {
+                // 1. Render interleaved buffer from callback.
+                let interleaved: &mut [A] = cast_slice_mut(interleaved);
+                match callback {
+                    None => interleaved.iter_mut().for_each(|s| *s = A::SILENCE),
+                    Some(callback) => {
+                        let buffer = A::unknown_type_output_buffer(interleaved);
+                        callback(stream_id, Ok(StreamData::Output { buffer }));
+                    }
+                }
+
+                // 2. Silence ASIO channels if necessary.
+                let n_channels = interleaved.len() / asio_stream.buffer_size as usize;
+                if silence_asio_buffer {
+                    for ch_ix in 0..n_channels {
+                        let asio_channel =
+                            asio_channel_slice_mut::<B>(asio_stream, buffer_index, ch_ix);
+                        asio_channel.iter_mut().for_each(|s| *s = to_endianness(B::SILENCE));
+                    }
+                }
+
+                // 3. Write interleaved samples to ASIO channels, one channel at a time.
+                for ch_ix in 0..n_channels {
+                    let asio_channel =
+                        asio_channel_slice_mut::<B>(asio_stream, buffer_index, ch_ix);
+                    for (frame, s_asio) in interleaved.chunks(n_channels).zip(asio_channel) {
+                        *s_asio = *s_asio + to_endianness(to_asio_sample(frame[ch_ix]));
+                    }
+                }
+            }
+
+            match (data_type, &stream_type) {
+                (SampleFormat::I16, &sys::AsioSampleType::ASIOSTInt16LSB) => {
+                    process_output_callback::<i16, i16, _, _>(
+                        stream_id,
+                        callback,
+                        &mut interleaved,
+                        silence,
+                        asio_stream,
+                        buffer_index as usize,
+                        std::convert::identity::<i16>,
+                        to_le,
+                    );
+                }
+                (SampleFormat::I16, &sys::AsioSampleType::ASIOSTInt16MSB) => {
+                    process_output_callback::<i16, i16, _, _>(
+                        stream_id,
+                        callback,
+                        &mut interleaved,
+                        silence,
+                        asio_stream,
+                        buffer_index as usize,
+                        std::convert::identity::<i16>,
+                        to_be,
+                    );
+                }
+
+                // TODO: Handle endianness conversion for floats? We currently use the `PrimInt`
+                // trait for the `to_le` and `to_be` methods, but this does not support floats.
+                (SampleFormat::F32, &sys::AsioSampleType::ASIOSTFloat32LSB) |
+                (SampleFormat::F32, &sys::AsioSampleType::ASIOSTFloat32MSB) => {
+                    process_output_callback::<f32, f32, _, _>(
+                        stream_id,
+                        callback,
+                        &mut interleaved,
+                        silence,
+                        asio_stream,
+                        buffer_index as usize,
+                        std::convert::identity::<f32>,
+                        std::convert::identity::<f32>,
+                    );
+                }
+
+                // TODO: Add support for the following sample formats to CPAL and simplify the
+                // `process_output_callback` function above by removing the unnecessary sample
+                // conversion function.
+                (SampleFormat::I16, &sys::AsioSampleType::ASIOSTInt32LSB) => {
+                    process_output_callback::<i16, i32, _, _>(
+                        stream_id,
+                        callback,
+                        &mut interleaved,
+                        silence,
+                        asio_stream,
+                        buffer_index as usize,
+                        |s| (s as i32) << 16,
+                        to_le,
+                    );
+                }
+                (SampleFormat::I16, &sys::AsioSampleType::ASIOSTInt32MSB) => {
+                    process_output_callback::<i16, i32, _, _>(
+                        stream_id,
+                        callback,
+                        &mut interleaved,
+                        silence,
+                        asio_stream,
+                        buffer_index as usize,
+                        |s| (s as i32) << 16,
+                        to_be,
+                    );
+                }
+                // TODO: Handle endianness conversion for floats? We currently use the `PrimInt`
+                // trait for the `to_le` and `to_be` methods, but this does not support floats.
+                (SampleFormat::F32, &sys::AsioSampleType::ASIOSTFloat64LSB) |
+                (SampleFormat::F32, &sys::AsioSampleType::ASIOSTFloat64MSB) => {
+                    process_output_callback::<f32, f64, _, _>(
+                        stream_id,
+                        callback,
+                        &mut interleaved,
+                        silence,
+                        asio_stream,
+                        buffer_index as usize,
+                        |s| s as f64,
+                        std::convert::identity::<f64>,
+                    );
+                }
+
+                unsupported_format_pair => {
+                    unreachable!("`build_output_stream` should have returned with unsupported \
+                                 format {:?}", unsupported_format_pair)
+                }
+            }
+        });
+
+        // Create the stream paused
+        self.cpal_streams
+            .lock()
+            .unwrap()
+            .push(Some(Stream { driver: driver.clone(), playing: false }));
+
+        // Give the ID based on the stream count
+        Ok(StreamId(count))
+    }
+
+    /// Play the cpal stream for the given ID.
+    pub fn play_stream(&self, stream_id: StreamId) -> Result<(), PlayStreamError> {
+        let mut streams = self.cpal_streams.lock().unwrap();
+        if let Some(s) = streams.get_mut(stream_id.0).expect("Bad play stream index") {
+            s.playing = true;
+            // Calling play when already playing is a no-op
+            s.driver.start().map_err(play_stream_err)?;
+        }
+        Ok(())
+    }
+
+    /// Pause the cpal stream for the given ID.
+    ///
+    /// Pause the ASIO streams if there are no other CPAL streams playing, as ASIO only allows
+    /// stopping the entire driver.
+    pub fn pause_stream(&self, stream_id: StreamId) -> Result<(), PauseStreamError> {
+        let mut streams = self.cpal_streams.lock().unwrap();
+        let streams_playing = streams.iter()
+            .filter(|s| s.as_ref().map(|s| s.playing).unwrap_or(false))
+            .count();
+        if let Some(s) = streams.get_mut(stream_id.0).expect("Bad pause stream index") {
+            if streams_playing <= 1 {
+                s.driver.stop().map_err(pause_stream_err)?;
+            }
+            s.playing = false;
+        }
+        Ok(())
+    }
+
+    /// Destroy the cpal stream based on the ID.
+    pub fn destroy_stream(&self, stream_id: StreamId) {
+        // TODO: Should we not also remove an ASIO stream here?
+        //       Yes, and we should update the logic in the callbacks to search for the stream with
+        //       the matching ID, rather than assuming the index associated with the ID is valid.
+        let mut streams = self.cpal_streams.lock().unwrap();
+        streams.get_mut(stream_id.0).take();
+    }
+
+    /// Run the cpal callbacks
+    pub fn run<F>(&self, mut callback: F) -> !
+    where
+        F: FnMut(StreamId, StreamDataResult) + Send,
+    {
+        let callback: &mut (FnMut(StreamId, StreamDataResult) + Send) = &mut callback;
+        // Transmute needed to convince the compiler that the callback has a static lifetime
+        *self.callbacks.lock().unwrap() = Some(unsafe { mem::transmute(callback) });
+        loop {
+            // A sleep here to prevent the loop being
+            // removed in --release
+            thread::sleep(Duration::new(1u64, 0u32));
+        }
+    }
+}
+
+/// Clean up if event loop is dropped.
+/// Currently event loop is never dropped.
+impl Drop for EventLoop {
+    fn drop(&mut self) {
+        *self.asio_streams.lock().unwrap() = sys::AsioStreams {
+            output: None,
+            input: None,
+        };
+    }
+}
+
+impl Silence for i16 {
+    const SILENCE: Self = 0;
+}
+
+impl Silence for i32 {
+    const SILENCE: Self = 0;
+}
+
+impl Silence for f32 {
+    const SILENCE: Self = 0.0;
+}
+
+impl Silence for f64 {
+    const SILENCE: Self = 0.0;
+}
+
+impl InterleavedSample for i16 {
+    fn unknown_type_input_buffer(buffer: &[Self]) -> UnknownTypeInputBuffer {
+        UnknownTypeInputBuffer::I16(::InputBuffer { buffer })
+    }
+
+    fn unknown_type_output_buffer(buffer: &mut [Self]) -> UnknownTypeOutputBuffer {
+        UnknownTypeOutputBuffer::I16(::OutputBuffer { buffer })
+    }
+}
+
+impl InterleavedSample for f32 {
+    fn unknown_type_input_buffer(buffer: &[Self]) -> UnknownTypeInputBuffer {
+        UnknownTypeInputBuffer::F32(::InputBuffer { buffer })
+    }
+
+    fn unknown_type_output_buffer(buffer: &mut [Self]) -> UnknownTypeOutputBuffer {
+        UnknownTypeOutputBuffer::F32(::OutputBuffer { buffer })
+    }
+}
+
+impl AsioSample for i16 {}
+
+impl AsioSample for i32 {}
+
+impl AsioSample for f32 {}
+
+impl AsioSample for f64 {}
+
+/// Check whether or not the desired format is supported by the stream.
+///
+/// Checks sample rate, data type and then finally the number of channels.
+fn check_format(
+    driver: &sys::Driver,
+    format: &Format,
+    num_asio_channels: u16,
+) -> Result<(), BuildStreamError> {
+    let Format {
+        channels,
+        sample_rate,
+        data_type,
+    } = format;
+    // Try and set the sample rate to what the user selected.
+    let sample_rate = sample_rate.0.into();
+    if sample_rate != driver.sample_rate().map_err(build_stream_err)? {
+        if driver.can_sample_rate(sample_rate).map_err(build_stream_err)? {
+            driver
+                .set_sample_rate(sample_rate)
+                .map_err(build_stream_err)?;
+        } else {
+            return Err(BuildStreamError::FormatNotSupported);
+        }
+    }
+    // unsigned formats are not supported by asio
+    match data_type {
+        SampleFormat::I16 | SampleFormat::F32 => (),
+        SampleFormat::U16 => return Err(BuildStreamError::FormatNotSupported),
+    }
+    if *channels > num_asio_channels {
+        return Err(BuildStreamError::FormatNotSupported);
+    }
+    Ok(())
+}
+
+/// Cast a byte slice into a mutable slice of desired type.
+///
+/// Safety: it's up to the caller to ensure that the input slice has valid bit representations.
+unsafe fn cast_slice_mut<T>(v: &mut [u8]) -> &mut [T] {
+    debug_assert!(v.len() % std::mem::size_of::<T>() == 0);
+    std::slice::from_raw_parts_mut(v.as_mut_ptr() as *mut T, v.len() / std::mem::size_of::<T>())
+}
+
+/// Helper function to convert to little endianness.
+fn to_le<T: PrimInt>(t: T) -> T {
+    t.to_le()
+}
+
+/// Helper function to convert to big endianness.
+fn to_be<T: PrimInt>(t: T) -> T {
+    t.to_be()
+}
+
+/// Helper function to convert from little endianness.
+fn from_le<T: PrimInt>(t: T) -> T {
+    T::from_le(t)
+}
+
+/// Helper function to convert from little endianness.
+fn from_be<T: PrimInt>(t: T) -> T {
+    T::from_be(t)
+}
+
+/// Shorthand for retrieving the asio buffer slice associated with a channel.
+///
+/// Safety: it's up to the user to ensure that this function is not called multiple times for the
+/// same channel.
+unsafe fn asio_channel_slice<T>(
+    asio_stream: &sys::AsioStream,
+    buffer_index: usize,
+    channel_index: usize,
+) -> &[T] {
+    asio_channel_slice_mut(asio_stream, buffer_index, channel_index)
+}
+
+/// Shorthand for retrieving the asio buffer slice associated with a channel.
+///
+/// Safety: it's up to the user to ensure that this function is not called multiple times for the
+/// same channel.
+unsafe fn asio_channel_slice_mut<T>(
+    asio_stream: &sys::AsioStream,
+    buffer_index: usize,
+    channel_index: usize,
+) -> &mut [T] {
+    let buff_ptr: *mut T = asio_stream
+        .buffer_infos[channel_index]
+        .buffers[buffer_index as usize]
+        as *mut _;
+    std::slice::from_raw_parts_mut(buff_ptr, asio_stream.buffer_size as usize)
+}
+
+fn build_stream_err(e: sys::AsioError) -> BuildStreamError {
+    match e {
+        sys::AsioError::NoDrivers |
+        sys::AsioError::HardwareMalfunction => BuildStreamError::DeviceNotAvailable,
+        sys::AsioError::InvalidInput |
+        sys::AsioError::BadMode => BuildStreamError::InvalidArgument,
+        err => {
+            let description = format!("{}", err);
+            BackendSpecificError { description }.into()
+        }
+    }
+}
+
+fn pause_stream_err(e: sys::AsioError) -> PauseStreamError {
+    match e {
+        sys::AsioError::NoDrivers |
+        sys::AsioError::HardwareMalfunction => PauseStreamError::DeviceNotAvailable,
+        err => {
+            let description = format!("{}", err);
+            BackendSpecificError { description }.into()
+        }
+    }
+}
+
+fn play_stream_err(e: sys::AsioError) -> PlayStreamError {
+    match e {
+        sys::AsioError::NoDrivers |
+        sys::AsioError::HardwareMalfunction => PlayStreamError::DeviceNotAvailable,
+        err => {
+            let description = format!("{}", err);
+            BackendSpecificError { description }.into()
+        }
+    }
+}

src/host/mod.rs

@@ -1,5 +1,7 @@
 #[cfg(any(target_os = "linux", target_os = "freebsd"))]
 pub(crate) mod alsa;
+#[cfg(all(windows, feature = "asio"))]
+pub(crate) mod asio;
 #[cfg(any(target_os = "macos", target_os = "ios"))]
 pub(crate) mod coreaudio;
 #[cfg(target_os = "emscripten")]

src/host/wasapi/device.rs

@@ -490,7 +490,6 @@ impl Device {
                 format.sample_rate = SampleRate(rate as _);
                 supported_formats.push(SupportedFormat::from(format.clone()));
             }
-
             Ok(supported_formats.into_iter())
         }
     }

src/lib.rs

@@ -377,6 +377,9 @@ pub enum BuildStreamError {
 /// them immediately.
 #[derive(Debug, Fail)]
 pub enum PlayStreamError {
+    /// The device associated with the stream is no longer available.
+    #[fail(display = "the device associated with the stream is no longer available")]
+    DeviceNotAvailable,
     /// See the `BackendSpecificError` docs for more information about this error variant.
     #[fail(display = "{}", err)]
     BackendSpecific {
@@ -392,6 +395,9 @@ pub enum PlayStreamError {
 /// them immediately.
 #[derive(Debug, Fail)]
 pub enum PauseStreamError {
+    /// The device associated with the stream is no longer available.
+    #[fail(display = "the device associated with the stream is no longer available")]
+    DeviceNotAvailable,
     /// See the `BackendSpecificError` docs for more information about this error variant.
     #[fail(display = "{}", err)]
     BackendSpecific {

src/platform/mod.rs

@@ -256,6 +256,7 @@ macro_rules! impl_platform_host {
             type StreamId = StreamId;
             type Device = Device;
 
+            #[allow(unreachable_patterns)]
             fn build_input_stream(
                 &self,
                 device: &Self::Device,
@@ -269,9 +270,11 @@ macro_rules! impl_platform_host {
                                 .map(StreamId)
                         }
                     )*
+                    _ => panic!("tried to build a stream with a device from another host"),
                 }
             }
 
+            #[allow(unreachable_patterns)]
             fn build_output_stream(
                 &self,
                 device: &Self::Device,
@@ -285,9 +288,11 @@ macro_rules! impl_platform_host {
                                 .map(StreamId)
                         }
                     )*
+                    _ => panic!("tried to build a stream with a device from another host"),
                 }
             }
 
+            #[allow(unreachable_patterns)]
             fn play_stream(&self, stream: Self::StreamId) -> Result<(), crate::PlayStreamError> {
                 match (&self.0, stream.0) {
                     $(
@@ -295,9 +300,11 @@ macro_rules! impl_platform_host {
                             e.play_stream(s.clone())
                         }
                     )*
+                    _ => panic!("tried to play a stream with an ID associated with another host"),
                 }
             }
 
+            #[allow(unreachable_patterns)]
             fn pause_stream(&self, stream: Self::StreamId) -> Result<(), crate::PauseStreamError> {
                 match (&self.0, stream.0) {
                     $(
@@ -305,9 +312,11 @@ macro_rules! impl_platform_host {
                             e.pause_stream(s.clone())
                         }
                     )*
+                    _ => panic!("tried to pause a stream with an ID associated with another host"),
                 }
             }
 
+            #[allow(unreachable_patterns)]
             fn destroy_stream(&self, stream: Self::StreamId) {
                 match (&self.0, stream.0) {
                     $(
@@ -315,6 +324,7 @@ macro_rules! impl_platform_host {
                             e.destroy_stream(s.clone())
                         }
                     )*
+                    _ => panic!("tried to destroy a stream with an ID associated with another host"),
                 }
             }
 
@@ -513,9 +523,18 @@ mod platform_impl {
     }
 }
 
-// TODO: Add `Asio asio` once #221 lands.
 #[cfg(windows)]
 mod platform_impl {
+    #[cfg(feature = "asio")]
+    pub use crate::host::asio::{
+        Device as AsioDevice,
+        Devices as AsioDevices,
+        EventLoop as AsioEventLoop,
+        Host as AsioHost,
+        StreamId as AsioStreamId,
+        SupportedInputFormats as AsioSupportedInputFormats,
+        SupportedOutputFormats as AsioSupportedOutputFormats,
+    };
     pub use crate::host::wasapi::{
         Device as WasapiDevice,
         Devices as WasapiDevices,
@@ -526,6 +545,10 @@ mod platform_impl {
         SupportedOutputFormats as WasapiSupportedOutputFormats,
     };
 
+    #[cfg(feature = "asio")]
+    impl_platform_host!(Asio asio, Wasapi wasapi);
+
+    #[cfg(not(feature = "asio"))]
     impl_platform_host!(Wasapi wasapi);
 
     /// The default host for the current compilation target platform.