HudCanvas: extract HudCanvasState

frontend/src/App.tsx

@@ -11,6 +11,8 @@ import { AudioFormat } from './generated/media_set';
 import { VideoPreview } from './VideoPreview';
 import { Overview, CanvasLogicalWidth } from './Overview';
 import { Waveform } from './Waveform';
+import { SelectionChangeEvent } from './HudCanvas';
+import { Selection, SelectionMode } from './HudCanvasState';
 import { ControlBar } from './ControlBar';
 import { SeekBar } from './SeekBar';
 import { firstValueFrom, from, Observable } from 'rxjs';
@@ -107,6 +109,7 @@ function App(): JSX.Element {
 
       // check if the end of selection has been passed, and pause if so:
       if (
+        selection.start != selection.end &&
         currentTimeToFrame(positionRef.current.currentTime) < selection.end &&
         currentTimeToFrame(currTime) >= selection.end
       ) {
@@ -205,7 +208,9 @@ function App(): JSX.Element {
   };
 
   // handler called when the selection in the overview (zoom setting) is changed.
-  const handleOverviewSelectionChange = (newViewport: Frames) => {
+  const handleOverviewSelectionChange = ({
+    selection: newViewport,
+  }: SelectionChangeEvent) => {
     if (mediaSet == null) {
       return;
     }
@@ -219,28 +224,39 @@ function App(): JSX.Element {
     setPositionFromFrame(newViewport.start);
   };
 
-  // handler called when the selection in the main waveform view is changed.
+  const setPositionAfterSelectionChange = (
-  const handleWaveformSelectionChange = (
+    newSelection: Selection,
-    newSelection: Frames,
+    mode: SelectionMode,
-    final: boolean
+    prevMode: SelectionMode
-  ) => {
+  ): boolean => {
-    if (mediaSet == null) {
+    // if creating a new selection from scratch, reset position on mouseup.
-      return;
+    if (prevMode == SelectionMode.Selecting && mode == SelectionMode.Normal) {
+      return true;
     }
 
+    // if re-entering normal mode, reset position if the current position is
+    // outside the new selection on mouseup.
+    if (prevMode != SelectionMode.Normal && mode == SelectionMode.Normal) {
+      const currFrame = currentTimeToFrame(positionRef.current.currentTime);
+      if (currFrame < newSelection.start || currFrame > newSelection.end) {
+        return true;
+      }
+    }
+
+    return false;
+  };
+
+  // handler called when the selection in the main waveform view is changed.
+  const handleWaveformSelectionChange = ({
+    selection: newSelection,
+    mode,
+    prevMode,
+  }: SelectionChangeEvent) => {
     setSelection(newSelection);
 
-    // only update playback position when the selection is final.
+    if (setPositionAfterSelectionChange(newSelection, mode, prevMode)) {
-    if (!final) {
+      setPositionFromFrame(newSelection.start);
-      return;
     }
-
-    // move playback position to start of selection
-    const ratio = newSelection.start / mediaSet.audioFrames;
-    const currentTime =
-      (mediaSet.audioFrames / mediaSet.audioSampleRate) * ratio;
-    audio.currentTime = currentTime;
-    video.currentTime = currentTime;
   };
 
   const togglePlay = () => {
@@ -262,9 +278,7 @@ function App(): JSX.Element {
     video.pause();
     audio.pause();
 
-    if (selection.start != selection.end) {
     setPositionFromFrame(selection.start);
-    }
 
     setPlayState(PlayState.Paused);
   };
@@ -454,7 +468,10 @@ function App(): JSX.Element {
             <ControlBar
               playState={playState}
               zoomInEnabled={canZoomViewportIn(viewport, selection, zoomFactor)}
-              zoomOutEnabled={canZoomViewportOut(viewport, mediaSet.audioFrames)}
+              zoomOutEnabled={canZoomViewportOut(
+                viewport,
+                mediaSet.audioFrames
+              )}
               onTogglePlay={togglePlay}
               onClip={handleClip}
               onZoomIn={handleZoomIn}

frontend/src/HudCanvas.tsx

@@ -1,4 +1,14 @@
-import { useState, useEffect, useRef, MouseEvent } from 'react';
+import { useEffect, useRef, useReducer, MouseEvent } from 'react';
+import {
+  stateReducer,
+  State,
+  SelectionMode,
+  HoverState,
+  Selection,
+  EmptySelectionAction,
+} from './HudCanvasState';
+import constrainNumeric from './helpers/constrainNumeric';
+export { EmptySelectionAction } from './HudCanvasState';
 
 interface Styles {
   borderLineWidth: number;
@@ -15,36 +25,39 @@ interface Props {
   styles: Styles;
   position: number | null;
   selection: Selection;
-  onSelectionChange: (selection: Selection, final: boolean) => void;
+  onSelectionChange: (selectionState: SelectionChangeEvent) => void;
 }
 
-enum Mode {
+export interface SelectionChangeEvent {
-  Normal,
+  selection: Selection;
-  Selecting,
+  mode: SelectionMode;
-  Dragging,
+  prevMode: SelectionMode;
-  ResizingStart,
-  ResizingEnd,
-}
-
-enum HoverState {
-  Normal,
-  OverSelectionStart,
-  OverSelectionEnd,
-  OverSelection,
-}
-
-export enum EmptySelectionAction {
-  SelectNothing,
-  SelectPrevious,
-}
-
-export interface Selection {
-  start: number;
-  end: number;
 }
 
 const emptySelection: Selection = { start: 0, end: 0 };
 
+const initialState: State = {
+  width: 0,
+  emptySelectionAction: EmptySelectionAction.SelectNothing,
+  hoverX: 0,
+  selection: emptySelection,
+  origSelection: emptySelection,
+  mousedownX: 0,
+  mode: SelectionMode.Normal,
+  prevMode: SelectionMode.Normal,
+  cursorClass: 'cursor-auto',
+  hoverState: HoverState.Normal,
+  shouldPublish: false,
+};
+
+const getCanvasX = (evt: MouseEvent<HTMLCanvasElement>): number => {
+  const rect = evt.currentTarget.getBoundingClientRect();
+  const x = Math.round(
+    ((evt.clientX - rect.left) / rect.width) * evt.currentTarget.width
+  );
+  return constrainNumeric(x, evt.currentTarget.width);
+};
+
 export const HudCanvas: React.FC<Props> = ({
   width,
   height,
@@ -57,34 +70,45 @@ export const HudCanvas: React.FC<Props> = ({
     hoverPositionStrokeStyle,
   },
   position,
-  selection,
+  selection: initialSelection,
   onSelectionChange,
 }: Props) => {
-  // selection and newSelection are in canvas logical pixels:
-  const [newSelection, setNewSelection] = useState({
-    ...emptySelection,
-  });
-  const [hoverPosition, setHoverPosition] = useState<number | null>(null);
-  const [mode, setMode] = useState(Mode.Normal);
-  const [hoverState, setHoverState] = useState(HoverState.Normal);
-  const [cursor, setCursor] = useState('cursor-auto');
-
   const canvasRef = useRef<HTMLCanvasElement>(null);
-  const moveOffsetX = useRef(0);
+
+  const [state, dispatch] = useReducer(stateReducer, {
+    ...initialState,
+    width,
+    emptySelectionAction,
+    selection: initialSelection,
+  });
 
   // side effects
 
+  useEffect(() => {
+    dispatch({ selection: initialSelection, x: 0, type: 'setselection' });
+  }, [initialSelection]);
+
   // handle global mouse up
   useEffect(() => {
     window.addEventListener('mouseup', handleMouseUp);
     return () => {
       window.removeEventListener('mouseup', handleMouseUp);
     };
-  }, [mode, newSelection]);
+  }, [state]);
 
+  // trigger onSelectionChange callback.
   useEffect(() => {
-    onSelectionChange({ ...newSelection }, mode == Mode.Normal);
+    // really we only care if hoverX hasn't changed, not checking this will
-  }, [mode, newSelection]);
+    // result in a flood of events being published.
+    if (!state.shouldPublish) {
+      return;
+    }
+    onSelectionChange({
+      selection: state.selection,
+      mode: state.mode,
+      prevMode: state.prevMode,
+    });
+  }, [state]);
 
   // draw the overview HUD
   useEffect(() => {
@@ -103,22 +127,13 @@ export const HudCanvas: React.FC<Props> = ({
 
       // draw selection
 
-      let currentSelection: Selection;
+      const currentSelection = state.selection;
-      if (
-        mode == Mode.Selecting ||
-        mode == Mode.Dragging ||
-        mode == Mode.ResizingStart ||
-        mode == Mode.ResizingEnd
-      ) {
-        currentSelection = newSelection;
-      } else {
-        currentSelection = selection;
-      }
 
       ctx.beginPath();
       ctx.strokeStyle = borderStrokeStyle;
       ctx.lineWidth = borderLineWidth;
-      const alpha = hoverState == HoverState.OverSelection ? '0.15' : '0.13';
+      const alpha =
+        state.hoverState == HoverState.OverSelection ? '0.15' : '0.13';
       ctx.fillStyle = `rgba(255, 255, 255, ${alpha})`;
       ctx.rect(
         currentSelection.start,
@@ -130,16 +145,17 @@ export const HudCanvas: React.FC<Props> = ({
       ctx.stroke();
 
       // draw hover position
+
+      const hoverX = state.hoverX;
       if (
-        hoverPosition != null &&
+        hoverX != null &&
-        (hoverPosition < currentSelection.start ||
+        (hoverX < currentSelection.start || hoverX > currentSelection.end)
-          hoverPosition > currentSelection.end)
       ) {
         ctx.beginPath();
         ctx.strokeStyle = hoverPositionStrokeStyle;
         ctx.lineWidth = 2;
-        ctx.moveTo(hoverPosition, 0);
+        ctx.moveTo(hoverX, 0);
-        ctx.lineTo(hoverPosition, canvas.height);
+        ctx.lineTo(hoverX, canvas.height);
         ctx.stroke();
       }
 
@@ -157,174 +173,35 @@ export const HudCanvas: React.FC<Props> = ({
       ctx.lineTo(position, canvas.height);
       ctx.stroke();
     });
-  }, [selection, newSelection, position, hoverPosition]);
+  }, [state, position]);
-
-  // handlers
-
-  const hoverOffset = 10;
-
-  const isHoveringSelectionStart = (x: number): boolean => {
-    return (
-      x > selection.start - hoverOffset && x < selection.start + hoverOffset
-    );
-  };
-
-  const isHoveringSelectionEnd = (x: number): boolean => {
-    return x > selection.end - hoverOffset && x < selection.end + hoverOffset;
-  };
-
-  const isHoveringSelection = (x: number): boolean => {
-    return x >= selection.start && x <= selection.end;
-  };
-
-  const getCanvasX = (evt: MouseEvent<HTMLCanvasElement>): number => {
-    const rect = evt.currentTarget.getBoundingClientRect();
-    const x = Math.round(((evt.clientX - rect.left) / rect.width) * width);
-    return constrainXToCanvas(x);
-  };
-
-  const constrainXToCanvas = (x: number): number => {
-    if (x < 0) {
-      return 0;
-    }
-    if (x > width) {
-      return width;
-    }
-    return x;
-  };
 
   const handleMouseDown = (evt: MouseEvent<HTMLCanvasElement>) => {
-    if (mode != Mode.Normal) {
+    if (state.mode != SelectionMode.Normal) {
       return;
     }
-
+    dispatch({ x: getCanvasX(evt), type: 'mousedown' });
-    const x = getCanvasX(evt);
-
-    if (isHoveringSelectionStart(x)) {
-      setMode(Mode.ResizingStart);
-      moveOffsetX.current = x;
-    } else if (isHoveringSelectionEnd(x)) {
-      setMode(Mode.ResizingEnd);
-      moveOffsetX.current = x;
-    } else if (isHoveringSelection(x)) {
-      setMode(Mode.Dragging);
-      setCursor('cursor-move');
-      moveOffsetX.current = x;
-    } else {
-      setMode(Mode.Selecting);
-      setCursor('cursor-col-resize');
-      moveOffsetX.current = x;
-      setNewSelection({ start: x, end: x });
-    }
   };
 
   const handleMouseMove = (evt: MouseEvent<HTMLCanvasElement>) => {
-    const x = getCanvasX(evt);
+    dispatch({ x: getCanvasX(evt), type: 'mousemove' });
-    setHoverPosition(x);
-
-    switch (mode) {
-      case Mode.Normal: {
-        if (isHoveringSelectionStart(x)) {
-          setHoverState(HoverState.OverSelectionStart);
-          setCursor('cursor-col-resize');
-        } else if (isHoveringSelectionEnd(x)) {
-          setHoverState(HoverState.OverSelectionEnd);
-          setCursor('cursor-col-resize');
-        } else if (isHoveringSelection(x)) {
-          setHoverState(HoverState.OverSelection);
-          setCursor('cursor-move');
-        } else {
-          setCursor('cursor-auto');
-        }
-        break;
-      }
-      case Mode.ResizingStart: {
-        const diff = x - moveOffsetX.current;
-        const start = constrainXToCanvas(selection.start + diff);
-
-        if (start > selection.end) {
-          setNewSelection({ start: selection.end, end: start });
-          break;
-        }
-
-        setNewSelection({ ...selection, start: start });
-        break;
-      }
-      case Mode.ResizingEnd: {
-        const diff = x - moveOffsetX.current;
-        const end = constrainXToCanvas(selection.end + diff);
-
-        if (end < selection.start) {
-          setNewSelection({ start: Math.max(0, end), end: selection.start });
-          break;
-        }
-
-        setNewSelection({ ...selection, end: end });
-        break;
-      }
-      case Mode.Dragging: {
-        const diff = x - moveOffsetX.current;
-        const selectionWidth = selection.end - selection.start;
-        let start = Math.max(0, selection.start + diff);
-        let end = start + selectionWidth;
-        if (end > width) {
-          end = width;
-          start = end - selectionWidth;
-        }
-
-        setNewSelection({ start: start, end: end });
-        break;
-      }
-      case Mode.Selecting: {
-        if (x < moveOffsetX.current) {
-          setNewSelection({
-            start: x,
-            end: moveOffsetX.current,
-          });
-        } else {
-          setNewSelection({ start: moveOffsetX.current, end: x });
-        }
-        break;
-      }
-    }
   };
 
   const handleMouseUp = () => {
-    if (mode == Mode.Normal) {
+    if (state.mode == SelectionMode.Normal) {
-      return;
-    }
-
-    setMode(Mode.Normal);
-    setCursor('cursor-auto');
-
-    if (newSelection.start == newSelection.end) {
-      handleEmptySelectionAction();
       return;
     }
+    dispatch({ x: state.hoverX, type: 'mouseup' });
   };
 
-  const handleEmptySelectionAction = () => {
+  const handleMouseLeave = () => {
-    switch (emptySelectionAction) {
+    dispatch({ x: state.hoverX, type: 'mouseleave' });
-      case EmptySelectionAction.SelectPrevious:
-        setNewSelection({ ...selection });
-        break;
-      case EmptySelectionAction.SelectNothing:
-        setMode(Mode.Normal);
-        setNewSelection({ start: 0, end: 0 });
-        break;
-    }
-  };
-
-  const handleMouseLeave = (_evt: MouseEvent<HTMLCanvasElement>) => {
-    setHoverState(HoverState.Normal);
-    setHoverPosition(0);
   };
 
   return (
     <>
       <canvas
         ref={canvasRef}
-        className={`block absolute w-full h-full ${cursor} z-20`}
+        className={`block absolute w-full h-full ${state.cursorClass} z-20`}
         width={width}
         height={height}
         onMouseDown={handleMouseDown}

frontend/src/HudCanvasState.test.ts

@@ -0,0 +1,371 @@
+import {
+  stateReducer,
+  SelectionMode,
+  EmptySelectionAction,
+  HoverState,
+} from './HudCanvasState';
+
+const initialState = {
+  width: 5000,
+  emptySelectionAction: EmptySelectionAction.SelectNothing,
+  hoverX: 0,
+  selection: { start: 0, end: 0 },
+  origSelection: { start: 0, end: 0 },
+  mousedownX: 0,
+  mode: SelectionMode.Normal,
+  prevMode: SelectionMode.Normal,
+  cursorClass: 'cursor-auto',
+  hoverState: HoverState.Normal,
+  shouldPublish: false,
+};
+
+describe('stateReducer', () => {
+  describe('setselection', () => {
+    it('sets the selection', () => {
+      const state = stateReducer(
+        { ...initialState },
+        { type: 'setselection', x: 0, selection: { start: 100, end: 200 } }
+      );
+      expect(state.selection).toEqual({ start: 100, end: 200 });
+    });
+  });
+
+  describe('mousedown', () => {
+    describe('when hovering over the selection start', () => {
+      it('updates the state', () => {
+        const state = stateReducer(
+          { ...initialState, selection: { start: 1000, end: 2000 } },
+          { type: 'mousedown', x: 995 }
+        );
+        expect(state.mode).toEqual(SelectionMode.ResizingStart);
+        expect(state.selection).toEqual({ start: 1000, end: 2000 });
+        expect(state.mousedownX).toEqual(995);
+      });
+    });
+    describe('when hovering over the selection end', () => {
+      it('updates the state', () => {
+        const state = stateReducer(
+          { ...initialState, selection: { start: 1000, end: 2000 } },
+          { type: 'mousedown', x: 2003 }
+        );
+        expect(state.mode).toEqual(SelectionMode.ResizingEnd);
+        expect(state.selection).toEqual({ start: 1000, end: 2000 });
+        expect(state.mousedownX).toEqual(2003);
+      });
+    });
+    describe('when hovering over the selection', () => {
+      it('updates the state', () => {
+        const state = stateReducer(
+          { ...initialState, selection: { start: 1000, end: 2000 } },
+          { type: 'mousedown', x: 1500 }
+        );
+        expect(state.mode).toEqual(SelectionMode.Dragging);
+        expect(state.selection).toEqual({ start: 1000, end: 2000 });
+        expect(state.mousedownX).toEqual(1500);
+      });
+    });
+    describe('when not hovering over the selection', () => {
+      it('updates the state', () => {
+        const state = stateReducer(
+          { ...initialState, selection: { start: 1000, end: 2000 } },
+          { type: 'mousedown', x: 3000 }
+        );
+        expect(state.mode).toEqual(SelectionMode.Selecting);
+        expect(state.selection).toEqual({ start: 3000, end: 3000 });
+        expect(state.mousedownX).toEqual(3000);
+      });
+    });
+  });
+
+  describe('mouseup', () => {
+    describe('when re-entering normal mode', () => {
+      it('updates the state', () => {
+        const state = stateReducer(
+          {
+            ...initialState,
+            selection: { start: 1000, end: 2000 },
+            mode: SelectionMode.Selecting,
+            mousedownX: 1200,
+          },
+          { type: 'mouseup', x: 0 }
+        );
+        expect(state.mode).toEqual(SelectionMode.Normal);
+        expect(state.selection).toEqual({ start: 1000, end: 2000 });
+        expect(state.mousedownX).toEqual(1200);
+      });
+    });
+
+    describe('nothing is selected and emptySelectionAction is SelectNothing', () => {
+      it('clears the selection at the mouse x coord', () => {
+        const state = stateReducer(
+          {
+            ...initialState,
+            selection: { start: 1000, end: 1000 },
+            mode: SelectionMode.Selecting,
+            emptySelectionAction: EmptySelectionAction.SelectNothing,
+          },
+          { type: 'mouseup', x: 500 }
+        );
+        expect(state.mode).toEqual(SelectionMode.Normal);
+        expect(state.selection).toEqual({ start: 500, end: 500 });
+      });
+    });
+
+    describe('nothing is selected and emptySelectionAction is SelectPrevious', () => {
+      it('reverts to the previous selection', () => {
+        const state = stateReducer(
+          {
+            ...initialState,
+            selection: { start: 1000, end: 2000 },
+            mode: SelectionMode.Selecting,
+            emptySelectionAction: EmptySelectionAction.SelectNothing,
+          },
+          { type: 'mouseup', x: 0 }
+        );
+        expect(state.mode).toEqual(SelectionMode.Normal);
+        expect(state.selection).toEqual({ start: 1000, end: 2000 });
+      });
+    });
+  });
+
+  describe('mouseup', () => {
+    it('sets the state', () => {
+      const state = stateReducer(
+        {
+          ...initialState,
+          selection: { start: 2000, end: 3000 },
+          mode: SelectionMode.Dragging,
+          mousedownX: 475,
+        },
+        { type: 'mouseleave', x: 500 }
+      );
+      expect(state.mode).toEqual(SelectionMode.Dragging);
+      expect(state.selection).toEqual({ start: 2000, end: 3000 });
+      expect(state.mousedownX).toEqual(475);
+    });
+  });
+
+  describe('mousemove', () => {
+    describe('in normal mode', () => {
+      describe('when hovering over the selection start', () => {
+        it('updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              selection: { start: 1000, end: 3000 },
+              mode: SelectionMode.Normal,
+            },
+            { type: 'mousemove', x: 997 }
+          );
+          expect(state.mode).toEqual(SelectionMode.Normal);
+          expect(state.selection).toEqual({ start: 1000, end: 3000 });
+          expect(state.hoverState).toEqual(HoverState.OverSelectionStart);
+        });
+      });
+
+      describe('when hovering over the selection end', () => {
+        it('updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              selection: { start: 1000, end: 3000 },
+              mode: SelectionMode.Normal,
+            },
+            { type: 'mousemove', x: 3009 }
+          );
+          expect(state.mode).toEqual(SelectionMode.Normal);
+          expect(state.selection).toEqual({ start: 1000, end: 3000 });
+          expect(state.hoverState).toEqual(HoverState.OverSelectionEnd);
+        });
+      });
+
+      describe('when hovering over the selection', () => {
+        it('updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              selection: { start: 1000, end: 3000 },
+              mode: SelectionMode.Normal,
+            },
+            { type: 'mousemove', x: 1200 }
+          );
+          expect(state.mode).toEqual(SelectionMode.Normal);
+          expect(state.selection).toEqual({ start: 1000, end: 3000 });
+          expect(state.hoverState).toEqual(HoverState.OverSelection);
+        });
+      });
+
+      describe('when hovering elsewhere', () => {
+        it('updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              selection: { start: 1000, end: 3000 },
+              mode: SelectionMode.Normal,
+            },
+            { type: 'mousemove', x: 10 }
+          );
+          expect(state.mode).toEqual(SelectionMode.Normal);
+          expect(state.selection).toEqual({ start: 1000, end: 3000 });
+          expect(state.hoverState).toEqual(HoverState.Normal);
+        });
+      });
+    });
+
+    describe('in selecting mode', () => {
+      describe('a normal selection', () => {
+        it('updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              selection: { start: 2000, end: 3000 },
+              mode: SelectionMode.Selecting,
+              mousedownX: 2000,
+            },
+            { type: 'mousemove', x: 3005 }
+          );
+          expect(state.mode).toEqual(SelectionMode.Selecting);
+          expect(state.selection).toEqual({ start: 2000, end: 3005 });
+        });
+      });
+
+      describe('when crossing over', () => {
+        it('updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              selection: { start: 2000, end: 2002 },
+              mode: SelectionMode.Selecting,
+              mousedownX: 2000,
+            },
+            { type: 'mousemove', x: 1995 }
+          );
+          expect(state.mode).toEqual(SelectionMode.Selecting);
+          expect(state.selection).toEqual({ start: 1995, end: 2000 });
+        });
+      });
+    });
+
+    describe('in dragging mode', () => {
+      describe('in the middle of the canvas', () => {
+        it('updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              selection: { start: 1000, end: 1500 },
+              origSelection: { start: 1000, end: 1500 },
+              mode: SelectionMode.Dragging,
+              mousedownX: 1200,
+            },
+            { type: 'mousemove', x: 1220 }
+          );
+          expect(state.mode).toEqual(SelectionMode.Dragging);
+          expect(state.selection).toEqual({ start: 1020, end: 1520 });
+        });
+      });
+
+      describe('at the start of the canvas', () => {
+        it('constrains the movement and updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              selection: { start: 10, end: 210 },
+              origSelection: { start: 10, end: 210 },
+              mode: SelectionMode.Dragging,
+              mousedownX: 50,
+            },
+            { type: 'mousemove', x: 30 }
+          );
+          expect(state.mode).toEqual(SelectionMode.Dragging);
+          expect(state.selection).toEqual({ start: 0, end: 200 });
+        });
+      });
+
+      describe('at the end of the canvas', () => {
+        it('constrains the movement and updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              width: 3000,
+              selection: { start: 2800, end: 2900 },
+              origSelection: { start: 2800, end: 2900 },
+              mode: SelectionMode.Dragging,
+              mousedownX: 1200,
+            },
+            { type: 'mousemove', x: 1350 }
+          );
+          expect(state.mode).toEqual(SelectionMode.Dragging);
+          expect(state.selection).toEqual({ start: 2900, end: 3000 });
+        });
+      });
+    });
+
+    describe('in resizing start mode', () => {
+      describe('a normal resize', () => {
+        it('updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              selection: { start: 2000, end: 3000 },
+              origSelection: { start: 2000, end: 3000 },
+              mode: SelectionMode.ResizingStart,
+            },
+            { type: 'mousemove', x: 2020 }
+          );
+          expect(state.mode).toEqual(SelectionMode.ResizingStart);
+          expect(state.selection).toEqual({ start: 2020, end: 3000 });
+        });
+      });
+
+      describe('when crossing over', () => {
+        it('updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              selection: { start: 2000, end: 2002 },
+              origSelection: { start: 2000, end: 2002 },
+              mode: SelectionMode.ResizingStart,
+            },
+            { type: 'mousemove', x: 2010 }
+          );
+          expect(state.mode).toEqual(SelectionMode.ResizingStart);
+          expect(state.selection).toEqual({ start: 2002, end: 2010 });
+        });
+      });
+    });
+
+    describe('in resizing end mode', () => {
+      describe('a normal resize', () => {
+        it('updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              selection: { start: 1000, end: 2000 },
+              origSelection: { start: 1000, end: 2000 },
+              mode: SelectionMode.ResizingEnd,
+            },
+            { type: 'mousemove', x: 2007 }
+          );
+          expect(state.mode).toEqual(SelectionMode.ResizingEnd);
+          expect(state.selection).toEqual({ start: 1000, end: 2007 });
+        });
+      });
+
+      describe('when crossing over', () => {
+        it('updates the state', () => {
+          const state = stateReducer(
+            {
+              ...initialState,
+              selection: { start: 2000, end: 2002 },
+              origSelection: { start: 2000, end: 2002 },
+              mode: SelectionMode.ResizingEnd,
+            },
+            { type: 'mousemove', x: 1995 }
+          );
+          expect(state.mode).toEqual(SelectionMode.ResizingEnd);
+          expect(state.selection).toEqual({ start: 1995, end: 2000 });
+        });
+      });
+    });
+  });
+});

frontend/src/HudCanvasState.ts

@@ -0,0 +1,247 @@
+import constrainNumeric from './helpers/constrainNumeric';
+
+export enum HoverState {
+  Normal,
+  OverSelectionStart,
+  OverSelectionEnd,
+  OverSelection,
+}
+
+export enum EmptySelectionAction {
+  SelectNothing,
+  SelectPrevious,
+}
+
+export interface Selection {
+  start: number;
+  end: number;
+}
+
+export enum SelectionMode {
+  Normal,
+  Selecting,
+  Dragging,
+  ResizingStart,
+  ResizingEnd,
+}
+
+export interface State {
+  width: number;
+  emptySelectionAction: EmptySelectionAction;
+  hoverX: number;
+  selection: Selection;
+  origSelection: Selection;
+  mousedownX: number;
+  mode: SelectionMode;
+  prevMode: SelectionMode;
+  cursorClass: string;
+  hoverState: HoverState;
+  shouldPublish: boolean;
+}
+
+interface SelectionAction {
+  type: string;
+  x: number;
+  // TODO: selection is only used for the setselection SelectionAction. Improve
+  // the typing here.
+  selection?: Selection;
+}
+
+export const stateReducer = (
+  {
+    selection: prevSelection,
+    origSelection,
+    mousedownX: prevMousedownX,
+    mode: prevMode,
+    width,
+    emptySelectionAction,
+  }: State,
+  { x, type, selection: selectionToSet }: SelectionAction
+): State => {
+  let mode: SelectionMode;
+  let newSelection: Selection;
+  let mousedownX: number;
+  let cursorClass: string;
+  let hoverState: HoverState;
+
+  switch (type) {
+    case 'setselection':
+      newSelection = selectionToSet || { start: 0, end: 0 };
+      mousedownX = prevMousedownX;
+      mode = SelectionMode.Normal;
+      cursorClass = 'cursor-auto';
+      hoverState = HoverState.Normal;
+
+      break;
+    case 'mousedown':
+      mousedownX = x;
+      cursorClass = 'cursor-auto';
+      hoverState = HoverState.Normal;
+
+      if (isHoveringSelectionStart(x, prevSelection)) {
+        newSelection = prevSelection;
+        mode = SelectionMode.ResizingStart;
+      } else if (isHoveringSelectionEnd(x, prevSelection)) {
+        newSelection = prevSelection;
+        mode = SelectionMode.ResizingEnd;
+      } else if (isHoveringSelection(x, prevSelection)) {
+        newSelection = prevSelection;
+        mode = SelectionMode.Dragging;
+        cursorClass = 'cursor-move';
+      } else {
+        newSelection = { start: x, end: x };
+        mode = SelectionMode.Selecting;
+        cursorClass = 'cursor-col-resize';
+      }
+
+      origSelection = newSelection;
+
+      break;
+    case 'mouseup':
+      newSelection = prevSelection;
+      mousedownX = prevMousedownX;
+      mode = SelectionMode.Normal;
+      cursorClass = 'cursor-auto';
+      hoverState = HoverState.Normal;
+
+      if (
+        newSelection.start == newSelection.end &&
+        emptySelectionAction == EmptySelectionAction.SelectNothing
+      ) {
+        newSelection = { start: x, end: x };
+      }
+
+      break;
+    case 'mouseleave':
+      newSelection = prevSelection;
+      mousedownX = prevMousedownX;
+      mode = prevMode;
+      cursorClass = 'cursor-auto';
+      hoverState = HoverState.Normal;
+
+      break;
+    case 'mousemove':
+      mousedownX = prevMousedownX;
+      hoverState = HoverState.Normal;
+
+      switch (prevMode) {
+        case SelectionMode.Normal: {
+          newSelection = prevSelection;
+          mode = SelectionMode.Normal;
+
+          if (isHoveringSelectionStart(x, prevSelection)) {
+            cursorClass = 'cursor-col-resize';
+            hoverState = HoverState.OverSelectionStart;
+          } else if (isHoveringSelectionEnd(x, prevSelection)) {
+            cursorClass = 'cursor-col-resize';
+            hoverState = HoverState.OverSelectionEnd;
+          } else if (isHoveringSelection(x, prevSelection)) {
+            cursorClass = 'cursor-move';
+            hoverState = HoverState.OverSelection;
+          } else {
+            cursorClass = 'cursor-auto';
+            hoverState = HoverState.Normal;
+          }
+
+          break;
+        }
+        case SelectionMode.Selecting: {
+          cursorClass = 'cursor-col-resize';
+          mode = SelectionMode.Selecting;
+          if (x < prevMousedownX) {
+            newSelection = {
+              start: x,
+              end: prevMousedownX,
+            };
+          } else {
+            newSelection = {
+              start: prevMousedownX,
+              end: x,
+            };
+          }
+
+          break;
+        }
+        case SelectionMode.Dragging: {
+          mode = SelectionMode.Dragging;
+          cursorClass = 'cursor-move';
+
+          const diff = x - prevMousedownX;
+          const selectionWidth = origSelection.end - origSelection.start;
+
+          let start = Math.max(0, origSelection.start + diff);
+          let end = start + selectionWidth;
+          if (end > width) {
+            end = width;
+            start = end - selectionWidth;
+          }
+          newSelection = { start: start, end: end };
+
+          break;
+        }
+        case SelectionMode.ResizingStart: {
+          mode = SelectionMode.ResizingStart;
+          cursorClass = 'cursor-col-resize';
+
+          const start = constrainNumeric(x, width);
+          if (start > origSelection.end) {
+            newSelection = { start: origSelection.end, end: start };
+          } else {
+            newSelection = { ...origSelection, start: start };
+          }
+
+          break;
+        }
+        case SelectionMode.ResizingEnd: {
+          mode = SelectionMode.ResizingEnd;
+          cursorClass = 'cursor-col-resize';
+
+          const end = constrainNumeric(x, width);
+          if (end < origSelection.start) {
+            newSelection = {
+              start: end,
+              end: origSelection.start,
+            };
+          } else {
+            newSelection = { ...origSelection, end: x };
+          }
+
+          break;
+        }
+      }
+
+      break;
+    default:
+      throw new Error();
+  }
+
+  return {
+    width: width,
+    emptySelectionAction: emptySelectionAction,
+    hoverX: x,
+    selection: newSelection,
+    origSelection: origSelection,
+    mousedownX: mousedownX,
+    mode: mode,
+    prevMode: prevMode,
+    cursorClass: cursorClass,
+    hoverState: hoverState,
+    shouldPublish: newSelection != prevSelection || mode != prevMode,
+  };
+};
+
+// helpers
+
+const hoverOffset = 10;
+
+const isHoveringSelectionStart = (x: number, selection: Selection): boolean => {
+  return x > selection.start - hoverOffset && x < selection.start + hoverOffset;
+};
+
+const isHoveringSelectionEnd = (x: number, selection: Selection): boolean => {
+  return x > selection.end - hoverOffset && x < selection.end + hoverOffset;
+};
+
+const isHoveringSelection = (x: number, selection: Selection): boolean => {
+  return x >= selection.start && x <= selection.end;
+};

frontend/src/Overview.tsx

@@ -2,7 +2,12 @@ import { useState, useEffect } from 'react';
 import { MediaSet } from './generated/media_set';
 import { Frames, VideoPosition } from './App';
 import { WaveformCanvas } from './WaveformCanvas';
-import { HudCanvas, EmptySelectionAction } from './HudCanvas';
+import {
+  HudCanvas,
+  EmptySelectionAction,
+  SelectionChangeEvent,
+} from './HudCanvas';
+import { SelectionMode } from './HudCanvasState';
 import { Observable } from 'rxjs';
 
 export interface Selection {
@@ -15,7 +20,7 @@ interface Props {
   mediaSet: MediaSet;
   position: VideoPosition;
   viewport: Frames;
-  onSelectionChange: (selection: Selection) => void;
+  onSelectionChange: (selectionState: SelectionChangeEvent) => void;
 }
 
 export const CanvasLogicalWidth = 2_000;
@@ -34,6 +39,7 @@ export const Overview: React.FC<Props> = ({
   // side effects
 
   // convert viewport from frames to canvas pixels.
+  // TODO: consider an adapter component to handle this.
   useEffect(() => {
     setSelectedPixels({
       start: Math.round(
@@ -46,6 +52,7 @@ export const Overview: React.FC<Props> = ({
   }, [viewport, mediaSet]);
 
   // convert position from frames to canvas pixels:
+  // TODO: consider an adapter component to handle this.
   useEffect(() => {
     const ratio =
       position.currentTime / (mediaSet.audioFrames / mediaSet.audioSampleRate);
@@ -56,13 +63,23 @@ export const Overview: React.FC<Props> = ({
   // handlers
 
   // convert selection change from canvas pixels to frames, and trigger callback.
-  const handleSelectionChange = ({ start, end }: Selection, final: boolean) => {
+  const handleSelectionChange = (selectionState: SelectionChangeEvent) => {
-    if (!final) {
+    const {
+      mode,
+      prevMode,
+      selection: { start, end },
+    } = selectionState;
+
+    if (mode != SelectionMode.Normal || prevMode == SelectionMode.Normal) {
       return;
     }
+
     onSelectionChange({
+      ...selectionState,
+      selection: {
         start: Math.round((start / CanvasLogicalWidth) * mediaSet.audioFrames),
         end: Math.round((end / CanvasLogicalWidth) * mediaSet.audioFrames),
+      },
     });
   };
 

frontend/src/Waveform.tsx

@@ -2,7 +2,8 @@ import { useEffect, useState } from 'react';
 import { Frames, VideoPosition, newRPC } from './App';
 import { MediaSetServiceClientImpl, MediaSet } from './generated/media_set';
 import { WaveformCanvas } from './WaveformCanvas';
-import { Selection, HudCanvas, EmptySelectionAction } from './HudCanvas';
+import { HudCanvas, SelectionChangeEvent } from './HudCanvas';
+import { EmptySelectionAction, SelectionMode } from './HudCanvasState';
 import { from, Observable } from 'rxjs';
 import { bufferCount } from 'rxjs/operators';
 
@@ -10,7 +11,7 @@ interface Props {
   mediaSet: MediaSet;
   position: VideoPosition;
   viewport: Frames;
-  onSelectionChange: (selection: Selection, final: boolean) => void;
+  onSelectionChange: (selectionState: SelectionChangeEvent) => void;
 }
 
 export const CanvasLogicalWidth = 2000;
@@ -43,13 +44,6 @@ export const Waveform: React.FC<Props> = ({
         return;
       }
 
-      console.log(
-        'fetch audio segment, range',
-        viewport,
-        'numFrames',
-        viewport.end - viewport.start
-      );
-
       const service = new MediaSetServiceClientImpl(newRPC());
       const segment = await service.GetPeaksForSegment({
         id: mediaSet.id,
@@ -91,19 +85,25 @@ export const Waveform: React.FC<Props> = ({
 
   // handlers
 
-  const handleSelectionChange = (selection: Selection, final: boolean) => {
+  // convert selection change from canvas pixels to frames, and trigger callback.
+  const handleSelectionChange = (selectionState: SelectionChangeEvent) => {
+    const { mode, prevMode, selection } = selectionState;
+
     const framesPerPixel = (viewport.end - viewport.start) / CanvasLogicalWidth;
     const selectedFrames = {
       start: Math.round(viewport.start + selection.start * framesPerPixel),
       end: Math.round(viewport.start + selection.end * framesPerPixel),
     };
 
-    if (final) {
+    if (mode == SelectionMode.Normal && prevMode != SelectionMode.Normal) {
       setSelectedPixels(selection);
       setSelectedFrames(selectedFrames);
     }
 
-    onSelectionChange(selectedFrames, final);
+    onSelectionChange({
+      ...selectionState,
+      selection: selectedFrames,
+    });
   };
 
   // helpers

frontend/src/helpers/constrainNumeric.test.ts

@@ -0,0 +1,15 @@
+import constrainNumeric from './constrainNumeric';
+
+describe('constrainNumeric', () => {
+  it('constrains the value when it is less than 0', () => {
+    expect(constrainNumeric(-1, 10)).toEqual(0);
+  });
+
+  it('constrains the value when it is greater than max', () => {
+    expect(constrainNumeric(11, 10)).toEqual(10);
+  });
+
+  it('does not constrain an acceptable value', () => {
+    expect(constrainNumeric(3, 10)).toEqual(3);
+  });
+});

frontend/src/helpers/constrainNumeric.ts

@@ -0,0 +1,11 @@
+function constrainNumeric(x: number, max: number): number {
+  if (x < 0) {
+    return 0;
+  }
+  if (x > max) {
+    return max;
+  }
+  return x;
+}
+
+export default constrainNumeric;