Tutorial: 2 React Music Box

In this tutorial, we implement a simple multiplayer music box to illustrate realtime view side update and how to use other hooks. We also use TypeScript in this example. The app has a number of "balls", each of which represents the timing and pitch of a note. A participant can manipulate them to compose a loop. The timing for the bar to wrap is synchronized by the model side logic, but the view interpolates the position of the bar and plays a sound when the bar passes a ball.

We define our main model class, MusicBoxModel as a subclass of Model. The model contains a set of balls stored in Map<BallId, BallData>, keyed by a BallId-type value and logical position and the state of interaction of the ball as the value. The balls may be added and deleted and we still have to enumerate them in deterministic order; this is another reason why we use a Map.

The model has methods for each type of user interaction, such as grab, move, addBall, etc. wrap() is called every 2000 milliseconds and publishes a message called wrap.

class MusicBoxModel extends Model {
  width = 720;
  height = 480;
  wrapTime = 0;
  balls: Map<BallId, BallData> = new Map();
  currentId = 0;

The move() method, for example, receives a new ball position from a participant, update balls, and publishes a "moved" message with the same argument.

move(data: MoveData) {
    const { viewId, id, x, y } = data;
    const ball = this.balls.get(id);
    if (!ball) {
      return;
    }
    if (ball.grabbed !== viewId) {
      return;
    }
    ball.x = x;
    ball.y = y;
    this.publish(this.id, "moved", data);
  }

Moving over to the view side, we first define our top-level component, MusicBoxApp, which starts a Croquet session.

function MusicBoxApp() {
  return (
    <InCroquetSession
      name="musicbox"
      tps={10}
      apiKey="1_k2xgbwsmtplovtjbknerd53i73otnqvlwwjvix0f"
      appId="io.croquet.react.codesandbox.musicbox"
      password="abc"
      model={MusicBoxModel}
      eventRateLimit={60}}>
      <MusicBoxField/>
    </InCroquetSession>
  );
}

The view side logic is written in the MusicBoxField component. For the view to rerender without having to wait for a network round trip, the component has a state called viewBalls, which is "almost" the copy of the model's balls, except that modifications occur in the local view are included. Because balls may be added and deleted, the data structure is a Map, but to obey React's state update rule, we wrap it in an object.

 const [viewBalls, setViewBalls]
    = useState<{data: Map<BallId, BallData>}>({data: new Map(model.balls)});

grabInfo stores the transient state when a ball is grabbed by the local user.

  const [grabInfo, setGrabInfo]
    = useState<{data: Map<PointerId, {ballId:BallId, grabPoint:Point, translation:Point}>}>({data: new Map()});

The component subscribes to the message for each type of interaction, beside the wrap message.

  useSubscribe<number>(model.id, "wrap", (time) => setWrapTime(time));
  useSubscribe<GrabData>(model.id, "grabbed", grabBall);
  useSubscribe<MoveData>(model.id, "moved", moveBall);
  useSubscribe<GrabData>(model.id, "released", releaseBall);
  useSubscribe<MoveData>(model.id, "added", addBall);
  useSubscribe<GrabData>(model.id, "removed", removeBall);

Notice that each call to useSubscribe has a type parameter to ensure that handlers have matching types.

A message handler, such as moveBall, is defined as follows.

  const moveBall = useCallback((data:MoveData, viewSide?:boolean) => {
    const {viewId, id, x, y} = data;

    if (!viewSide && viewId === myViewId) {return;}

    setViewBalls((oldViewBalls) =>{
      const map = oldViewBalls.data;
      map.set(id, {x, y, grabbed: viewId});
      return {data: map};
    });
  }, [myViewId]);

As described later, the same moveBall function is called directly from the view side user event handler as well. The second argument specifies if it is handling the view side event or a Croquet message from the model. The call to setViewBalls state updater mutates the wrapped Map but creates a new object to trigger rerendering.

We also define publish hooks.

  const publishGrab = usePublish<GrabData>((id) => [
    model.id, 'grab', {viewId: myViewId, id}]);
  const publishMove = usePublish<MoveData>((id, newTranslation) => [
    model.id, 'move', {viewId: myViewId, id, x: newTranslation.x, y: newTranslation.y}]});
  const publishRelease = usePublish<GrabData>((id) => [
    model.id, 'release', {viewId: myViewId, id}]);
  const publishAddBall = usePublish((x, y) => [
    model.id, 'addBall', {viewId: myViewId, x, y}]);
  const publishRemoveBall = usePublish<GrabData>((id) => [
    model.id, 'removeBall', {id, viewId: myViewId}]);

The functions take some arguments such as id, or x, y and publish a message. The type parameter for usePublish ensures that the data to be publised conforms to the type.

There are three user event handlers, namely pointerDown, pointerMove and pointerUp. We use pointerEvents to support multi touch interaction . Due to React's platform-neutral synthetic event restrictions, and due to the need for allowing the MusicBoxField to be transformed to fit into different screen sizes of participants, we need to attach the event handlers to the MusicBoxField component, and implement the hit detection logic ourselves in the findBall() function. The pointerDown callback, for example, computes the original translation in the reference of the MusicBoxField, updates the grabInfo state, first by directly mutating data in the wrapped Map, then by calling setGrabInfo. The call to grabBall() has the second argument (viewSide) so it updates the viewBalls data before publishing a message by calling publishGrab.

  const pointerDown = useCallback((evt) => {
    enableSound();
    const x = evt.nativeEvent.offsetX;
    const y = evt.nativeEvent.offsetY;
    const pointerId:PointerId = evt.pointerId;
    const balls = model.balls;
    const entry = findBall(x, y, balls);
    if (!entry) {return;}
    const [ballId, ballData] = entry;
    if (ballData.grabbed && ballData.grabbed !== myViewId) {return;}
    const info = grabInfo.data.get(pointerId);
    if (info) {return;}
    const g = {ballId: entry[0], grabPoint: {x: x, y: y} as Point,
               translation: {x: ballData.x, y: ballData.y} as Point};

    grabInfo.data.set(evt.pointerId, g);
    setGrabInfo({data: grabInfo.data});
    grabBall({viewId: myViewId, id: ballId}, true);

    publishGrab(ballId);
    evt.target.setPointerCapture(evt.pointerId);
  }, [grabInfo, findBall, grabBall, model.balls, publishGrab, myViewId]);

The pointerMove handler follows the similar structure. The latter part computes the new position for the ball, move the ball locally by calling moveBall() with the second argument, and publish the move message to other participants.

  const pointerMove = useCallback((evt) => {
    if (evt.buttons === 0) {return;}
    const pointerId:PointerId = evt.pointerId;
    const info = grabInfo.data.get(pointerId);
    if (!info) {return;}
  
    const ball = model.balls.get(info.ballId);
    if (!ball) {return;}
    if (ball.grabbed && ball.grabbed !== myViewId) {return;}

    let x = evt.nativeEvent.offsetX - info.grabPoint.x + info.translation.x;
    let y = evt.nativeEvent.offsetY - info.grabPoint.y + info.translation.y;
    if (x <= 0) {x = 0;}
    // if (x > model.width - BallDiameter) {x = model.width - BallDiameter;}
    if (y <= 0) {y = 0}
    if (y > model.height - BallDiameter * 2) {y = model.height - BallDiameter * 2;}

    moveBall({x, y, viewId: myViewId, id: info.ballId}, true);
  
    publishMove(info.ballId, {x, y});
  }, [grabInfo, moveBall, publishMove, model.height, model.balls, myViewId]);

An important part is the condition in the middle.

    const ball = model.balls.get(info.ballId);
    if (!ball) {return;}
    if (ball.grabbed && ball.grabbed !== myViewId) {return;}

Imagine if two or more participants tried to click on the same ball almost at the same time. The pointerDown handler for each participant would successfully store data into grabInfo, and then publishes the grab message. However, the reflector decides who actually get there first. Other participants need to stop dragging the ball if the reflector decided against their favor. In the line above, pointerMove checks if the data in the model agrees with the local view state, and if not, avoid executing further logic in pointerMove.

(Note that this kind of speculative execution is an optimization and may not be always necessary to implement. If you application does not require smooth movement, first write the logic as clean as possible without optimization and try it. Only then consider adding optimizations.)

Because we want the bar to keep moving smoothly at 60 fps (or more), we need to tap into the Croquet.View's update() method. The useUpdateCallback hook "injects" a function into update() and has it invoked from each update() call. The argument for the hook typically needs to be a fresh function, thus is defined as a function in the component.

useUpdateCallback(update);

There are other hooks to handle synced event and detach() method invocation. In this example, we use useSyncedCallback to log a message to the console.

 useSyncedCallback((flag:boolean) => {
    console.log("synced", flag, barPos);
  });

The updateCallback effectively invokes MusicBoxField for each animation frame. MusicBoxField returns a Fragment with an appropriate scale and other style parameters and updated list of Ball components.

return (
    <>
      <div
        id="field"
        style={style as any}
        onPointerDown={pointerDown}
        onPointerMove={pointerMove}
        onPointerUp={pointerUp}
      >
        <Bar pos={barPos}></Bar>
        {balls}
      </div>
      <BallContainer publishAddBall={publishAddBall} />
    </>
  );

A word of caution here, however, is that a Croquet application may as well be easier to develop on top of the vanilla Croquet library or the Virtual DOM framework. As you can see above, the view side smoothing logic requires a separate data source for components and makes imperative udpates on the data source. Handling a list of components whose properties may be changed by more than one client requires more computation than simply setting values into elements. A careful deliberation on the trade-offs between frameworks is something one should do before picking the @croquet/react framework.