Stage

autoClear:true

Indicates whether the stage should automatically clear the canvas before each render. You can set this to false to manually control clearing (for generative art, or when pointing multiple stages at the same canvas for example).

var stage = new createjs.Stage("canvasId");
stage.autoClear = false;

p.update = function(props) {
  if (!this.canvas) { return; }
  if (this.tickOnUpdate) { this.tick(props); }
  if (this.dispatchEvent("drawstart", false, true) === false) { return; }
  createjs.DisplayObject._snapToPixelEnabled = this.snapToPixelEnabled;
  var r = this.drawRect, ctx = this.canvas.getContext("2d");
  ctx.setTransform(1, 0, 0, 1, 0, 0);

  if (this.autoClear) { // 在render前自动清除canvas
    if (r) { ctx.clearRect(r.x, r.y, r.width, r.height); }
    else { ctx.clearRect(0, 0, this.canvas.width+1, this.canvas.height+1); }
  }

  ctx.save();
  if (this.drawRect) {
    ctx.beginPath();
    ctx.rect(r.x, r.y, r.width, r.height);
    ctx.clip();
  }
  this.updateContext(ctx);
  this.draw(ctx, false);
  ctx.restore();
  this.dispatchEvent("drawend");
};

handleEvent()

当收到Tick事件时调用阶段更新方法的默认事件处理程序。这允许您直接在ticker上将阶段实例注册为事件侦听器

Ticker.addEventListener("tick", myStage); 
// 直接把Stage当参数放入，会自动执行stage.update()方法

p.handleEvent = function(evt) {
    if (evt.type == "tick") { this.update(evt); }
};

为什么Stage类可以自动调用handleEvent方法呢？原来在EventDispatcher类中

p._dispatchEvent = function(eventObj, eventPhase) {
  var l, arr, listeners = (eventPhase <= 2) ? this._captureListeners : this._listeners;
  if (eventObj && listeners && (arr = listeners[eventObj.type]) && (l=arr.length)) {
    try { eventObj.currentTarget = this; } catch (e) {}
    try { eventObj.eventPhase = eventPhase|0; } catch (e) {}
    eventObj.removed = false;

    arr = arr.slice(); // to avoid issues with items being removed or added during the dispatch
    for (var i=0; i<l && !eventObj.immediatePropagationStopped; i++) {
      var o = arr[i];

      if (o.handleEvent) { o.handleEvent(eventObj); } // 如果有handleEvent方法，则执行
      else { o(eventObj); }
      if (eventObj.removed) {
        this.off(eventObj.type, o, eventPhase==1);
        eventObj.removed = false;
      }
    }
  }
  if (eventPhase === 2) { this._dispatchEvent(eventObj, 2.1); }
};