對一個矢量圖動畫，開啟位圖緩存能大大提高運行效率。所謂開啟位圖緩存，其實要自己動手，先創建一個臨時canvas，然后把矢量圖繪制到這個canvas上，到了實際繪制時，直接把這個臨時canvas拷貝到真正canvas上。而位圖拷貝的速度是非常快的，比重新繪制矢量圖要快很多。
三部曲：
1、建立臨時canvas（位圖緩存）
p.cache = function(x, y, width, height, scale) {// draw to canvas.scale = scale||1;if (!this.cacheCanvas) { this.cacheCanvas = document.createElement("canvas");}this._cacheWidth = width;this._cacheHeight = height;this._cacheOffsetX = x;this._cacheOffsetY = y;this._cacheScale = scale;this.updateCache();}
2、繪制到臨時canvas
p.updateCache = function(compositeOperation) {var cacheCanvas = this.cacheCanvas, scale = this._cacheScale, offX = this._cacheOffsetX*scale, offY = this._cacheOffsetY*scale;var w = this._cacheWidth, h = this._cacheHeight, fBounds;if (!cacheCanvas) return;var ctx = cacheCanvas.getContext("2d");w = Math.ceil(w*scale);h = Math.ceil(h*scale);if (w != cacheCanvas.width || h != cacheCanvas.height) {cacheCanvas.width = w;cacheCanvas.height = h;} else if (!compositeOperation) {ctx.clearRect(0, 0, w+1, h+1);}ctx.save();ctx.globalCompositeOperation = compositeOperation;ctx.setTransform(scale, 0, 0, scale, -offX, -offY);this.draw(ctx, true);this._applyFilters();ctx.restore();};
3、copy到真正canvas
p.drawFromCache = function(ctx) {var cacheCanvas = this.cacheCanvas;if (!cacheCanvas) { return false; }var scale = this._cacheScale, offX = this._cacheOffsetX, offY = this._cacheOffsetY, fBounds;if (fBounds = this._applyFilterBounds(offX, offY, 0, 0)) {offX = fBounds.x;offY = fBounds.y;}ctx.drawImage(cacheCanvas, offX, offY, cacheCanvas.width/scale, cacheCanvas.height/scale);return true;};
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但是，這樣會引起問題。在Android上運行，可以發現webview native層的內存占用飛漲，關鍵因素就是這個位圖緩存。
雖然矢量圖可能在舞臺上被移除了，但由于JS層和DOM層兩個關聯，導致垃圾回收機制沒有正常發揮。
需要注意的是，在矢量圖被移除的時候，必須在JS側顯式地把臨時canvas置為null
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