你还在抱怨自己写的canvas demo徘徊在10帧以下吗？你还在烦恼打开自己写的应用就听见CUP风扇转吗？你正在写一个javascript Canvas库吗？那么下面九点就是你必须知道的！

一.预渲染

错误代码：

      var canvas = document.getElementById("myCanvas");

      var context = this.canvas.getContext('2d');

      var drawAsync = eval(Jscex.compile("async", function () {

          while (true) {

              drawMario(context);

              $await(Jscex.Async.sleep(1000));

          }

      }))

      drawAsync().start();

正确代码：

      var canvas = document.getElementById("myCanvas");

      var context = this.canvas.getContext('2d');

      var m_canvas = document.createElement('canvas');

      m_canvas.width = 64;

      m_canvas.height = 64;

      var m_context = m_canvas.getContext('2d');

      drawMario(m_context);

      var drawAsync = eval(Jscex.compile("async", function () {

          while (true) {

              context.drawImage(m_canvas, 0, 0);

              $await(Jscex.Async.sleep(1000));

          }

      }))

      drawAsync().start();

这里m_canvas的宽度和高度控制得越小越好。

二.尽量少调用canvasAPI

错误代码：

    for (var i = 0; i < points.length - 1; i++) {

          var p1 = points[i];

          var p2 = points[i + 1];

          context.beginPath();

          context.moveTo(p1.x, p1.y);

          context.lineTo(p2.x, p2.y);

          context.stroke();

      }

正确代码：

      context.beginPath();

      for (var i = 0; i < points.length - 1; i++) {

          var p1 = points[i];

          var p2 = points[i + 1];

          context.moveTo(p1.x, p1.y);

          context.lineTo(p2.x, p2.y);

      }

      context.stroke();

三.尽量少改变CANVAS状态

错误代码：

      for (var i = 0; i < STRIPES; i++) {

          context.fillStyle = (i % 2 ? COLOR1 : COLOR2);

          context.fillRect(i * GAP, 0, GAP, 480);

      }

正确代码：

      context.fillStyle = COLOR1;

      for (var i = 0; i < STRIPES / 2; i++) {

          context.fillRect((i * 2) * GAP, 0, GAP, 480);

      }

      context.fillStyle = COLOR2;

      for (var i = 0; i < STRIPES / 2; i++) {

          context.fillRect((i * 2 + 1) * GAP, 0, GAP, 480);

      }

四.重新渲染的范围尽量小

错误代码：

  context.fillRect(0, 0, canvas.width, canvas.height);

正确代码：

      context.fillRect(20, 20, 100, 100);

五.复杂场景使用多层画布

 <canvas  width="600" height="400" style="position: absolute; z-index: 0">

</canvas>

<canvas  width="600" height="400" style="position: absolute; z-index: 1">

</canvas>

六.不要使用阴影

      context.shadowOffsetX = 5;

      context.shadowOffsetY = 5;

      context.shadowBlur = 4;

      context.shadowColor = 'rgba(255, 0, 0, 0.5)';

      context.fillRect(20, 20, 150, 100);

七.清除画布

详细性能差别： 
http://simonsarris.com/blog/346-how-you-clear-your-canvas-matters 
一般情况下：clearRect的性能优于fillRect优于canvas.width = canvas.width;

八.像素级别操作尽量用整数

几种取整数的方法：

      rounded = (0.5 + somenum) | 0;

      rounded = ~ ~(0.5 + somenum);

      rounded = (0.5 + somenum) << 0;

九.使用requestAnimationFrame制作游戏或动画

        (function () {

            var lastTime = 0;

            var vendors = ['ms', 'moz', 'webkit', 'o'];

            for (var x = 0; x < vendors.length && !window.requestAnimationFrame; ++x) {

                window.requestAnimationFrame = window[vendors[x] + 'RequestAnimationFrame'];

                window.cancelAnimationFrame =

          window[vendors[x] + 'CancelAnimationFrame'] || window[vendors[x] + 'CancelRequestAnimationFrame'];

            }

            if (!window.requestAnimationFrame)

                window.requestAnimationFrame = function (callback, element) {

                    var currTime = new Date().getTime();

                    var timeToCall = Math.max(0, 16 - (currTime - lastTime));

                    var id = window.setTimeout(function () { callback(currTime + timeToCall); },

              timeToCall);

                    lastTime = currTime + timeToCall;

                    return id;

                };

            if (!window.cancelAnimationFrame)

                window.cancelAnimationFrame = function (id) {

                    clearTimeout(id);

                };

        } ());

十.其他

与渲染无关的计算交给worker

复杂的计算交给引擎（自己写，或者用开源的），比如3D、物理

缓存load好的图片，canvas上画canvas,而不是画image