实现Animating使用requestAnimationFrame，尽量不用setInterval,setTimeout

 1 // handle multiple browsers for requestAnimationFrame()
 2 window.requestAFrame = (function () {
 3     return window.requestAnimationFrame ||
 4             window.webkitRequestAnimationFrame ||
 5             window.mozRequestAnimationFrame ||
 6             window.oRequestAnimationFrame ||
 7             // if all else fails, use setTimeout
 8             function (callback) {
 9                 return window.setTimeout(callback, 1000 / 60); // shoot for 60 fps
10             };
11 })();
12  
13 // handle multiple browsers for cancelAnimationFrame()
14 window.cancelAFrame = (function () {
15     return window.cancelAnimationFrame ||
16             window.webkitCancelAnimationFrame ||
17             window.mozCancelAnimationFrame ||
18             window.oCancelAnimationFrame ||
19             function (id) {
20                 window.clearTimeout(id);
21             };
22 })();