requestAnimationFrame.js

var lastTime = 0;
var prefixes = 'webkit moz ms o'.split(' '); //各浏览器前缀

var requestAnimationFrame = window.requestAnimationFrame;
var cancelAnimationFrame = window.cancelAnimationFrame;

var prefix;
//通过遍历各浏览器前缀，来得到requestAnimationFrame和cancelAnimationFrame在当前浏览器的实现形式
for( var i = 0; i < prefixes.length; i++ ) {
    if ( requestAnimationFrame && cancelAnimationFrame ) {
      break;
    }
    prefix = prefixes[i];
    requestAnimationFrame = requestAnimationFrame || window[ prefix + 'RequestAnimationFrame' ];
    cancelAnimationFrame  = cancelAnimationFrame  || window[ prefix + 'CancelAnimationFrame' ] || window[ prefix + 'CancelRequestAnimationFrame' ];
}

//如果当前浏览器不支持requestAnimationFrame和cancelAnimationFrame，则会退到setTimeout
if ( !requestAnimationFrame || !cancelAnimationFrame ) {
    requestAnimationFrame = function( callback, element ) {
      var currTime = new Date().getTime();
      //为了使setTimteout的尽可能的接近每秒60帧的效果
      var timeToCall = Math.max( 0, 16 - ( currTime - lastTime ) ); 
      var id = window.setTimeout( function() {
        callback( currTime + timeToCall );
      }, timeToCall );
      lastTime = currTime + timeToCall;
      return id;
    };
    
    cancelAnimationFrame = function( id ) {
      window.clearTimeout( id );
    };
}

//得到兼容各浏览器的API
window.requestAnimationFrame = requestAnimationFrame; 
window.cancelAnimationFrame = cancelAnimationFrame;