// Adapted from https://gist.github.com/paulirish/1579671 which derived from 
// http://paulirish.com/2011/requestanimationframe-for-smart-animating/
// http://my.opera.com/emoller/blog/2011/12/20/requestanimationframe-for-smart-er-animating

// requestAnimationFrame polyfill by Erik Möller.
// Fixes from Paul Irish, Tino Zijdel, Andrew Mao, Klemen Slavič, Darius Bacon

// MIT license

if (!Date.now)
 Date.now = function() { return new Date().getTime(); };

(function() {
 'use strict';

 var vendors = ['ms', 'moz', 'webkit', 'o'];
 //浏览器兼容
 for (var i = 0; i < vendors.length && !window.requestAnimationFrame; ++i) {
 //对于支持的浏览器直接统一接口
  var vp = vendors[i];
  window.requestAnimationFrame = window[vp+'RequestAnimationFrame'];
  window.cancelAnimationFrame = (window[vp+'CancelAnimationFrame']
         || window[vp+'CancelRequestAnimationFrame']);
 }
 if (/iP(ad|hone|od).*OS 6/.test(window.navigator.userAgent) // iOS6 is buggy
  || !window.requestAnimationFrame || !window.cancelAnimationFrame) {
  //对于不支持的浏览器退而使用setTimeout和clearTimeout
  var lastTime = 0;
  window.requestAnimationFrame = function(callback) {
   var now = Date.now();
   var nextTime = Math.max(lastTime + 16, now);
   return setTimeout(function() { callback(lastTime = nextTime); },
        nextTime - now);
  };
  window.cancelAnimationFrame = clearTimeout;
 }
}());

function animate() {
  console.log("animation");
  window.requestAnimationFrame(animate);
 }
animate();