requestAnimationFrame 优雅降级

if (!Date.now)    Date.now = function() { return new Date().getTime(); };(function() {    'use strict';    var vendors = ['webkit', 'moz'];    for (var i = 0; i < vendors.length && !window.requestAnimationFrame; ++i) {        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) {        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;    }}());