requestanimation()兼容处理

window.requestNextAnimationFrame = (function () {
var originalWebkitRequestAnimationFrame = undefined,
wrapper = undefined,
callback = undefined,
geckoVersion = 0,
userAgent = navigator.userAgent,
index = 0,
self = this;

// Workaround for Chrome 10 bug where Chrome
// does not pass the time to the animation function

if (window.webkitRequestAnimationFrame) {
// Define the wrapper

wrapper = function (time) {
  if (time === undefined) {
    time = +new Date();
  }
  self.callback(time);
};

// Make the switch

originalWebkitRequestAnimationFrame = window.webkitRequestAnimationFrame;

window.webkitRequestAnimationFrame = function (callback, element) {
  self.callback = callback;

  // Browser calls the wrapper and wrapper calls the callback

  originalWebkitRequestAnimationFrame(wrapper, element);
}

}

// Workaround for Gecko 2.0, which has a bug in
// mozRequestAnimationFrame() that restricts animations
// to 30-40 fps.

if (window.mozRequestAnimationFrame) {
// Check the Gecko version. Gecko is used by browsers
// other than Firefox. Gecko 2.0 corresponds to
// Firefox 4.0.

index = userAgent.indexOf('rv:');

if (userAgent.indexOf('Gecko') != -1) {
  geckoVersion = userAgent.substr(index + 3, 3);

  if (geckoVersion === '2.0') {
    // Forces the return statement to fall through
    // to the setTimeout() function.

    window.mozRequestAnimationFrame = undefined;
  }
}

}

return window.requestAnimationFrame ||
window.webkitRequestAnimationFrame ||
window.mozRequestAnimationFrame ||
window.oRequestAnimationFrame ||
window.msRequestAnimationFrame ||

  function (callback, element) {
    var start,
        finish;

    window.setTimeout(function () {
      start = +new Date();
      callback(start);
      finish = +new Date();

      self.timeout = 1000 / 60 - (finish - start);

    }, self.timeout);
  };

}());

window.cancelNextRequestAnimationFrame = window.cancelRequestAnimationFrame
|| window.webkitCancelAnimationFrame
|| window.webkitCancelRequestAnimationFrame
|| window.mozCancelRequestAnimationFrame
|| window.oCancelRequestAnimationFrame
|| window.msCancelRequestAnimationFrame
|| clearTimeout;