/*
 * 動畫幀函數(shù)
 *
 * */
  var requestFrame=function(){
  var prefixList=['webkit','moz','ms'];
  var func;
  for(var i=0;i<prefixList.length;i++){
    func=window[prefixList[i]+"RequestAnimationFrame"];
    if(func){
      return function(callback){
        func(callback);
      }
    }
  }
  return function(callback){
    setTimeout(callback,67);
  }
}();
/*
 * 勻加速運動
 *
 * */
function animate_easeIn(element,from,to,duration,callback){
  var time=+new Date;
  var distance=to-from;
  var a=2*distance/(duration*duration); //加速度a=2x/t^2（包含方向）
    var func=function(){
    var time2,offsetDis,durTime;
    time2=+new Date;
    durTime=time2-time; //運動的時間間隔
    offsetDis=Math.ceil(a*durTime*durTime/2);//X=a*t^2/2
        if(duration<durTime){
      element.css('left',to+'px');
      callback();
    }else{
      element.css('left',from+offsetDis+'px');
      requestFrame(func);
    }
  }
  func();
}
/*
 * 勻減速運動
 *
 * */
function animate_easeOut(element,from,to,duration,callback){
  var time=+new Date;
  var distance=Math.abs(to-from);
  var a=2*distance/(duration*duration); //x=a*t^2/2 求出加速度
  var v0=Math.sqrt(distance*2*a); // 根據(jù)公式：2as=v^2求出初速度
    var func=function(){
    var time2,offsetDis,durTime,pos;
    time2=+new Date;
    durTime=time2-time;
    offsetDis=Math.ceil(v0*durTime-a*durTime*durTime/2); //根據(jù)s=v0*t+1/2*a*t^2求出位移x
    if(duration<durTime){
      element.css('left',to+'px');
      callback();
    }else{
      pos=from>to? from-offsetDis : from+offsetDis;
      element.css('left',pos+'px');
      requestFrame(func);
    }
    }
    func();
}