text: //  动画函数
  var fx = function( fn , begin , end ){
  
      //  渐出特效
      fx.easeOut = function(t,b,c,d){
              return -c *(t /= d)*(t-2) + b;
      }
  
      var options = arguments[3] || {};
      var duration = options.duration || 500;
      var ease = options.ease || fx.easeOut;
  
      startTime = new Date().getTime();
  
      (function(){
          setTimeout(function(){
              timestamp = new Date().getTime() - startTime;
              fn( ease( timestamp,begin, ( end - begin),duration) , 'step' );
  
              if(duration <= timestamp){
                  fn( end , 'end' );
              }else{
                  setTimeout(arguments.callee,25);
              }
          },25)
      })();
  }