function getStyle(obj,attr){ //兼容性写法获得样式
        if(window.getComputedStyle){ 
          return window.getComputedStyle(obj, null)[attr];
        }else{
          return obj.currentStyle[attr];
        }
      }
       function animate(obj, json, speed, callback){
        clearInterval(obj.timer);
        obj.timer = setInterval(function(){
          var flag = true;
          for(var attr in json){
          var current = parseInt(getStyle(obj,attr));
          var step = (json[attr] - current) / 10;
           step = step > 0 ? Math.ceil(step) : Math.floor(step);
           obj.style[attr] = current + step +'px';
          if(current != json[attr]){
            flag = false;
          }
        }
         if(flag){ 
          clearInterval(obj.timer);
          if(callback && typeof callback == 'function'){ //验证callback是否传递，传递的话看是否是函数类型
           callback();
          }
         }
        },speed);
      }