原生javascript 实现 animate

//animate
			function getstyle(obj,name){
				if(obj.currentStyle){
					return obj.currentStyle[name];
				}else{
					return getComputedStyle(obj,false)[name];
				}
			}
			function startrun(obj,attr,target,fn){
				 clearInterval(obj.timer);
				 obj.timer = setInterval(function(){
				  var cur = 0;
				  if(attr == "opacity"){
				   cur = Math.round(parseFloat(getstyle(obj,attr))*100);
				  }else{
				   cur = parseInt(getstyle(obj,attr));
				  }
				  var speed = (target-cur)/8;
				  speed = speed>0?Math.ceil(speed):Math.floor(speed);
				   
				  if(cur == target){
				   clearInterval(obj.timer);
				   if(fn){
				    fn();
				   }
				  }else{
				   if(attr == "opacity"){
				    obj.style.filter = "alpha(opacity="+(cur+speed)+")";
				    obj.style.opacity = (cur+speed)/100;
				   }else{
					obj.style[attr] = cur + speed + "px";
				   }
				  }
				 },30)
				}