简单的动画引擎

<div id="box" style="position:absolute;">Hello!</div>

var timers = {

       timerID:0,
       timers:[],

       add:function(fn){
           this.timers.push(fn);
       },

       start:function(){
           if(this.timerID) return;
           (function runNext(){
               if(timers.timers.length > 0){
                   for(var i=0; i<timers.timers.length; i++){
                       if(timers.timers[i]() === false){
                           timers.timers.splice(i,1);
                           i--;
                       }
                   }
                   timers.timersID = setTimeout(runNext,0);
               }
           })();
       },

       stop:function(){
           clearTimeout(this.timerID);
           this.timerID = 0;
       }
};


var box = document.getElementById('box'), x = 0, y = 20;

timers.add(function(){
   box.style.left = x + 'px';
   if(++x > 50) return false;
});

timers.add(function(){
   box.style.top = y + 'px';
   y += 2;
   if(y > 120) return false;
});

timers.start();

在此基础上再扩展一下：

var timers = function(speed,callback){

       var cacheID = 0,
           cache = [];
               
       var timers = {
                   
           add:function(fn){
               cache.push(fn);
           },

           start:function(){
               if(cacheID) return;
               (function runNext(){
                   if(cache.length > 0){
                       for(var i=0; i<cache.length; i++){
                           if(cache[i]() === false){
                               cache.splice(i,1);
                               i--;
                           }
                       }
                       cacheID = setTimeout(runNext,speed || 0);
                   }else{
                       timers.stop();
                   }
               })();
           },

           stop:function(){
               clearTimeout(cacheID);
               cacheID = 0;
               callback && callback();
           }
       };

       return timers;
};


   var animate = function(el,params,speed,callback){
       if(!el) return;

       var t = timers(speed,callback);

       for(var n in params){

           var target = parseFloat(params[n]) || 0;
        var b =  parseFloat(self.currentStyle(el)[n]) || el["offset" + n.substring(0,1).toUpperCase() + n.substring(1)] || 0;

        (function(target,b,n){
               t.add(function(){
                   el.style[n] = b + 'px';
                   if(++b > target) return false;
               });
           })(target,b,n);
       }
               
       t.start();
};


   var box = document.getElementById('box');
   var box2 = document.getElementById('box2');

   animate(box,{'left':200,'top':500,'fontSize':40},10,function(){
       alert('动画结束啦');
   });

   animate(box2,{'left':400,'top':300,'fontSize':80},10,function(){
       alert('动画结束啦');
   });