简单事件委托实现

function _delegate(obj,className,fn){
    var dc = " "+className+ " ";

    function cb(e){
        var target = e.target,
            c = "";

        while(target != obj){
            c = " "+target.getAttribute("class")+" ";
            if(c.indexOf(dc) != -1){
                fn.call(target,e);
            }
            target = target.parentNode;
        }
    }
    obj.addEventListener("click",cb,false);
}