封装drag方法

function addEvent(elem,type,handle) {
    if(elem.addEventListener) {//通用 IE9以下不兼容
        elem.addEventListener(type,handle,false);
    }else if(elem.attachEvent) {//兼容IE
        elem.attachEvent('on' + type,function() {
            handle.call(elem);//this本身指向window，加上call(elem)使this指向elem
        })
    }else{//兼容性最好，但一个事件只能绑定一个处理函数（缺陷）
        elem['on' + type] = handle;
    }
}

function removeEvent(elem,type,handle){
    if(elem.removeEventListener){
        elem.removeEventListener(type,handle,false);
    }else if(elem.detachEvent){
        elem.detachEvent('on'+type,handle);
    }else{
        elem['on'+type] = null;
    }
}

function stopBubble(event) {
    if(event.stopPropagation) {
        event.stopPropagation();
    }else{
        event.cancelBubble = true;
    }
}

function cancelHandler(event) {
    if(event.preventDefault) {
        event.preventDefault();
    }else{
        event.returnValue = false;
    }
}

function drag(elem) {
    var disX,
        disY;
    addEvent(elem,'mousedown',function(e) {
        var event = e || window.event;
        disX = event.clientX - parseInt(getstyle(elem,'left'));
        disY = event.clientY - parseInt(getstyle(elem,'top'));
        addEvent(document,'mousemove',mouseMove);
        addEvent(document,'mouseup',mouseUp);
        stopBubble(event);
        cancelHandler(event);
    });
    function mouseMove(e) {
        var event = e || window.event;
        elem.style.left = event.clientX - disX + "px";
        elem.style.top = event.clientY - disY + "px";
    }
    function mouseUp(e) {
        var event = e || window.event;
        removeEvent(document,'mousemove',mouseMove);
        removeEvent(document,'mouseup',mouseUp);
    }
}