最近一直在看javascript的书籍，有些东西在书上看着貌似理解了，但是在真正动手实践时，其实有些细节你根本不了解。所以看起来就算是一个简单的效果，写起来也未必简单，就算写起来简单，写的代码也未必规范、可维护性等等。无奈，一直学习编程以来都是眼高手低，导致什么都写不出来。在自己没有足够的功底写出好的代码出来，学习大牛的代码也是一直高效的方式。以下是一个拖放的效果，参考的代码，重构以下，加以理解学习。
首先来看效果：
拖动div
拖放状态：未开始
【程序说明】
拖动原理：其实就是在拖动块上监听mousedown事件，鼠标点击时，通过事件对象获取对应的坐标参数。然后鼠标移动时再监听document上的mousemove事件，获取鼠标的clientX 和clientY坐标然后设置拖动块的left 和 top。
首先是监听mousedown事件
EventUtil.addEventHandler(this.Drag, "mousedown", BindAsEventListener(this, this.Start));
然后在Start上添加mousemove 和 mouseup 事件
//监听mousemove 和 mouseup事件 EventUtil.addEventHandler(document, "mousemove", this._fM); EventUtil.addEventHandler(document, "mouseup", this._fS);
鼠标移动时，设置拖动块的left 和 top 属性 ：
if(!this.LockX)this.Drag.style.left = iLeft + "px"; if(!this.LockY)this.Drag.style.top = iTop + "px";
水平和垂直锁定：通过判断LockX 和lockY属性来限制对于的top 和 left 属性即可。
范围限制锁定：通过计算容器的宽高和拖动块的宽高差值来设定最大left值和top值，来限制拖动块的left值和top值会在一定的范围里。
完整DEMO：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>JavaScript拖放效果</title>
<style type="text/css">
</style>
</head>
<body>
<div id="drag-wrap" style="height: 300px;margin:10px;border:5px solid #FF8000;background:#8080C0;position: relative;">
<div class="draggable" id="drag" style="100px;height: 100px;position: absolute;top: 100px;left:100px;background:#eee;border:1px solid #aceaac;cursor: move;">拖动div</div>
</div>
<input id="idReset" type="button" value="复位" />
<input id="idLock" type="button" value="锁定全部" />
<input id="idLockX" type="button" value="锁定水平" />
<input id="idLockY" type="button" value="锁定垂直" />
<input id="idLimit" type="button" value="范围锁定" />
<input id="idLimitOff" type="button" value="取消范围锁定" />
<br />拖放状态：<span id="idShow">未开始</span>
<script type="text/javascript">
/**
*工具函数
*/
var isIE = (document.all) ? true : false ;
var $$ = function(id){
return "string" == typeof id ? document.getElementById(id) : id;
};
var Class = {
create: function() {
return function() { this.initialize.apply(this, arguments); }
}
};
var Extend = function(destination,source){
for(var property in source){
destination[property] = source[property];
}
};
var BindAsEventListener = function(object,func){
return function(event){
return func.call(object,event || window.event);
}
};
var Bind = function(object,func){
return function(){
return func.apply(object,arguments);
}
};
/**
*跨浏览器事件对象
*/
var EventUtil = {
//事件注册处理程序
addEventHandler:function(oTarget,sEventType,fnHandler){
if(oTarget.addEventListener){
oTarget.addEventListener(sEventType,fnHandler,false);
}else if(oTarget.attachEvent){
oTarget.attachEvent("on"+sEventType,fnHandler);
}else{
oTarget["on"+sEventType] = fnHandler;
}
},
//事件移除处理程序
removeEventHandler:function(oTarget,sEventType,fnHandler){
if(oTarget.removeEventListener){
oTarget.removeEventListener(sEventType,fnHandler,false);
}else if(oTarget.detachEvent){
oTarget.detachEvent("on"+sEventType,fnHandler);
}else{
oTarget["on"+sEventType] = null;
}
},
getEvent:function(event){
return event ? event : window.event;
},
getTarget:function(event){
return event.target || event.srcElement;
}
};
/**
*拖放程序
*/
var Drag= Class.create();
Drag.prototype = {
//初始化对象
initialize:function(drag,options){
this.Drag = $$(drag);//拖放对象
this._x = this._y = 0;//记录鼠标相对于拖放对象的位置
//事件对象(用于绑定移除事件)
this._fM = BindAsEventListener(this, this.Move);
this._fS = Bind(this, this.Stop);
this.Drag.style.position = "absolute";
this.marginLeft = this.marginTop = 0;//记录margin
//设置参数
this.setOptions(options);
//获取相关参数及类型转换
this.Limit = !!this.options.Limit;//转换为布尔型
this.mxLeft = parseInt(this.options.mxLeft);
this.mxRight = parseInt(this.options.mxRight);
this.mxTop = parseInt(this.options.mxTop);
this.mxBottom = parseInt(this.options.mxBottom);
this.Lock = !!this.options.Lock;
this.LockX = !!this.options.LockX;
this.LockY = !!this.options.LockY;
this.onStart = this.options.onStart;
this.onMove = this.options.onMove;
this.onStop = this.options.onStop;
this._Handle = $$(this.options.Handle) || this.Drag;
this._mxContainer = $$(this.options.mxContainer) || null;
//监听拖动对象mousedown事件
EventUtil.addEventHandler(this.Drag, "mousedown", BindAsEventListener(this, this.Start));
},
//准备拖动
Start:function(oEvent){
if(this.Lock){return;}//如果锁定则不执行
//记录mousedown触发时鼠标相对于拖放对象的位置
this._x = oEvent.clientX - this.Drag.offsetLeft;
this._y = oEvent.clientY - this.Drag.offsetTop;
//监听mousemove 和 mouseup事件
EventUtil.addEventHandler(document, "mousemove", this._fM);
EventUtil.addEventHandler(document, "mouseup", this._fS);
},
//拖动
Move:function(oEvent){
//设置移动参数
var iLeft = oEvent.clientX - this._x , iTop = oEvent.clientY - this._y;
//设置范围限制
if(this.Limit){
//设置范围参数
var mxLeft = this.mxLeft, mxRight = this.mxRight, mxTop = this.mxTop, mxBottom = this.mxBottom;
//如果设置了容器，再修正范围参数
if(!!this._mxContainer){
mxLeft = Math.max(mxLeft, 0);
mxTop = Math.max(mxTop, 0);
mxRight = Math.min(mxRight, this._mxContainer.clientWidth);
mxBottom = Math.min(mxBottom, this._mxContainer.clientHeight);
};
//修正移动参数
iLeft = Math.max(Math.min(iLeft, mxRight - this.Drag.offsetWidth), mxLeft);
iTop = Math.max(Math.min(iTop, mxBottom - this.Drag.offsetHeight), mxTop);
}
//XY锁定
if(!this.LockX)this.Drag.style.left = iLeft + "px";
if(!this.LockY)this.Drag.style.top = iTop + "px";
//执行附加程序
this.onMove();
},
//停止拖动
Stop:function(){
EventUtil.removeEventHandler(document, "mousemove", this._fM);
EventUtil.removeEventHandler(document, "mouseup", this._fS);
//执行附加程序
this.onStop();
},
//设置默认参数
setOptions:function(options){
this.options = {//默认值
Handle: "",//设置触发对象（不设置则使用拖放对象）
Limit: false,//是否设置范围限制(为true时下面参数有用,可以是负数)
mxLeft: 0,//左边限制
mxRight: 9999,//右边限制
mxTop: 0,//上边限制
mxBottom: 9999,//下边限制
mxContainer: "",//指定限制在容器内
LockX: false,//是否锁定水平方向拖放
LockY: false,//是否锁定垂直方向拖放
Lock: false,//是否锁定
Transparent: false,//是否透明
onStart: function(){},//开始移动时执行
onMove: function(){},//移动时执行
onStop: function(){}//结束移动时执行
};
Extend(this.options, options || {});
}
};
//初始化拖动对象
var drag = new Drag('drag',{mxContainer:'drag-wrap',Limit:true,
onStart: function(){ $$("idShow").innerHTML = "开始拖放"; },
onMove: function(){ $$("idShow").innerHTML = "left："+this.Drag.offsetLeft+"；top:"+this.Drag.offsetTop; },
onStop: function(){ $$("idShow").innerHTML = "结束拖放"; }
});
$$("idReset").onclick = function(){
drag.Limit = true;
drag.mxLeft = drag.mxTop = 0;
drag.mxRight = drag.mxBottom = 9999;
drag.LockX = drag.LockY = drag.Lock = false;
$$("idShow").innerHTML = "复位";
}
$$("idLock").onclick = function(){ drag.Lock = true;$$("idShow").innerHTML = "锁定全部";}
$$("idLockX").onclick = function(){ drag.LockX = true; $$("idShow").innerHTML = "锁定水平";}
$$("idLockY").onclick = function(){ drag.LockY = true; $$("idShow").innerHTML = "锁定垂直";}
$$("idLimit").onclick = function(){ drag.mxRight = drag.mxBottom = 200;drag.Limit = true;$$("idShow").innerHTML = "范围锁定"; }
$$("idLimitOff").onclick = function(){ drag.Limit = false; $$("idShow").innerHTML = "取消范围锁定";}
</script>
</body>
</html>
实例下载：
原文出处：
http://www.cnblogs.com/cloudgamer/archive/2008/11/17/Drag.html