js实现可拖动Div

       随着时代的变化，越来越感觉到js的重要性，js不仅可以做web页面(如Ext框架)，还可以做一些web的特效，这些特效不仅兼容PC，而且兼容手机端，毕竟是基于浏览器的，和平台没关系。现在微软的windows8 系统的App都可以用js开发了，大家有时间可以去尝试一下。

       现在切入正题，说一下js 实现可拖动Div.实现这个功能我们先说一下思路：

     1.捕捉鼠标div的mousedown事件

     2.捕捉 document的   mousemove事件

      3.取消事件

然后我们看一下代码：

function Drag(id) {
            var $ = function (flag) {
                return document.getElementById(flag);
            }
            $(id).onmousedown = function (e) {
                var d = document;
                var page = {
                    event: function (evt) {
                        var ev = evt || window.event;
                        return ev;
                    },
                    pageX: function (evt) {
                        var e = this.event(evt);
                        return e.pageX || (e.clientX + document.body.scrollLeft - document.body.clientLeft);
                    },
                    pageY: function (evt) {
                        var e = this.event(evt);
                        return e.pageY || (e.clientY + document.body.scrollTop - document.body.clientTop);

                    },
                    layerX: function (evt) {
                        var e = this.event(evt);
                        return e.layerX || e.offsetX;
                    },
                    layerY: function (evt) {
                        var e = this.event(evt);
                        return e.layerY || e.offsetY;
                    }
                }             
                var x = page.layerX(e);
                var y = page.layerY(e);        
                if (dv.setCapture) {
                    dv.setCapture();
                }
                else if (window.captureEvents) {
                    window.captureEvents(Event.MOUSEMOVE | Event.MOUSEUP);
                }
                d.onmousemove = function (e) {                    
                    var tx = page.pageX(e) - x;
                    var ty = page.pageY(e) - y;
                    dv.style.left = tx + "px";
                    dv.style.top = ty + "px";
                }
                d.onmouseup = function () {
                    if (dv.releaseCapture) {
                        dv.releaseCapture();
                    }
                    else if (window.releaseEvents) {
                        window.releaseEvents(Event.MOUSEMOVE | Event.MOUSEUP);
                    }
                    d.onmousemove = null;
                    d.onmouseup = null;
                }
            }
        }

代码分析：

1.  

   获取div对象

   var $ = function (flag) {
                   return document.getElementById(flag);
               }    

2. 捕捉document的mousedown事件：

   里面有这么一段代码：

        var page = {
                       event: function (evt) {
                           var ev = evt || window.event;
                           return ev;
                       },
                       pageX: function (evt) {
                           var e = this.event(evt);
                           return e.pageX || (e.clientX + document.body.scrollLeft - document.body.clientLeft);
                       },
                       pageY: function (evt) {
                           var e = this.event(evt);
                           return e.pageY || (e.clientY + document.body.scrollTop - document.body.clientTop);
   
                       },
                       layerX: function (evt) {
                           var e = this.event(evt);
                           return e.layerX || e.offsetX;
                       },
                       layerY: function (evt) {
                           var e = this.event(evt);
                           return e.layerY || e.offsetY;
                       }
                   }

   其中event获取鼠标事件，pageX，pageY获取鼠标的坐标，layerX，layerY获取鼠标距离div边框的距离。

   还有一段代码：

                if (dv.setCapture) {
                       dv.setCapture();
                   }
                   else if (window.captureEvents) {
                       window.captureEvents(Event.MOUSEMOVE | Event.MOUSEUP);
                   }

   这个就是捕捉div的MouseMove和MouseUp事件，不懂得tx可以到网上查查。

3.  document的MouseMove和mouseUp事件： 

d.onmousemove = function (e) {                    
                    var tx = page.pageX(e) - x;
                    var ty = page.pageY(e) - y;
                    dv.style.left = tx + "px";
                    dv.style.top = ty + "px";
                }   
                d.onmouseup = function () {
                    if (dv.releaseCapture) {
                        dv.releaseCapture();
                    }
                    else if (window.releaseEvents) {
                        window.releaseEvents(Event.MOUSEMOVE | Event.MOUSEUP);
                    }
                    d.onmousemove = null;
                    d.onmouseup = null;
                }

其中的tx,ty就是最重要的代码了，是设置div坐标的

有的人可能会问为什么要-x,-y?

x,y其实就是获取鼠标距离div边框的距离，如果不减掉的话

鼠标箭头的坐标和div的x,y坐标一样了，这样拖动之后，鼠标的位置会偏到左上角，效果就是，拖动之后会弹动一下。

                if (dv.releaseCapture) {
                        dv.releaseCapture();
                    }
                    else if (window.releaseEvents) {
                        window.releaseEvents(Event.MOUSEMOVE | Event.MOUSEUP);
                    }
                    d.onmousemove = null;
                    d.onmouseup = null;

上面这段代码就是在鼠标松开之后取消document的onmousemove，onmouseup事件。

图片来源：http://www.cnblogs.com/panjun-Donet/articles/1294033.html