window.onload=function(){
    var div=document.querySelector('div');
    //页面中拖拽
    /*
    div.onmousedown=move;
    function move(e){
        var ox=e.offsetX;
        var oy=e.offsetY;
        var ow=div.offsetWidth;
        var oh=div.offsetHeight;
        var cw=document.documentElement.clientWidth;
        var ch=document.documentElement.clientHeight;
        document.onmousemove=function(e){
            var cx=e.clientX;
            var cy=e.clientY;
            var lefts=cx-ox;
            var tops=cy-oy;
            if(lefts<=0){
                lefts=0
            }
            if(tops<=0){
                tops=0;
            }
            if(lefts>cw-ow){
                lefts=cw-ow;
            }
            if(tops>ch-oh){
                tops=ch-oh;
            }
            div.style.left=lefts+'px';
            div.style.top=tops+'px';
        }
    }
    div.onmouseup=function(){
        document.onmousemove=null;
    }
    */




    //元素拖拽（创建对象的方法封装函数）
    /*
    function Drag(domobj){
        this.domobj=domobj;
        this.cw=document.documentElement.clientWidth;
        this.ch=document.documentElement.clientHeight;
        this.ow=domobj.offsetWidth;
        this.oh=domobj.offsetHeight;
    }
    Drag.prototype={
        drag:function(){
            var that=this;
            this.domobj.onmousedown=function(e){
                that.ox=e.offsetX;
                that.oy=e.offsetY;
                document.onmousemove=function(e){
                    that.cx=e.clientX;
                    that.cy=e.clientY;
                    var lefts=that.cx-that.ox;
                    var tops=that.cy-that.ox;
                    if(lefts<=0){
                        lefts=0;
                    }
                    if(tops<=0){
                        tops=0;
                    }
                    if(lefts>that.cw-that.ow){
                        lefts=that.cw-that.ow;
                    }
                    if(tops>that.ch-that.oh){
                        tops=that.ch-that.oh;
                    }
                    that.domobj.style.left=lefts+'px';
                    that.domobj.style.top=tops+'px';
                }
            }
            this.domobj.onmouseup=function(){
                document.onmousemove=null;
            }
        }
    }
    var obj=new Drag(div);
    var div=document.querySelector('div');
    obj.drag();
    */






 //元素拖拽（创建对象的方法封装函数）

    function Drag(domobj){
        this.domobj=domobj;
        this.cw=document.documentElement.clientWidth;
        this.ch=document.documentElement.clientHeight;
        this.ow=domobj.offsetWidth;
        this.oh=domobj.offsetHeight;
    }
    Drag.prototype={
        drag:function(){
            this.down();
        },
        down:function(){
            var that=this;
            this.domobj.onmousedown=function(e){
                that.ox=e.offsetX;
                that.oy=e.offsetY;
                that.move();
            }
            
            this.up();
        },  
        move:function(){
            var that=this;
            document.onmousemove=function(e){
                that.cx=e.clientX;
                that.cy=e.clientY;
                var lefts=that.cx-that.ox;
                var tops=that.cy-that.ox;
                if(lefts<=0){
                    lefts=0;
                }
                if(tops<=0){
                    tops=0;
                }
                if(lefts>that.cw-that.ow){
                    lefts=that.cw-that.ow;
                }
                if(tops>that.ch-that.oh){
                    tops=that.ch-that.oh;
                }
                that.domobj.style.left=lefts+'px';
                that.domobj.style.top=tops+'px';
            }
        },
        up:function(){
            this.domobj.onmouseup=function(){
                document.onmousemove=null;
            }
        }
    }
    var div=document.querySelector('div');
    var obj=new Drag(div);
    obj.drag();
    
}