面向对象--拖拽

function Drag(id){
    this.obj = document.getElementById(id);
    this.disX = 0;
    this.disY = 0;
}

Drag.prototype.init = function(){
    var self = this;
    
    this.obj.onmousedown = function(ev){
        var event = ev || window.event;
        self.mouseDown(event);
    }
    
    return false;
};

Drag.prototype.mouseDown = function(ev){
    var self = this;
    this.disX = event.clientX - this.obj.offsetLeft;
    this.disY = event.clientY - this.obj.offsetTop;
    
    document.onmousemove = function(ev){
        self.mouseMove();
    }
    
    document.onmouseup = function(){
        self.mouseUp();
    }
};


Drag.prototype.mouseMove = function(){
    this.obj.style.left = (event.clientX - this.disX) + 'px';
    this.obj.style.top = (event.clientY - this.disY) + 'px';
};

Drag.prototype.mouseUp = function(){
    document.onmousemove = null;
    document.onmouseup = null;
}