jquery 插件 drag drop

(function ($)
{
    var methods = {
        init: function (options)
        {
            options.obj = this;
            options.obj.children().bind("mousedown", options, methods.ondrag).unbind("click").css({ cursor: "move" });
        },
        destroy: function (options)
        {
            this.children().unbind("mousedown").css({ cursor: "hand" }); ;
            if (options && options.fn)
            {
                this.children().bind("click", options.fn);
            }
        },
        ondrag: function (event)
        {
            this.setCapture();
            var $this = $(this);
            $this.css({ "z-index": setdate.topindex, "filter": "alpha(opacity=50);-moz-opacity: 0.5;opacity: 0.5;" });
            setdate.topindex++;
            setdate.mouseOffest = { top: event.offsetY, left: event.offsetX };
            $this.bind("mousemove.drag", event.data, methods.onmove);
            $this.bind("mouseup.drag", event.data, methods.ondrop);
        },
        onmove: function (event)
        {
            var $this = $(this);
            $this.offset({ top: event.clientY - setdate.mouseOffest.top, left: event.clientX - setdate.mouseOffest.left });
        },
        ondrop: function (event)
        {
            var $this = $(this);
            $this.unbind(".drag").css({ "filter": "alpha(opacity=100);-moz-opacity: 1;opacity: 1;" });
            this.releaseCapture();
        },
        checkpos: function (event)
        {

        }
    };
    var setdate = { topindex: 1 };
    $.fn.DragDropAuto = function (method)
    {
        if (this.length == 0) return;
        if (methods[method])
        {
            return methods[method].apply(this, Array.prototype.slice.call(arguments, 1));
        }
        else if (typeof method === 'object' || !method)
        {
            return methods.init.apply(this, arguments);
        }
        else { $.error('Method ' + method + ' does not exist on jQuery.DragDropAuto'); }
    };
})(jQuery);