jq实现拖拽

$("body").delegate(
            ".msg-layer",{
                mousedown: function (e) {
                    var el = $(".msg-layer");
                    var os = el.offset(),
                    dx = e.pageX - os.left, dy = e.pageY - os.top;
                    $(document).on('mousemove.drag', function (e) {
                        el.offset({top: e.pageY - dy, left: e.pageX - dx});
                    });
                },
                mouseup: function (e) {
                    $(document).off('mousemove.drag');
                }
            }
    );

".msg-layer"为被拖拽的元素