让某标签始终出现在屏幕中间

function setDivCenter(obj) {
            var top = ($(window).height() - $(obj).height()) / 2;
            var left = ($(window).width() - $(obj).width()) / 2;
            var scrollTop = $(document).scrollTop();
            var scrollLeft = $(document).scrollLeft();
            $(obj).css({ position: 'absolute', 'top': top + scrollTop, left: left + scrollLeft }).show();
            $(window).scroll(function () {
                screenWidth = $(window).width();
                screenHeight = $(window).height();
                scrolltop = $(document).scrollTop();

                objLeft = (screenWidth - obj.width()) / 2;
                objTop = (screenHeight - obj.height()) / 2 + scrolltop;

                obj.css({ left: objLeft + 'px', top: objTop + 'px' });
            });
        }