html中div居中并移动

(function($) {  
	   jQuery.fn.center = function () {  
            this.css('position','absolute');  
             this.css('top', ( $(window).height() - this.height() ) /2 +$(window).scrollTop() + 'px');  
             this.css('left', ( $(window).width() - this.width() ) / 2+$(window).scrollLeft() + 'px');  
             return this;  
        } 
		
		$("#bannerBar").center();
		
		
		$('#banner').mousedown( function (event) {
	
			var isMove = true; 
			var abs_x = event.pageX - $('#bannerBar').offset().left; 
			var abs_y = event.pageY - $('#bannerBar').offset().top;
		
			$(document).mousemove(function (event) { 
				if (isMove) { 
					var obj = $('#bannerBar'); 
					obj.css({'left':event.pageX - abs_x, 'top':event.pageY - abs_y}); 
				}
			}).mouseup(function () { 
				isMove = false; 
			}); 
		
		}); 
	})(jQuery);