dialog

 　　　　function showDialog(){
             var $by = $(window),
                 obj = $('.dialog'),
                 brsW = $by.width(),
                 brsH = $by.height(),
                 sclL = $by.scrollLeft(),
                 sclT = $by.scrollTop(),
                 curW = obj.width(),
                 curH = obj.height();
             //计算对话框居中时的左、上边距
             var left = sclL + (brsW - curW)/2;
             var top = sclT + (brsH - curH)/2;
             //设置对话框的位置
             obj.css({"left":left,"top":top});
             console.log(sclT,sclL)
         }
         showDialog();
         $(window).resize(function(){
             showDialog();
         })

 .dialog{
     width: 500px;
     height: 500px;
     border: 1px solid #ddd;
     position: absolute;
 }