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;

 }