Bootstrap Modal垂直居中

$('#modal').on('show.bs.modal', function(){
    var $this = $(this);
    var $modal_dialog = $this.find('.modal-dialog');
    $this.css('display', 'block');//如没将modal设置为 block，则$modala_dialog.height() 为零
    $modal_dialog.css({'margin-top': Math.max(0, ($(window).height() - $modal_dialog.height()) / 2) });
});