居中显示bootstrap modal

在使用 bootstrap modal　的时候，　默认的样式不是居中遮罩显示的，看着很别扭 而且还不想去修改它自身的代码，就在网上查了查，　看到这篇博客的方法还是很不错，转载记录于此 http://www.abeautifulsite.net/vertically-centering-bootstrap-modals/

只需要在你的js代码里面加入这样几个函数就可以，很清晰的code .

//最好在jquey的环境下
    function reposition() {
        var modal = $(this),
            dialog = modal.find('.modal-dialog');
        modal.css('display', 'block');
        
        // Dividing by two centers the modal exactly, but dividing by three 
        // or four works better for larger screens.
        dialog.css("margin-top", Math.max(0, ($(window).height() - dialog.height()) / 2));
    }
    // Reposition when a modal is shown
    $('.modal').on('show.bs.modal', reposition);
    // Reposition when the window is resized
    $(window).on('resize', function() {
        $('.modal:visible').each(reposition);
    });