模态框（modal）自动居中

// modal对用户可见
$('.modal').on('shown.bs.modal', function () {
    modalOnResize(this);
})

// 浏览器窗口大小改变事件
window.onresize = function(){
    // 已显示modal
    var node = $(".modal:visible");
    if (node.length) {
        modalOnResize(node);
    }
}

function modalOnResize(node){
    var modalHeight = ($(window).height() - $(node).find('.modal-dialog').height()) / 2;
    $(node).find('.modal-dialog').css('margin-top', modalHeight);
}