code: function (zIndex) {
      var $overlay = $('.mdui-overlay');
  
      if ($overlay.length) {
        $overlay.data('isDeleted', 0);
  
        if (zIndex !== undefined) {
          $overlay.css('z-index', zIndex);
        }
      } else {
        if (zIndex === undefined) {
          zIndex = 2000;
        }
  
        $overlay = $('<div class="mdui-overlay">')
          .appendTo(document.body)
          .reflow()
          .css('z-index', zIndex);
      }
  
      var level = $overlay.data('overlay-level') || 0;
      return $overlay
        .data('overlay-level', ++level)
        .addClass('mdui-overlay-show');
    }
docs: 创建并显示遮罩
  @param zIndex 遮罩层的 z-index