If a widget has a lot of child widgets, the performance will be better if you rerender only the portion(s) that don’t have a lot of child widgets (and are not changed).

For example, zul.wgt.Groupbox rerenders only itself and the caption child, if any, when setClosable is called, as follows.

setClosable: function (closable) {
    if (this._closable != closable) {
        this._closable = closable;
        if (this.desktop)
            this.rerender(zk.Skipper.nonCaptionSkipper);
   }
}

It invokes zk.Widget#rerender(zk.Skipper) with a skipper (an instance of zk.Skipper). The skipper decides what to skip (i.e., not to rerender), detach the skipped portion(s), and attach them back after rerendering. Thus, the skipped part won’t be rerendered, nor unbound/bound, so the performance is better.

In the above example, we use the zk.Skipper#nonCaptionSkipper instance to do the job. It skips all child widgets except the one called caption (i.e., child != this.caption).

In addition to passing a skipper to zk.Widget#rerender(zk.Skipper), the widget has to implement the mold method (redraw) to handle the skipper:

function (out, skipper) {
    out.push('<fieldset', this.domAttrs_(), '>');
    var cap = this.caption;
    if (cap) cap.redraw(out);

    out.push('<div id="',  this.uuid, '$cave"', this._contentAttrs(), '>');
    if (!skipper)
        for (var w = this.firstChild; w; w = w.nextSibling)
            if (w != cap) w.redraw(out);
    out.push('</div></fieldset>');
}

As shown above, the mold method is also called with the skipper, and the implementation should not redraw the skipped part. In this case, all child widgets except caption are not redrawn.

You can implement your own skipper. Refer to zk.Skipper for details.