how can I detect a zoom event on the map zk ?

Like this:http://blog.zkoss.org/index.php/2012/07/03/zk-openlayers-integration/

Just add the zoom callback function by override the ZK openlayer widget

    <script><![CDATA[
    zk.afterLoad('openlayers', function() {
   var _openlayers = {};
   zk.override(openlayers.Openlayers.prototype, _openlayers, {
       bind_: function (desktop, skipper, after) {
           _openlayers.bind_.apply(this, arguments);
           this.map.events.on({
               zoomend: this.proxy(this.zoomend)
           });
       },
       zoomend: function() {
           zk.log('current zoom level', this.map.zoom);
       }
   });
    });

    ]]></script>