Event Listening

Overview

ZK allows applications to handle events at both the server and client side. Handling events at the server side, as described in the previous sections, is more common, since the listeners can access the backend services directly. However, handling event at the client side improves the responsiveness. For example, it is better to be done with a client-side listener if you want to open the drop-down list when a comobox gains the focus.

The rule of thumb is to use server-side listeners first since it is easier, and then improve the responsiveness of the critical part, if any, with the client-side listener.

Here we describe how to handle events at the client. For client-side UI manipulation, please refer to the UI Composing and Widget Customization sections.

Declare a Client-side Listener in ZUML

Declaring a client-side listener in a ZUML document is similar to declaring a server-side listener, the steps are:

1. Declare client namespace first, URI is http://www.zkoss.org/2005/zk/client (aka., client)
2. write your logic in JavaScript

Implementation Notes:

• this references to the event target widget.
• Use this.$f() to reference fellow widgets (Widget.$f())
• event is referenced to zk.Event.

For example,

<combobox xmlns:w="client" w:onFocus="this.open()"/>

Notice that EL expressions are allowed in the JavaScript code (for the client-side listener). Thus, it is straightforward to embed the server-side data to the client-side listener. For example,

<window id="wnd" title="main">
<combobox xmlns:w="client" w:onFocus="zk.log('${wnd.title}')"/>
</window>

If you want to escape it, place a backslash between $ and {, such as w:onFocus="zk.log('$\{wnd.title}')".

For more information about manipulating widgets at the client, please refer to the UI Composing section.

Client-side Event Listener First then Server-side

It is allowed to register both the client and server-side event listeners. They will be both invoked. Of course, the client-side listener is called first, and then the server-side listener. For example,

  <combobox xmlns:w="client" w:onFocus="this.open()"
   onFocus='self.parent.appendChild(new Label("focus"))'/>

Client-side Event Controls Firing Behavior

If you want to stop the event propagation such that the server won't receive the event, you could invoke Event.stop(Map). For example, the server-side listener won't be invoked in the following example:

  <combobox xmlns:w="client" w:onFocus="this.open(); event.stop();"
   onFocus='self.parent.appendChild(new Label("focus"))'/>

Since ZK fires an event to the server-side based on the same Event, you can also override event.opts to affect event firing behavior.

Declare a Client-side Listener in Java

The other way to declare a client-side listener at the server is Component.setWidgetListener(String, String). For example,

combobox.setWidgetListener("onFocus", "this.open()");

Notice that it is Java and running at the server.

Also notice that EL expressions are not allowed (i.e., not interpreted) if you assign it directly. It is because EL expressions are interpreted by ZK Loader when loading a ZUL page. However, it is easy to construct a string to any content you want with Java.

Register a Client-side Listener in Client-Side JavaScript

Listening an event at the client could be done by calling Widget.listen(Map, int). For example,

<window>
	<bandbox id="bb"/>
	<script defer="true">
	this.$f().bb.listen({onFocus: function () {this.open();}});
	</script>
</window>

where

1. defer="true" is required such that the JavaScript code will be evaluated after all widgets are created successfully. Otherwise, it is not able to retreive the bandbox (bb).
2. script is a widget (unlike zscript), so this references to the script widget, rather than the parent.
3. Widget.$f(String) is equivalent to Component.getFellow(String), except it is a JavaScript method (accessible at the client).

Register DOM-level Event Listener

Notice that the event listener handling discussed in the previous sections is for handling so-called ZK widget event (Event). Though rare, you could register a DOM-level event too by the use of jQuery (API: jq).

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