Event Listening

From Documentation

There are two ways to listen an event: an event handler and an event listener.

What is Event

An event (Event) is used to abstract an activity made by user, a notification made by an application, and an invocation of server push. Thus, the application can handle different kind of notifications and sources with a universal mechanism. By and large, developers can even use the same approach to handle, say, message queues.

Listen by use of an Event Handler

An event handler is a method specified as an event attribute of a ZK page or as a member of a component class.

Declare an Event Handler in a ZUL page

An event handler can be declared in a ZUL page by specifying an event attribute[1]. For example,

<button label="hi" onClick='alert("Hello")'/>

where the content of the event handler is the code snippet in Java. The event handler will be interpreted at the run time (by use of BeanShell). If you prefer to use other language, you could specify the language name in front of it. For example, the following uses Groovy as the interpreter:

<button label="hi" onClick="groovy:alert('Hi, Groovy')"/>

Important Builtin Variables

  • self - the component receiving the event. In the previous example, it is the button itself.
  • event - the event being received. In the previous example, it is an instance of MouseEvent.

Notice that the event handler declared in this way is interpreted at the run time, so it inherits all advantages and disadvantage of interpreter-based execution.


  • It can be changed on the fly without recompiling and reloading the application.
  • Easy to maintain if the code snippet is small.


  • Slower to run.
  • Compilation error can not be known in advance.
  • Hard to maintain if mixing business logic with user interface.


  • It is generally suggested to use this approach for 1) prototyping, or 2) simple event handling.

Declare an Event Handler in Java

The other way to have an event listener is to declare it as a member of a component class. For example,

public class MyButton extends Button {
    public void onClick() {

If the event handler needs to handle the event, it can declare the event as the argument as follows.

public class MyButton extends Button {
    public void onClick(MouseEvent event) {
        Messagebox.show("Hello, "+event.getName());


  • It is suggested to use this approach for component development, since it is subtle for application developers to notice its existence. In additions, it requires to extend the component class.

  1. An event attribute is an attribute starting with on

Listen by Use of an Event Listener

Event Listener

An event listener is a class implementing EventListener. For example,

public class MyListener implements EventListener {
    public void onEvent(Event event) {

Then, you can register an event listener to the component that might receive the event by use of Component.addEventListener(String, EventListener). For example,

button.addEventListener("onClick", new MyListener());

This is a typical approach to handle events. However, it is a bit tedious to register event listeners one-by-one if there are a lot of listeners. Rather, it is suggested to use a composer as described in the following section.

Composer and Event Listener Autowiring

If there are a lot of event listeners to register, it is suggested to use the auto-wiring feature of a composer. For example,

public class MyComposer extends GenericForwardComposer {
    public void onClick$hi() {
    public void onClick$bye() {

As shown above, the method for lsitening an event shall be named by starting with the event name, separating with $, and ending with the component's ID. The composer will search all matched methods and register the event listener automatically. Then, in the ZUL page, you can specify the apply attribute to associate the composer with a component.

<window apply="MyComposer">
    <button id="hi"/>
    <button id="bye"/>

If the listener needs to access the event, just declare it as the argument:

    public void onClick$hi(MouseEvent event) {
      Messsagebox.show("Hello, " + event.getName());

Though not limited, a composer is usually associated with an ID space (such as Window) to handle events and component within the give ID space. You could associate any component that properly represents a scope of your application to manage.

Version History

Version Date Content

Last Update : 2010/11/02

Copyright © Potix Corporation. This article is licensed under GNU Free Documentation License.