Event Driven Programming

This article is out of date, please refer to http://books.zkoss.org/zkessentials-book/master/ for more up to date information.

In the previous section, we saw how components are declared in ZK User Interface Markup Language (ZUML) to compose an application's user interface. Now we'll look into ZK's event driven programming model to make all the components in an application work together to meet our cause.

How Event Driven Programming Works with Request/Response

Event driven programming is widely adopted for responsive GUI experience. Each ZK component can accept one or more event listeners registered to itself. A component registered with an event listener will listen to actions from users or events registered by the ZK framework itself. The event driven programming model in ZK works perfectly with the Request-Response model of HTTP. Take the sample we saw in the previous section:

<window title="ZK Essentials" mode="overlapped" border="normal" width="250px">
	<label id="lbl"/>World !
	<button label="Hello " onClick="lbl.value = self.label"/>
	<button label="Good-bye " onClick="lbl.value = self.label"/>
</window>

The functionality for this sample code displays the strings of text the user has selected and the text is displayed in a label.

Breaking down how the event driven model works

Register Event Listeners

• At line 3 and 4, we register the onClick events on the buttons so we'll know when and which button is clicked.

	<button label="Hello " onClick="lbl.value = self.label"/>
	<button label="Good-bye " onClick="lbl.value = self.label"/>

Event Sent with Request

• When the user clicks one of the buttons, its respective onClick event is sent to the server for processing by default (optionally, we could handle events from the client directly). As shown highlighted in blue, the "onClick" event and the button's UUID (Universal Unique Identifier) is sent by the Ajax request.

Event Handling Instructions

• From the request, the ZK upload engine knows which component fired the event and what event is fired. It also knows how to handle this request because we've specified the event handling code in ZUL using EL (Expression Language): "lbl.value = self.label", which translates into the following Java code:

 lbl.setValue(self.getLabel);

• • Where lbl is the ID we assigned to the Label component and self refers to the Button component itself (akin to this in Java)

Update Instruction Sent Back in Response

• The server then sends the instructions back to the client to reflect this change in the label's value:

Hello
Good-bye

• • Where the instructions tell the client engine to set the attribute "value" to "Hello" for the component with the UUID "z_d__2", which is the label component.

How to Register Event Listeners

Various ways are provided in ZK to register and handle events, we'll look into the most common usages here.

Event Handling in ZUL page

In a ZUL file, an event handler is registered to a component as the component's attribute.
For example, suppose we have a button labeled "Hello", and we want to create another button labeled "World !" dynamically when the "Hello" button is clicked.

We could program this behavior entirely in a ZUL file:

<window id="win" title="ZK Essentials" border="normal" width="250px">
	<button label="Hello">
		<attribute name="onClick">
		<![CDATA[
		     Button btn = new Button();
		     btn.setLabel("World !");
		     btn.setParent(win);
		]]>
		</attribute>
	</button>
</window>

Inside the button declaration, we declare the "onClick" event as the button's attribute and code its event handling instructions directly in ZUL. This is possible because the Java code is interpreted by BeanShell dynamically. In the Java code, we first created a new Button, set its label to "World !", and then set the Window component as its parent. Despite its convenience, registering event directly from ZUL is not recommended if your application is performance sensitive. As all the Java code needs to be interpreted dynamically.

Event Handling In a Controller

Following the ZK MVC pattern, we create a controller class where we can wire variables and listen to events in our UI.
So here we make the necessary changes:

• At line 1, we declared: apply=controller class name so that the events fired within the window is all forwarded to the controller for event handling.

<window id="win" title="ZK Essentials" border="normal" width="250px" apply="demo.zkoss.SampleCtrl">
	<button label="Hello"/>
</window>

We then implement our controller class:

• At line 9, we extend the SelectorComposer so we can wire variables and listen to events using annotations whose parameters are CSS-like selectors.
• At lines 11 and 12, we declare a Window component and wire it with the window in our ZUL file using annotation.
• At line 14, we use a selector to wire the Button whose label is "Hello" and listen to its onClick event.
• At line 15, the method createWorld contains the same Java code we did previously to dynamically create a button.

package demo.zkoss;

import org.zkoss.zk.ui.select.SelectorComposer;
import org.zkoss.zk.ui.select.annotation.Listen;
import org.zkoss.zk.ui.select.annotation.Wire;
import org.zkoss.zul.Button;
import org.zkoss.zul.Window;

public class SampleCtrl extends SelectorComposer {
	
	@Wire("window")
	Window win;
	
	@Listen("onClick = button[label='Hello']")
	public void createWorld(){
		Button btn = new Button();
		btn.setLabel("World !");
		btn.setParent(win);
	}
}

Event Handling of Dynamically Generated Components

Suppose we take our previous example a step further, we would like to clear off the "Hello" button in the window when we click the dynamically created "World !" button. We'll need to register an event listener to the dynamically created "World !" button.

• At line 12, we add a new event listener to the newly created button. The addEventListener method takes an event name (String) and an EventListener as its arguments.
• Within the anonymous EventListener class, we implement the onEvent method to have the Window component fetch its first child component, which is the button labelled "Hello", and call its detach method to clear it off the Window component.

public class SampleCtrl extends SelectorComposer {
	
	@Wire("window")
	Window win;
	
	@Listen("onClick = button[label='Hello']")
	public void createWorld(){
		Button btn = new Button();
		btn.setLabel("World !");
		btn.setParent(win);
		
		btn.addEventListener("onClick", new EventListener(){
			public void onEvent(Event event) throws Exception {
				Button helloBtn;
				win.getFirstChild().detach();
			};
		});
		
	}
}