ZK Essentials/Introduction/Component Based UI

This article is out of date, please refer to http://books.zkoss.org/zkessentials-book/master/ for more up to date information.

In ZK, we can declare UI components using either markup language or Java.

For example, here we declared a Window component, setting the border to normal and resizing its width to a definite 250 pixels. Enclosed in the Window are two Button components.

Where We Declare the Components

The components are declared in files with the extension ".zul". A ZUL page is interpreted dynamically on the server. We could think of it as a JSP empowered with Ajax capabilities. For instance, here we create a new ZUL file, ajax.zul, and we implement a sample, in which users' input in Textbox is reflected in the label below instantly when the text box loses focus:
ajax.zul

<window title="ZK Essentials" border="normal" width="250px">
	<vlayout>
	<textbox id="txtbx" onChange="lbl.value = txtbx.value"/>
	<label id="lbl"/>
	</vlayout>
</window>

The markup declaration above renders a sample program, see below:


ZK components can also be easily declared in Java source codes and are also compatible with Java. Please see the following.

 package org.zkoss.zkdemo;

 import org.zkoss.zk.ui.Page;
 import org.zkoss.zk.ui.GenericRichlet;
 import org.zkoss.zul.*;

 public class TestRichlet extends GenericRichlet {
     //Richlet//
     public void service(Page page) {
        
         final Window win = new Window("ZK Essentials", "normal", false);
         win.setWidth("250px");

         Vlayout vl = new Vlayout();
         vl.setParent(win);
		
         final Textbox txtbx = new Textbox();
	     txtbx.setParent(vl);
		
         final Label lbl = new Label();
	     lbl.setParent(vl);
		
         txtbx.addEventListener("onChange", new EventListener(){
			@Override
			public void onEvent(Event event) throws Exception {
				lbl.setValue(txtbx.getValue());
			}
		});
        

         win.setPage(page);
     }
 }

Please refer to Developer's Reference: Richlet for more details on programming with Richlets.

What the Components Declarations Become

Components declared using ZUML in a ZUL file are parsed by a ZK enhanced XML parser. The components declared are created as Plain Old Java Objects (POJO) in the JVM on the server. Suppose we have a ZUL page that contains nested components as shown below:

<window title="ZK Essentials" border="normal" width="250px">
	<button label="Hello"/>
	<button label="Good-bye "/>
</window>

which would render a window containing two buttons as shown in the image below:

The markup in ZUL is equivalent to the following POJO declarations in Java:

Window win = new Window();
	win.setTitle("ZK Essentials");
	win.setBorder("normal");
	win.setWidth("250px");
		
Button helloBtn = new Button();
	helloBtn.setLabel("Hello");
	helloBtn.setParent(win);
		
Button byeBtn = new Button();
	byeBtn.setLabel("Good-bye");
	byeBtn.setParent(win);

Components on the server are translated into instructions(in JSON) for widget (JavaScript objects) creation and then sent to the client.

Where the Components Belong

The Page

Imagine components are actors in a play, then Page is the stage where components play out their roles. A page is a space holder in a browser window where ZK components can be attached and detached. A Page is not a component and it does not implement a Component interface. A page is automatically created for a ZUL page when user makes the request for it.

The Desktop

Suppose we have a shopping cart application deployed on www.zkstore.com. When a user enters this URL in a browser, by default, the page index.zul is requested. A Desktop is created automatically when a Page is created. A desktop may contain one or more pages, serving requests for the same URL. The concept is illustrated below:

How to Access a Component

With the components nested and stacked up together to give us our application UI, we need a way to identify the necessary ones for manipulation. For example, we might need to dynamically append a component to an existing component, or program one component's behaviour to depend on that of another.
The sample below illustrates the case:

The markup source is:

<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 value of the label with ID "lbl" depends on which button the user clicks: "Hello", or "Good-bye". When a button is clicked, the value of the button's label is assigned to the value of the label. Note that the string "World !" is automatically converted to a label. ZK assigns each component with a UUID (Universal Unique Identifier) to keep track of all the components internally. This UUID is over-ridden when the developer provides it with a more legible ID.

Finding a Component Programmatically in Java

Suppose we have a POJO declaration for a simple UI that looks something like this:

Window outerWin = new Window();

Button outerBtn = new Button();
btn.setParent(outerWin);

Window innerWin = new Window();
innerWin.setParent(outerWin);

Button innerBtn = new Button();
innerBtn.setParent(innerWin);

For better readability, the equivalent declaration above using ZUML in ZUL file is given here:

<window id="outerWin">
       <button id="outerBtn">
        <window id="innerWin">
             <button id="innerBtn"/>
        </window>
</window>

Now suppose we have a controller class where we want to programmatically access and modify the children components (outerBtn, innerWin, innerBtn); how should we do it if we only have access to the Window component?

One of the approaches to accomplish this is to call the component's Component.getFellow() method. For example, if we wish to access the inner Window, we could do the following:

Window innerWin = (Window)outerWin.getFellow("innerWin");

We can call the getFellow method on any component to access another component in the same ID Space.

An ID Space is a way to group components into a more manageable collection in ZK so that we're not lost in trying to access a component in a large component tree. This concept is illustrated below:

The Window component and Page are Space Owners by default. We can make any components a space owner by implementing the IdSpace interface. To identify a component's space owner, call its getSpaceOwner method.

There are other methods to accomplish this, we summarize them in a table for clarity:

Component	method	Note
outerBtn	= (Button)outerWin.getFellow("outerBtn");	The components outerWin, outerBtn, and innerWin form an ID Space; with outerWin being the Space Owner. Components in the same ID Space can call each other by ID using the getFellow method.
innerWin	= (Window)outerWin.getFellow("innerWin");	innerWin belongs to the same ID Space as outerWin, hence, it could be called using the getFellow method.
innerBtn	= (Button)outerWin.getFellow("innerWin").getFellow("innerBtn");	innerWin and innerBtn belong to an ID Space of their own, with innerWin being the Space Owner. innerWin is also a member of the ID Space which outerWin is the Space Owner. Hence, we can call getFellow on outerWin to get innerWin, then call getFellow on innerWin to get innerBtn.
outerBtn	= (Button)Path.getComponent("/outerBtn");	The Path provides the utility method getComponent which takes the relative path of the component as its argument. /outerBtn is equivalent to outerWin/outerBtn
innerWin	= (Window)Path.getComponent("/innerWin");	innerWin and outerBtn both have outerWin as an ID Space Owner.
innerBtn	= (Button)Path.getComponent("/innerWin/innerBtn");	innerBtn has innerWin as its ID Space Owner, innerWin in turn has outerWin as its Space Owner. Hence, we write /innerWin/innerBtn, which is equivalent to outerWin/innerWin/innerBtn
outerBtn	= (Button)outerWin.getFirstChild();	The getFirstChild method returns the first child component of the caller. The advantage of using this method is that you don't even need to know the component ID to fetch the component.
innerWin	= (Window)outerWin.getFirstChild().getNextSibling();	The getFirstChild method gets the outerBtn since it's outerWin's first child component. We then call the getNextSibling method to find the innerWin.
innerBtn	= (Button)outerWin.getFirstChild().getNextSibling().getFirstChild();	We compound another getFirstChild method to get the first, and only, child component of innerWin.

Notes on XML Syntax

The language we use to declare the components is ZUML, an abbreviation for ZK UI Markup Language. ZUML follows the syntax of XML. Here are a couple of basic quick notes if you're not familiar with XML.

• Elements must be well formed
  • close declaration with an end tag:

<window></window>

• • close declaration without an end tag (equivalent to the above statement):

<window/>

• Elements must be properly nested:
  • Correct:

    <window>
	<groupbox>
		Hello World!
	</groupbox>
    </window>

• • Wrong:

    <window>
	<groupbox>
		Hello World!
	</window>
     </groupbox>

• Only a single "root" component is allowed:
  • one root - legal

    <button />

• • two roots - illegal

    <button/>
    <button/>

• • one root containing all other components - legal

    <window>
	<button/>
        <button/>
    </window>

• Attribute value must be quoted
  • Correct:

    <window width="600px"/>

• • Incorrect:

    <window width=600px/>

Using XML tags, we declare a component and set a component's attributes; as an alternative to coding in Java files, we could set a component's attributes to initialize values, evaluate conditions/expressions, and handle events. The figure belows shows an example of how we could easily dictate whether a component is to be displayed on a page when the "if" condition is declared as its attribute.

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