Tutorial

This tutorial guides you through the most fundamental features of ZK to help you understand ZK faster.

==Hello World!==
After ZK is installed on your favorite Web server<ref>Please refer to [[ZK Installation Guide]].</ref>, writing applications is straightforward. Just create a ZUML file, say hello.zul<ref>The other way to try examples is to use [http://www.zkoss.org/zksandbox/userguide ZK Sandbox] to run them.</ref>, under one of the Web application's directories just as you would do for an HTML file.

<syntax lang="xml" >
 <window title="My First ZK Application" border="normal">
 	Hello World!
 </window>
</syntax>


Then, go to the corresponding URL, say http://localhost/myapp/hello.zul, and you'll see your first ZK application running.

[[Image:dgGettingStartedHello.zul.png]]

In a ZUML page, an XML element describes what component to create while the XML attributes are used to assign values to a component's properties. In this example, a <tt>window</tt> component is created and its <tt>title</tt> is set to <tt>"My First ZK Application"</tt> and its <tt>border</tt> is set to <tt>normal</tt>.

The text enclosed in the XML elements is also interpreted as a special component called <tt>label</tt>. Thus, the above example is equivalent to the following.

<syntax lang="xml" >
 <window title="My First ZK Application" border="normal">
	<label value="Hello World!"/>
</window>
</syntax>

<blockquote>
----
<references/>
</blockquote>

==Say Hello in Ajax way==

Let us put some interactivity into it.

<syntax lang="xml" >
 <button label="Say Hello" onClick='Messagebox.show("Hello World!")'/>
</syntax>

Then, when you click the button, you'll see the following:

[[Image:DgGettingStartedHello2.png]]

The <tt>onClick</tt> attribute is a special attribute used to add an event listener to the component such that it is invoked when an end user clicks the component. The attribute value could be any legal Java code. Notice that it is not JavaScript, and we have to use the double quotes (") to represent a string. Furthermore, to specify a double quote in an XML attribute, we could use single quotes (') to enclose it<ref>If you are not familiar with XML, you might take a look at [[ZK_Developer%27s_Guide/Fundamental_ZK/ZK_User_Interface_Markup_Language/XML |the XML section]].</ref>.

Here we invoke <javadoc method="show(java.lang.String)">org.zkoss.zul.Messagebox</javadoc> to show a message box as depicted above.

The Java code is interpreted by [http://www.beanshell.org/ BeanShell] at run time. In additions to event handling, we could embed the code in a ZUML page by specifying it in a special element called <tt>zscript</tt>. For example, we could define a function to simply the code as follows.

<syntax lang="xml" >
 <window title="My First ZK Application" border="normal">
 	<button label="Say Hello" onClick='alert("Hello World!")'/>
 	<zscript>
 		void alert(String message){ //declare a function
 			Messagebox.show(message);
 		}
 	</zscript>
 </window>
</syntax>

In fact, <tt>alert</tt> is a built-in function that you can use directly in the embedded Java code.

<blockquote>
----
<references/>
</blockquote>

==It is Java and runs at the server==
The embedded Java code runs at the server so it could access any resource available at the server. For example,

<syntax lang="xml" >
<window title="Property Retrieval" border="normal">
    Enter a property name: <textbox/>
    <button label="Retrieve" onClick="alert(System.getProperty(self.getPreviousSibling().getValue()))"/>
</window>
</syntax>

where <tt>self</tt> is a built-in variable that references the component receiving the event.

If we entered <tt>java.version</tt> and then clicked the button, the result is as follows.

[[Image:DgGettingStartedProperty.png]]

==Identify a component==

A component is a POJO, so you can reference it any way you like. However, ZK provides a convenient way to identify and to retrieve a component: identifier. For example, the above code can be simplified if we named the textbox as <tt>input</tt> by assigning <tt>id</tt> to it, as follows.

<syntax lang="xml" >
<window title="Property Retrieval" border="normal">
    Enter a property name: <textbox id="input"/>
    <button label="Retrieve" onClick="alert(System.getProperty(input.getValue()))"/>
</window>
</syntax>

Once an identifier is assigned, it can be referenced directly in a ZUML page (such as <tt>onClick</tt> in the above example). In pure Java, it can be retrieved by use of <javadoc method="getFellow(java.lang.String)">org.zkoss.zk.ui.Component</javadoc>.

<syntax lang="xml">
Window win = new Window();
win.setId("main"); //assign an identifier
...
Grid grid = (Grid)wnd.getFellow("a_grid"); //retrieve a component by identifier
</syntax>

==A component is a POJO==

A component is a POJO. You could instantiate and manipulate directly. For example, we could generate the result by instantiating a component to represent it, and then append it to another component (an instance of [[ZK_Component_Reference/Layouts/Vlayout|vlayout]]).

<syntax lang="xml" >
<window title="Property Retrieval" border="normal">
    Enter a property name: <textbox id="input"/>
    <button label="Retrieve" onClick="result.appendChild(new Label(System.getProperty(input.getValue())))"/>
    <vlayout id="result"/>
</window>
</syntax>

Similarly you could change the state of a component directly. All modifications will be synchronized back to the client automatically.

<syntax lang="xml" >
<window title="Property Retrieval" border="normal">
    Enter a property name: <textbox id="input"/>
    <button label="Retrieve" onClick="result.setValue(System.getProperty(input.getValue()))"/>
    <separator/>
    <label id="result"/>
</window>
</syntax>

==MVC: Separating code from user interface==

Embedding Java code in a ZUML page is straightforward and easy to read. However, in a production environment, it is usually better to separate the code from the user interfaces. In additions, the compiled Java code runs much faster than the embedded code which is interpreted at the run time.

To separate code from UI, we could implement a Java class (aka., the controller) that implements <javadoc type="interface">org.zkoss.zk.ui.util.Composer</javadoc>, and then handle UI in <javadoc type="interface" method="doAfterCompose(org.zkoss.zk.ui.Component)">org.zkoss.zk.ui.util.Composer</javadoc>. For example, we can redo the previous example by registering an event listener in <javadoc type="interface" method="doAfterCompose(org.zkoss.zk.ui.Component)">org.zkoss.zk.ui.util.Composer</javadoc>, and then retrieve the result and instantiate a label to represent it in the event listener as follows.

<syntax lang="java">
package foo;
import org.zkoss.zk.ui.Component;
import org.zkoss.zk.ui.util.Composer;
import org.zkoss.zk.event.EventListener;
import org.zkoss.zul.Label;

public class PropertyRetriever implements Composer {
    public void doAfterCompose(Component target) { //handle UI here
        target.addEventListener("onClick", new EventListener() { //add a event listener in Java
            public void onEvent(Event event) {
                String prop = System.getProperty(target.getFellow("input").getValue());
                target.getFellow("result").appendChild(new Label(prop));
            }
        });
    }
}
</syntax>

As shown, an event listener could be registered by use of <javadoc type="interface" method="addEventListener(java.lang.String, org.zkoss.zk.ui.event.EventListener)">org.zkoss.zk.ui.Component</javadoc>. An event listener must implement <javadoc type="interface">org.zkoss.zk.ui.event.EventListener</javadoc>, and then handle the event in <javadoc type="interface" method="onEvent(org.zkoss.zk.ui.event.Event">org.zkoss.zk.ui.event.EventListener</javadoc>.

Also notice that a component assigned with an identifier could be retrieved by use of <javadoc type="interface" method="getFellow(java.lang.String)" type="interface">org.zkoss.zk.ui.Component</javadoc>.

Then, we could associate the controller (<tt>foo.PropertyRetriever</tt>) with a component by use of the <tt>apply</tt> attribute as shown below.

<syntax lang="xml">
<window title="Property Retrieval" border="normal">
    Enter a property name: <textbox id="input"/>
    <button label="Retrieve" apply="foo.PropertyRetriever"/>
    <vlayout id="result"/>
</window>
</syntax>

==MVC: Autowiring==

Implementing and registering event listeners is a bit tedious. Thus, ZK provides a feature called autowiring. By extending from <javadoc>org.zkoss.zk.ui.util.GenericForwardComposer</javadoc>, ZK will looks for the members if their names match the identifiers of components. For example, we could rewrite <tt>foo.PropertyRetriever</tt> by utilizing the autowriing as follows.

<syntax lang="java">
package foo;
import org.zkoss.zk.ui.Component;
import org.zkoss.zk.ui.util.GenericForwardComposer;
import org.zkoss.zul.*;

public class PropertyRetriever extends GenericForwardComposer {
    Textbox input;
    Vlayout result;

    public retrieve$onClick(Event event) {
        //handle onClick of the retrieve button
        String prop = System.getProperty(input.getValue());
        result.appendChild(new Label(prop));
    }
}
</syntax>

and the ZUL page is as follows.

<syntax lang="xml">
<window title="Property Retrieval" border="normal" apply="foo.PropertyRetriever">
    Enter a property name: <textbox id="input"/>
    <button label="Retrieve" id="retrieve"/>
    <vlayout id="result"/>
</window>
</syntax>

As shown above, <tt>input</tt> and <tt>result</tt> are automatically assigned such that we could access the real components directly.
Also <tt>retrieve$onClick</tt> indicates an event listener will be registered to the component called <tt>retrieve</tt> to handle the <tt>onClick</tt> event.

<blockquote style="border:1px solid orange">
Notice that MVC is recommended for a production application. On the other hand, to maintain readability, most examples in our documents embed code directly in ZUML pages.
</blockquote>

==Express data with variable resolver and EL expressions==

In a ZUML page, we could locate data with a variable resolver (<javadoc type="interface">org.zkoss.xel.VariableResolver</javadoc>), and then express it with [[ZK_Developer's_Guide/Fundamental_ZK/ZK_User_Interface_Markup_Language/Expression_Language_(EL)|EL expressions]].

For example, assumes we have a class called <tt>foo.User</tt>, and we can retrieve a list of users by its static method called <tt>getAll()</tt>. Then, we can implement a variable resolver as follows.

<syntax lang=" java">
package foo;
public UserResolver implements org.zkoss.xel.VariableResolver {
    public Object resolveVariable(String name) {
        return "users".equals(name) ? Users.getAll(): null;
}
</syntax>

And, we can list all users as follows.

<syntax lang="xml">
<?variable-resolver class="foo.UserResolver"?>
<grid>
    <columns>
        <column label="Name" sort="auto"/>
        <column label="Title" sort="auto"/>
        <column label="Age" sort="auto"/>
    </columns>
    <rows>
        <row forEach="${users}">
            <label value="${each.name}"/>
            <label value="${each.title}"/>
            <label value="${each.age}"/>
        </row>
    </rows>
</grid>
</syntax>

where we assume <tt>foo.User</tt> has three methods: <tt>getName()</tt>, <tt>getTitle()</tt> and <tt>getAge()</tt>. <tt>forEach</tt> is used to instantiate components by iterating through a collection of objects.

[[File:DgGettingStartedUsers.png]]

==Access with Data Binder==

EL expressions are convenient but it is limited to display the read-only data. If we allow the end users to modify data, we could use the data binder to handling the display and modification for us. We need to provide the model (POJO) with proper getter and setter methods (such as <tt>getName()</tt> and <tt>setName(String)</tt>).

First, we declare a initial class <javadoc>org.zkoss.zkplus.databind.AnnotateDataBinderInit</javadoc>. Then, express the data (read-only or writable) with annotation expressions<ref>Annotation expressions can be used by another tools, not limited to the data binder.</ref>.
The annotation expression is similar to EL expressions, exception it starts with <tt>@{</tt>.

<syntax lang="xml">
<?init class="org.zkoss.zkplus.databind.AnnotateDataBinderInit"?>
<?variable-resolver class="foo.VariableResolver"?>
<grid model="@{users}">
    <columns>
        <column label="Name" sort="auto"/>
        <column label="Title" sort="auto"/>
        <column label="Age" sort="auto"/>
    </columns>
    <rows>
        <row self="@{each='user'}">
            <textbox value="@{user.name}"/>
            <textbox value="@{user.title}"/>
            <intbox value="@{user.age}"/>
        </row>
    </rows>
</grid>
</syntax>

where a special annotation expression, <tt>self="@{each='user'}"</tt>, is used to iterate through a collection of users.

[[File:DgGettingStartedUsers2.png]]

Notice that developers need not to handle the display and modification. They only need to prepare a POJO (such as <tt>foo.User</tt>). Any modification made to each input (by the end user) will be stored back to the object (<tt>foo.User</tt>) automatically, assuming the POJO has the required setter methods, such as <tt>setName(String)</tt>.

<blockquote>
----
<references/>
</blockquote>