Basic Rules
If you are not familiar with XML, please take a look at XML Background first.
A XML Element Represents a Component
Each XML element represents a component, except special elements like <zk> and <attribute>. Thus, the following will cause three components (window, textbox and button) being created when ZK Loader processes it.
<window>
<textbox/>
<button/>
</window>
In additions, the parent-child relationship of the created components will follow the same hierarchical structure of the XML document. In the previous example, window will be the parent of textbox and button, while textbox is the first child and button is the second.
Special XML Elements
There are a few elements dedicated to special functionality rather than a component. For example,
<zk>...</zk>
The zk element is a special element used to aggregate other components. Unlike a real component (say, hbox or div), it is not part of the component tree being created. In other words, it doesn't represent any component. For example,
<window>
<zk if="${whatever}">
<textbox/>
<textbox/>
</zk>
</window>
is equivalent to
<window>
<textbox if="${whatever}"/>
<textbox if="${whatever}"/>
</window>
For more information about special elements, please refer to ZUML Reference.
A XML Attribute Assigns a Value to a Component's Property or Event Listener
Each attribute, except special attributes like if and forEach, represents a value that shall be assigned to a property of a component after it is created. The attribute name is the property name, while the attribute value is the value to assign. For example, the following assigns "Hello" to window's title property. More precisely, Window.setTitle(String) will be called with "Hello".
<window title="Hello"/>
Like JSP, you could use EL as the value of any attribute. For example, the following assigns the value of the request parameter called name to window's title.
<window title="${param.name}"/>
For more information about EL expressions, please refer to ZUML Reference.
Assign Event Listener if Name Starts With on
If the attribute name starts with on and the third letter is uppercase, an event listener is assigned. For example, we can register an event listener to handle the onClick event as follows.
<button onClick="do_something_in_Java())"/>
The attribute value must be a valid Java code, and it will be interpreted[1] when the event is received. You could specify different languages by prefixing the language name. For example, we could write the event listener in Groovy as follows.
<vlayout onClick="groovy:self.appendChild(new Label('New'));">
Click me!
</vlayout>
Special Attributes
There are a few special attributes dedicated to special functionality rather than assigning properties or handling events. For example, the forEach attribute is used to specify a collection of object such that the XML element it belongs will be evaluated repeatedly for each object of the collection.
<listbox>
<listitem forEach="${customers}" label="${each.name}"/>
</listbox>
For more information about special attributes, please refer to the forEach, if and unless section, and ZUML Reference
A XML Text Represents Label Component or Property's Value
In general, a XML text is interpreted as a label component. For example,
<window>
Begin ${foo.whatever}
</window>
is equivalent to
<window>
<label value="Begin ${foo.whatever}"/>
</window>
A XML Text as Property's Value
Depending on component's implementation, the text nested in a XML element could be interpreted as the value of a component's particular property. For example, Html is one of this kind of components, and
<html>Begin ${foo.whatever}</html>
is equivalent to
<html content="Begin ${foo.whatever}"/>
It is designed to make it easy to specify multiple-line value, so it is usually used by particular components that requires the multiple-lines value. For a complete list of components that interprets the XML text as a property's value, please refer to ZUML Reference.
A XML Processing Instruction Specifies the Page-wide Information
Each XML processing instruction specifies the instruction how to process the XML document. It is called directives in ZK. For example, the following specifies the page title and style.
<?page title="Grey background" style="background: grey"?>
Notice that there shall be no whitespace between the question mark and the processing instruction's name (i.e., page in the above example).
The other directives include the declaration of components, the class to initialize a page, the variable resolver for EL expressions, and so on. For more information about directives, please refer to ZUML Reference.
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