On-demand Evaluation

By default, ZK creates components based on what are defined in a ZUML document, when loading the document. However, we can defer the creation of some sections of components, until necessary, such as becoming visible. This technique is called load-on-demand or render-on-demand.

For example, you could split a ZUML document into multiple, and then load the required ones when necessary. Please refer to the Load ZUML in Java section for how to load a ZUML document dynamically.

It improves the performance in both the server and client sides. It is suggested to apply this technique when appropriate. In additions, ZK Loader provides a standard on-demand evaluation called fulfill to simplify the implementation as described in the following section.

Load-on-Demand with the fulfill attribute

The simplest way to defer the creation of the child components is to use the fulfill attribute. For example, comboitem in the following code snippet will not be created, until combobox receives the onOpen event, indicating comboitem is becoming visible.

<combobox fulfill="onOpen">
    <comboitem label="First Option"/>    
</combobox>

In other words, if a XML element is specified with the fulfill attribute, all of its child elements won't be processed until the event specified as the value of the fulfill attribute is received.

Specify Target with its ID

If the event to trigger the creation of children is targeted to another component, you can specify the target component's identifier in front of the event name as depicted below.

<button id="btn" label="show" onClick="content.visible = true"/>
<div id="content" fulfill="btn.onClick">
    Any content created automatically when btn is clicked    
</div>

Specify Target with its Path

If the components belong to different ID space, you can specify a path before the event name as follows.

<button id="btn" label="show" onClick="content.visible = true"/>
<window id="content" fulfill="../btn.onClick">
    Any content created automatically when btn is clicked    
</window>

Specify Target with EL Expressions

EL expressions are allowed to specify the target, and it must return a component, an identifier or a path.

<div fulfill="${foo}.onClick">
...
</div>

Specify Multiple Fulfill Conditions

If there are multiple conditions to fulfill, you could specify all of them in the fulfill attribute by separating them with a comma, such as

<div fulfill="b1.onClick, ${another}.onOpen">
...
</div>

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