ZK Developer's Reference/UI Composing/Shadow for MVC

Introduction

Since ZK 8.0.0, we have introduced shadow elements like a boilerplate code to help application developers compose html layouts with dynamic data. It is inspired from Shadow DOM to enable better composition of ZK components. For more details, please check out our Official ZK MVVM Book. Shadow elements cannot only be used with MVVM binding but also with MVC pattern; however, there are some differences. We will discuss this more in the following sections.

In MVC pattern, developers can declare shadow tags in zul files, but the behavior is very different without MVVM annotation. For Example,

<apply template="any" />
<template name="any">
    ...
</template>

The shadow element apply will not exist once the output is rendered to client, so developers can't dynamically change the template value. For this purpose, we provide two kinds of Java class for those who favor MVC, that is, ShadowTemplate and CollectionTemplate.

They are NOT like the typical shadow elements defined in zul but components you can only create in Java code.

Use ShadowTemplate

ShadowTemplate is a utility class that allows developers to apply shadow elements in Java class. It has similar behavior to Apply; for example, developers can specify the template or pass parameters. The difference is that developers must designate a boolean value, called autodrop, to indicate whether to drop those rendered children or not. If true, every time the user changes template or detaches from the original host, ShadowTemplate will HtmlShadowElement.recreate() or remove the children; otherwise, the rendered children will remain. After instantiating ShadowTemplate instance, developers can trigger ShadowTemplate.apply(Component) to compose the specified template with shadow host passed as parameter. Note: the passed host should be the same one if autodrop is true, or pass null to detach the original host first.

Example

Assume we have a zul file like this:

<zk>
	<div apply="DemoComposer">
		<div id="host1"></div>
	</div>
	<template name="labels">
		<label value="zul label"/>
		<x:label>xhtml label</x:label>
		<n:span>native span</n:span>
	</template>
</zk>

and in DemoComposer.java

@Wire
Div host1;

public void doAfterCompose(Component comp) throws Exception {
	super.doAfterCompose(comp);
	ShadowTemplate st = new ShadowTemplate(true); //autodrop = true
	st.setTemplate("labels");
	st.apply(host1);
}

In line 6, we instantiate a new ShadowTemplate with autodrop equal to true.

In line 7, assign the template name to st.

In line 8, call apply method and shadow host is Div host1.

Then, we can see template "labels" are rendered and the created components are attached to host1.

If we have a button to change the template,

	@Listen("onClick = #btn")
	public void clickBtn() {
		st.setTemplate("othertemplate");
		st.apply(st.getShadowHost());
	}

Those components rendered before will be detached before new ones are attached. Note: developers have to call apply(host) method again.

If developers wish to apply other shadow host, please apply null first and then reapply like this:

st.apply(null);
st.apply(otherHost);

And the rendered components will also be detached.

Another case is autodrop equal to false. Here, neither changing templates nor applying to other hosts(yes, you can apply whichever hosts you want) will cause rendered components to be detached.

Use CollectionTemplate

CollectionTemplate is similar with ShadowTemplate. Developers can assign ListModel and CollectionTemplateResolver for iteratively rendering.

Example

<zk>
	<div id="root" apply="DemoComposer">
		<div id="host1"></div>
	
		<template name="male">
			<div>
				<label>I'm male, my name is ${each.name}</label>
			</div>
		</template>
		<template name="female">
			<div>
				<label>I'm female, my name is ${each.name}</label>
			</div>
		</template>
	</div>
</zk>

The each in line 7, 12 represents each item in ListModel, and in DemoComposer.java

@Wire
Div host1;
ListModelList<Person> model = new ListModelList<Person>(new ArrayList<Person>() {{
	add(new Person(true));
	add(new Person(false));
	add(new Person(false));
	add(new Person(true));
}});

public void doAfterCompose(Component comp) throws Exception {
	super.doAfterCompose(comp);
	CollectionTemplate ct = new CollectionTemplate(true); //autodrop = true
	ct.setModel(model);
	ct.setTemplateResolver(new MyCollectionTemplateResolver<Person>());
	ct.apply(host1);
}

public class MyCollectionTemplateResolver<E extends Person> implements CollectionTemplateResolver<E> {
	public Template resolve(E o) {
		if (o.getGender())
			return root.getTemplate("male");
		else
			return root.getTemplate("female");
	}
}

public class Person {
	String name = "old name";
	boolean isMale = true;
	.... getter and setter
}

Developers have to prepare a ListModel and assign to the CollectionTemplate instance, then you will see the template is created multiple times. Besides, CollectionTemplate provides not only setTemplate and setTemplateURI but also supports determining template dynamically by giving CollectionTemplateResolver like line 14, so the template will be resolved by evaluating the variable reference from model in runtime. Similarly, either template or model is changed, apply(host) must be triggered to take the effect. The benefit of using CollectionTemplate is that every time the model's content changes the layout will change as well no matter autodrop is true or false.

---

Note that, ShadowTemplate don't support set template and template URI at the same time, any one of them should be null or empty string before set another. CollectionTemplate will only consider the last call to either setTemplate, setTemplateURI or setTemplateResolver.

Comparison

Although the behavior looks alike between ShadowTemplate and Macro component, there exist some differences.

	ShadowTemplate	Macro Component
change host/parent	if autodrop is true, the rendered components will change parent, otherwise, they will still stick with the same parent(or host).	no matter it is inline or not, the rendered components will change parent.
change template/uri	if autodrop is true, the rendered components will be detached, otherwise, they will still stick with the same parent(or host).	no matter it is inline or not, the rendered components will be detached.

In short, ShadowTemplate has more flexibility for templating, developers can render anywhere without losing those rendered components with only one ShadowTemplate instance, while we have to instantiate more than one to achieve this goal using Macro component. Not to mention CollectionTemplate, it can render template iteratively with ListModel which is impossible for Macro component.

Version History

Version	Date	Content