ZK8 Features for MVC - Shadow Elements - Part 1

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DocumentationSmall Talks2016OctoberZK8 Features for MVC - Shadow Elements - Part 1
ZK8 Features for MVC - Shadow Elements - Part 1

Robert Wenzel, Engineer, Potix Corporation
October 6, 2016
ZK 8.0


ZK 8 added several new features mainly improving component control when using the MVVM design pattern. The question I sometimes heard since was: (How) Can our project benefit from these features when using (only) MVC?

To answer the question I'll introduce the most prominent features which are:

Let's have a look where they can be used in the MVC world.

If you are missing a feature, please let us know in the Comments section below.

The text below will contain several links pointing to the technical documentation. Hence another goal for this article is to put these sometimes unrelated articles into a common context rather than being a complete documentation.

Shadow Elements in MVC

  • Available for ZK:
  • http://www.zkoss.org/product/zkhttp://www.zkoss.org/whyzk/zkeeVersion ee.png

Some of the Shadow Elements are more/less beneficial in MVC than others, here's a short summary:

  • <if>
most useful when used with dynamic @load expressions in MVVM
in MVC you can attach/detach components directly (so this is nothing new for MVC)
used statically you can render parts of your UI based on a conditional EL expression <if test="${some.condition}">...</if>
  • <choose>, <when>, <otherwise>
similar to <if>, no real benefit for MVC
  • <forEach>
very useful, this will be the topic in Part 2
  • <apply>
I'll show a few usage scenarios today ... just read on.

Some background

If you already know the details and differences between <include> and Macro component, you can just skip this paragraph.

Why a new shadow element? Let's compare what's been there previously.

For static UI composition you'd classically use an <include> component such as this:

<include src="customerDetails.zul" customerId="123"/>

Or a macro component:

<?component name="my-macro" macroURI="customerDetails.zul"?>
<my-macro customerId="123"/>

These 2 examples will roughly give the same output. Comparing the generated DOM, we'll see that the <include> creates:

<div class="z-include">...</div>

and the Macro generates:

<div class="z-macro">...</div>

So far so good. You might ask: "What's the difference?" "Which is better?" - The answer is often not that trivial and depends on what you need - and sometimes neither option fulfills all requirements.

If you don't need/want the additional <div> (e.g. because it breaks your layout, or you are inside a component hierarchy that only allows certain child components e.g. grid>rows>row) the case seems clear: use an inline macro (which in return has its own technical limitations). Main impact is the root component of the macro being detached from the component tree - losing all its properties/custom-attributes, which you might want to refer to inside the included zul template (this gets additionally painful when using MVVM and dynamic bind annotations - not the topic in this MVC dedicated article). At the same time <include> doesn't have an inline mode.

If you need a different DOM element other than <div> you can use the enclosingTag property of the macro (since 5.0.3) or <include/> since (7.0.4).

Often surprising (even if documented) are the side effects when using an <include> component with a url containing a query string ... while Macros simply ignore the query string. The query string with a changing parameter is often used as a workaround to reload the same included page.

<include src="customerDetails.zul?customerId=123"/>

After adding the url parameter the <include> component changes to "defer" mode resulting in a different component creation life cycle and a nested page. While necessary in certain cases this is usually more than you expect/need if you simply want to compose a page from smaller zul fragments.

Last but not least there is a Template Composition pattern which works again in a very unique way (... let's not go into details right now, we can do the same with <apply> without learning a separate syntax while gaining flexibility).

I assume you see where I am getting ... these grown components/features have diverged over time, causing quite some confusion and discussion about which to use (in which scenario) and how to work around the limitations in various cases.

<apply> for static layout composition

In order to unify the approaches described above and to reduce the unexpected side effects and limitations the <apply> shadow element was introduced in ZK 8. To enable it you have to add the zuti.jar to your project.

Shadow Diagram.PNG

Why "Shadow"? - Because it works behind the scenes (similar to a Shadow-DOM):

  • it represents a position inside the component tree used as an injection point for a <template>
  • it does not affect the parent-child-sibling relations between the real components
  • it does not create any DOM element by itself - it is totally transparent to the client side (no more unwanted <div>)
  • it will even disappear automatically if not needed anymore to save memory (this can be disabled on demand)

Static usage examples

Like an include / macro to insert a zul file (optionally with parameters)

    <apply templateURI="customerDetails.zul" customerId="123"/>

Insert inline templates

    <apply template="${currentUser.hasEditPermission ? 'editable' : 'readonly'}">
        <template name="readonly">
           <label value="${person.name}"/>
        <template name="editable">
           <textbox value="${person.name}"/>

Insert external templates

    <apply template="${currentUser.hasEditPermission ? 'editable' : 'readonly'}">
        <template name="readonly" src="personView.zul"/>
        <template name="editable" src="personEdit.zul"/>

Inject content(s) into a layout template

This is an example of a nested usage, to replace the template composition pattern mentioned above

1. Define a layout template (e.g. pageLayout.zul ) containing any reusable layout structure with nested <apply> elements to inject the actual content where needed.

    <div sclass="wrapper">
        <div sclass="header">
            <apply template="headerContent"/>
        <div sclass="content">
            <apply template="pageContent"/>

2. Use the pageLayout.zul anywhere and define the templates to be injected (inline or external zul file)

    <apply templateURI="pageLayout.zul">
        <template name="headerContent">
            My Application Overview <button label="logout"/>
        <template name="pageContent" src="applicationOverviewPage.zul"/>

This greatly helps to reduce the size of your zul pages, and split them into smaller maintainable fragments.

Named <apply>

As a syntax sugar you can define your own tags in a way very similar to macro components (inline or inside a lang-addon.zul). Internally the <apply> tag is extended adding a default templateURI (also other properties can be predefined).

Inline definition using the component processing-instruction

    <?component name="sidebar" templateURI="/WEB-INF/zul/layout/sidebar.zul"/>
    <?component name="footer" templateURI="/WEB-INF/zul/layout/footer.zul"/>
        <div sclass="wrapper">
            <apply template="someContent.zul" />

Or make the tags globally available via lang-addon.zul (Tip: using this method the zul fragments and configuration can also be packaged into a separate jar file)


Refactor repeated markup

Besides the larger page layout the <apply> element is also useful when refactoring otherwise repeated markup.

Here a simple example of the repeated layout for a customerBox - only differing by customer object and a label. (It doesn't really matter where the customer object is coming from - here I get it from a session attribute - any other EL resolving to this kind of object will work equivalently - the existing variable resolvers will also work here - so nothing really new here). Imagine you have used the same customerBox in various pages and you need to update the layout or add a new field. Search and replace will be difficult, as the fragment is obviously not the same for all usages.

        <div sclass="customerBox">
            <div>Current Customer id: ${sessionScope.currentCustomer.id}</div>
            <div>Current Customer name: ${sessionScope.currentCustomer.name}</div>
        <div sclass="customerBox">
            <div>Last Customer id: ${sessionScope.lastCustomer.id}</div>
            <div>Last Customer name: ${sessionScope.lastCustomer.name}</div>

The repeated layout can be avoided by defining a template (customerBox) and apply the same template using the varying parameters.

        <apply template="customerBox" customer="${sessionScope.currentCustomer}" label="Current Customer"/>
        <apply template="customerBox" customer="${sessionScope.lastCustomer}"  label="Last Customer"/>

    <template name="customerBox">
        <div sclass="customerBox">
            <div>${label} id: ${customer.id}</div>
            <div>${label} name: ${customer.name}</div>

    <!-- alternatively define the template content in a separate file
    <template name="customerBox" src="/WEB-INF/templates/customerBox.zul"/>

As described above you can also apply the templateURI directly and start using the customerBox in multiple pages.

        <apply templateURI="/WEB-INF/templates/customerBox.zul" 
                  customer="${sessionScope.currentCustomer}" label="Current Customer"/>

Finally if that looks too long - define a custom tag inline or in a lang-addon.xml.





resulting zul file

        <customerBox customer="${sessionScope.currentCustomer}" label="Current Customer"/>
        <customerBox customer="${sessionScope.lastCustomer}"  label="Last Customer"/>

Now whenever the customerBox layout needs to change only one template is affected.

Dynamic usage

Of course in your dynamic application parameters or templateURIs need to change from time to time. This means you'll need to control them in your composer.

Remember: To save memory the <apply> shadow element is removed completely from the server side component/shadow tree when used in a purely static way - (i.e. only raw string properties or EL ${...} parameters) The shadow elements are not removed when used with dynamic parameters:

  • in MVVM ZKBIND annotations (@load/@init/@ref) are automatically considered dynamic
  • in MVC the special attribute dynamicValue="true" needs to be added

Examples for a pure static <apply>:

  <apply templateURI="statusBox.zul" status="${c:l('statusBox.OK')}" />
  <apply templateURI="home.zul" />

If we now want to update the status or the templateURI we need to change the property, and re-render the template. To avoid automatic removal and gain access to the <apply> element in a composer we have to indicate to the zul processor we want to keep the shadow element for later updates.

Dynamic <apply> via dynamicValue:

  <apply id="statusBox" dynamicValue="true"
            templateURI="statusBox.zul" status="${c:l('statusBox.OK')}" />
  <apply id="pageContent" dynamicValue="true"
            templateURI="home.zul" />

In our SelectorComposer we can now wire the <apply> elemets by ID using the shadow selector syntax.

  private Apply errorBox;
  private Apply homeContent;

To re-render the content of an <apply> element - re-apply the template (with or without updated properties) - you need to call the recreate() method. Parameters can be set using setDynamicProperty(String, Object).

  public void navigateToSettings() {
    //some additional navigation code
    homeContent.setTemplateURI("settings.zul"); //update the template url
    homeContent.recreate(); //trigger the update - re-renders also when the templateURI was not changed

  public void updateStatus() {
    String newStatus = calculateCurrentStatus();
    statusBox.setDynamicProperty("status", newStatus);
    statusBox.recreate(); //re-apply template with updated status-property

Of course you can update multiple properties and template(URI/name) in any order before finally calling recreate(). This separation between changing the properties and calling recreate() gives your more render control. (It also solves the issue with <include> when trying to re-render the content without changing the include source). Also if you prefer real setters/getters for your dynamic properties you can subclass the Apply class and configure a custom implementation class in the zul file directly or in a lang-addon.xml.

inline in a zul file

  <apply use="my.package.MyExtendedApply" .../>

define as custom tag <my-apply> in lang addon



As shown above the <apply> element has multiple usages inside an MVC application, it doesn't depend on MVVM at all (still it matches the MVVM pattern since it allows view model based control over it's properties). It reduces the HTML markup rendered by avoiding unwanted

elements and it saves memory by destroying itself if used purely static. Besides it allows dynamic control and component re-creation on demand.

This time there's no runnable example code as the code fragments above are meant to illustrate the idea and the reasons when to use either approach. You'll know better which templates you want to <apply> in your application ;)

Let me know your experiences and feedback in the Comments section below.

The next article will show how the <forEach> element is equally useful in an MVC application in reducing the effort to render collection based markup.


Copyright © Potix Corporation. This article is licensed under GNU Free Documentation License.