Sunday, March 18, 2012

jQuery Selector Inspired Controller in ZK 6's MVC Pattern

The MVC pattern is adopted pervasively among Web frameworks. Various flavours exist for the pattern but the common goal is to achieve separation of concerns.

Under the ZK framework, MVC implementation ultimately requires the Controller to gain reference to, and listen events coming from, the UI components in View. An interesting bit of ZK 6's MVC pattern is the CSS/jQuery Selector inspired mechanism in its Controller that makes this plumbing task simpler and more flexible.

ZK Primer


In simplicity, ZK is a component based and event driven Ajax Java framework. Understanding this basic description alone is enough to take us through its MVC pattern.

Component based

A component either declared in XML or in Java has its state maintained as a POJO in the JVM. A component is then rendered with a set of JavaScript instructions at client side.


Event Driven

Each UI component can listen to event(s). There's a variety of ways to register event listeners to the components.
Just to name a few here,
in XML:
<button onclick="...">
</button>
in Java:
Button btn = new Button();
btn.addEventListener("onClick", new EventListener(){...});       

MVC Nomenclature in ZK

The component declarations make up the View. Although it's possible to construct the UI in Java, akin to GWT or Swing, most would prefer writing mark-up in XML. A ZUL in ZK is an XML compliant page that contains the UI mark-up. One can consider a ZUL page as an Ajax enabled JSP page.

The Controller in ZK is a Java class that implements the Composer interface or extends any one of Composer's implementations. SelectorComposer is the target of our investigation in this post.

Selector Inspired Controller in Action


Consider a simple window that prompts a user to enter her name, email address, and select the journal she'd like to subscribe:

We'll examine how the server-side selector mechanism works in our controller class as we implement the following features:
  1. Displaying a list of the available journals for subscription in a combo box
  2. Clear all fields in this simple form when the "Clear" button is clicked

Let's first see the components in mark-up which our controller will work with:
<grid apply="lab.zkoss.mvc.SubscriptionComposer">
        ...
    <rows>
        <row>
            <label value="User Name"/>
            <textbox mold="rounded"/> 
        </row>
        <row>
            <label value="email"/>
            <textbox mold="rounded"/> 
        </row>
        <row>
            <cell ...>
                <label value="Please subscribe me to "></label>
                <combobox model="${journalListModel}">
                    <template name="model">
                       <comboitem label="${each.title}"/>
                    </template>
                </combobox>
            </cell>
        </row>
        <row>
            <cell ...>
                <button label="Clear" ..."/>
                <button label="Submit" ..."/>
            </cell>
        </row>
        </rows>
</grid>

In our controller class, to implement the said features under the ZK framework, we'd need to gain reference to the UI components so the list of available journals can be rendered in the combo box and the onClick event for the Clear button can be handled.

public class SubscriptionComposer extends SelectorComposer{

 @Wire("combobox")
 Combobox journalbox;

 @Wire("textbox, combobox")
 List<InputElement> inputs;
 
 
 public void doAfterCompose(Component comp) throws Exception{ 
  super.doAfterCompose(comp);
  JournalDAO jdao = new JournalDAO();
  List<Journal> journalList = jdao.findAll();
  ListModelList journalListModel = new ListModelList(journalList);
  journalbox.setModel(journalListModel);
  
 }
 
 @Listen("onClick = button[label='Clear']")
 public void clearAll(){
  for(InputElement i:inputs) i.setText("");
 }
}
Let's elaborate on how selectors are used.

The Controller's Scope

When a controller is "applied" to a component, all of the component's children components also become accessible to the controller. 

In our implementation, the grid component is applied with our controller:
<grid apply="lab.zkoss.mvc.SubscriptionComposer">
...
</grid>
Hence the grid and all of its children components define the scope which our controller SubscriptionComposer can take effect.

Component Wiring

The @Wire annotation on line 3 and 6 take in a CSS selector pattern as its parameter. With the pattern "combobox, the annotation associates the sole combo box in our UI mark-up with the Combobox instance, journalbox declared in the controller class.
One of the many alternatives to achieve the same exact wiring is to give the Combox an ID, for instance:

<combobox id="thisworks2">
...
</combobox>
and the parameter for the annotation would be:
@Wire("#thisworks2")
Combobox journalbox;

Component Initialization

Once we've obtained references to the UI components in View, we could initialize them accordingly.
The doAfterCompose method allows developers to insert instructions to be executed right after the component under effect and its children are created. It's a method invoked by the framework and must be implemented for all classes implementing the Composer interface; such as the SelectorComposer class which we're extending our SubscriptionComposer from.

For our hypothetical feature, we need to initialize our combo box by populating it with a list of journals available for subscription.
@Wire("combobox")
 Combobox journalbox;

 ...
 
 public void doAfterCompose(Component comp) throws Exception{ 
  super.doAfterCompose(comp);
  JournalDAO jdao = new JournalDAO();
  List<Journal> journalList = jdao.findAll();
  ListModelList<Journal> journalListModel = new ListModelList(journalList);
  journalbox.setModel(journalListModel);
  
 }

On line 11, the combo box which we obtained reference to via the selector mechanism, is given the model data we prepared on line 10. ListModelList is a wraper class that enables changes made in its wrapped collection to be updated on its host UI component accordingly.

<combobox model="${journalListModel}">
        <template name="model">
            <comboitem label="${each.title}"/>
        </template>
    </combobox>

Once the combo box is supplied with a list model, the template tag will iteratively create a combo item for each entry in the list model.

Event Listening

The @Listen annotation adds method clearAll, as an event listener for the onClick event, to the button matching the pattern button[label="Clear"].

@Wire("textbox, combobox")
 List<InputElement> inputs;
 
 @Listen("onClick = button[label='Clear']")
 public void clearAll(){
  for(InputElement i:inputs) i.setText("");
 }


As before, there're many alternatives to the selector pattern shown here. One possibility is cell:first-child button, since the "Clear" button is the only component that matches this pattern.

<cell ...>
        <button label="Clear" ..."/>
        <button label="Submit" ..."/>
    </cell>

The brevity of the clearAll method is made possible because a single annotation @Wire("textbox, combobox") in fact wired all fields in the UI to a list of ZK components.

In a Nutshell


In a typical ZK controller class implementation, before we can initialize a UI component, listen to its events, or change its state, we must first obtain a reference to that component.
This CSS/jQuery selector inspired controller gives us great flexibility in referencing the UI components of interest. A reference can be made by matching a component's ID, class name, component attributes, or by traversing through the component tree.
With this flexibility, changes made in the UI cause us minimum grief since the controller code can be updated as easily as coming up with new selector patterns.

References

ZK SmallTalks
ZK Developer's Reference

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