In Controller

In this section, we will demonstrate how to redirect users to an external site with event listeners in a Controller when they click an item in the sidebar.

MVC Pattern

A well-known design pattern is MVC (Model-View-Controller) which separates an application into 3 roles:

  • Model is responsible for exposing data while performing business logic which is usually implemented by users.
  • View is responsible for displaying data which is what ZUL does.
  • Controller can change the View's presentation and handle events sent from the View.

Such architecture follows SRP (Single Responsibility Principle), and its benefit is that your application is more modularized. Therefore, modifying one part doesn't have to modify another part.

ZK MVC Approach

By following this pattern, ZK traditionally supports MVC approach which controls components by calling their API. Under ZK context, the relationship of 3 roles looks like:

  • ZK SelectorComposer, that implements Composer, plays Controller.
  • ZK UI components plays View.
  • MyServiceClass is not a real class name. It represents any class which is usually implemented by you performs business logic like searching or authentication.


Let's start from making the sidebar work.

I build it with <grid> and 2 columns, a table-looking component.

<grid hflex="1" vflex="1" sclass="sidebar">
        <column width="36px"/>

In real application, the items in the sidebar are usually built dynamically from a database or upon a user's permission. Hence, I also don't put static <row> in the zul, and I will create them dynamically based on data at the server.

When users click a row, they will be brought to the corresponding site.

We implement our sidebar configuration including its name, image icon path, and corresponding URL, etc. We will create each <row> based on SidePage.

public class SidebarPageConfigChapter2Impl implements SidebarPageConfig{

    HashMap<String,SidebarPage> pageMap = new LinkedHashMap<String,SidebarPage>();
    public SidebarPageConfigChapter2Impl(){        
        pageMap.put("fn1",new SidebarPage("zk","ZK","/imgs/site.png",""));
        pageMap.put("fn2",new SidebarPage("demo","ZK Demo","/imgs/demo.png",""));
        pageMap.put("fn3",new SidebarPage("devref","ZK Developer Reference","/imgs/doc.png","'s_Reference"));


Create a Controller

In ZK world, a org.zkoss.zk.ui.util.Composer plays the same role as the Controller, and you can apply it to a target component. Through the composer, you can handle events of the target component (and its children) to change View's presentation according to your requirement. To create a Controller in ZK you simply create a class that inherits

public class SidebarChapter2Controller extends SelectorComposer<Component>{
    //other codes...

Then "associate" the controller with a component in the zul by specifying fully qualified class name in apply attribute. After that the component and all its child components are under the controller's control.


<grid apply="org.zkoss.essentials.chapter2.SidebarChapter2Controller"
    hflex="1" vflex="1" sclass="sidebar">

Wire Components

To control a component, we must get its object reference. In, when you specify a @Wire annotation on a field or setter method, the SelectorComposer will automatically find the component and assign it to the field or pass it into the setter method. By default SelectorComposer locates the component whose ID and type both match the variable name and type respectively in the zul, and @Wire also supports selector syntax to wire.

I can specify a component's ID to get wired by default selector in a controller. chapter2/sidebar.zul

<grid id="sidebar"
public class SidebarChapter2Controller extends SelectorComposer<Component>{

    //wire components
    private Grid sidebar;


SelectorComposer looks for a Grid whose ID is "sidebar" and assign it to the variable sidebar.

Initialize the View

It is very common that we need to initialize components when a zul file is loaded. In our example, we need to create <row> in <grid> for the sidebar, therefore we should override a composer life-cycle method doAfterCompose(Component). The passed argument, comp, is the component that the controller applies to, which in our example is the Grid. ZK will call this method after the applied component, <grid> , and its all child components are created, so we can change components' attributes or even create other components in it.

public class SidebarChapter2Controller extends SelectorComposer<Component>{

    //wire components
    private Grid sidebar;

    private SidebarPageConfig pageConfig = new SidebarPageConfigChapter2Impl();

    public void doAfterCompose(Component comp) throws Exception{

        //initialize view after view construction.
        Rows rows = sidebar.getRows();

        for(SidebarPage page:pageConfig.getPages()){
            Row row = constructSidebarRow(page.getLabel(),page.getIconUri(),page.getUri());
  • Line 8: Here we demonstrate a configurable architecture, the SidebarPageConfig stores hyperlink's configuration such as URL, and label and we use this configuration to create and setup components in the sidebar.
  • Line 12: You have to call super class doAfterCompose() method , because it performs initialization like wiring components for you.
  • Line 15 - 20: These codes involve the concept that we have not talked about yet. All you have to know for now is these codes create <row>s with event listeners and put them into Grid. We will show you the source in the next section.

Events & Event Listeners

A ZK event (org.zkoss.zk.event.Event) is an abstraction of an activity made by user, a notification made by an application, and an invocation of server push. For example, a user clicks a button on the browser, this will trigger onClick sent to the server. If there is an event listener registered for the button's onClick event, ZK will pass the event to the listener to handle it. The event-listener mechanism allows us to handle all user interaction at server side.

Create Components & Event Listeners Programmatically

Manipulating components is the most powerful feature of ZK. You can change the user interface by creating, removing, or changing components and all changes you made will reflect to browsers.

Now we are going to explain how to create components and add an event listener to respond to users' clicking. Basically, there are 3 steps to create a component:

  1. Create a component object.
  2. Setup the component's attributes.
  3. Append to the target parent component.

In constructSidebarRow() method, we create Rows and add an event listener to each of them.

public class SidebarChapter2Controller extends SelectorComposer<Component>{


    private Row constructSidebarRow(String name,String label, String imageSrc, final String locationUri) {

        //construct component and hierarchy
        Row row = new Row();
        Image image = new Image(imageSrc);
        Label lab = new Label(label);


        //set style attribute

        //create and register event listener
        EventListener<Event> actionListener = new SerializableEventListener<Event>() {
            private static final long serialVersionUID = 1L;

            public void onEvent(Event event) throws Exception {
                //redirect current url to new location

        row.addEventListener(Events.ON_CLICK, actionListener);

        return row;

  • Line 8: The first step to create a component is instantiating its class.
  • Line 12: Append a component to establish the parent-child relationship.
  • Line 16: You can change a component's attributes by various setter methods and their method names correspond to tag's attribute name.
  • Line 19: We create an EventListener anonymous class for convenience. Under a clustering environment, your event listener class should implement org.zkoss.zk.ui.event.SerializableEventListener.
  • Line 24: Implement the business logic inonEvent() method, and ZK will call this method when the listened event is sent to the server. Here we get current execution by org.zkoss.zk.ui.Executions and redirect a client to a new URL.
  • Line 28: Add the event listener to a <row> for listening Events.ON_CLICK event triggered by a mouse clicking action.

In Line 8 ~ 16, those codes work equally to writing a zul as follows:

<row sclass="sidebar-fn">

After completing above steps, when a user clicks a <row> on the sidebar, ZK will call a corresponding actionListener then the browser will be redirected to a specified URL. You can see the result via /chapter2/index.zul.