[OmniFaces utilities (2.1/2.0)] Subscribe to view system events using serializable/un-serializable Callbacks

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[OmniFaces utilities] The subscribeToViewEvent() methods subscribes the given callback (serializable (in 2.1) /un-serializable (in 2.0) void without arguments/void with one argument) to the current view that get invoked every time when the given system event type is published on the current view.

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Method:

- using OmniFaces 2.1 serializable callback

- using OmniFaces 2.0 un-serializable callback

Note By OmniFaces implementation, the listened emitter of system events is the UIViewRoot, so use only system events types that can be emitted by the component tree's view root. OmniFaces relies on its org.omnifaces.eventlistener.DefaultViewEventListener, listed below:

public abstract class DefaultViewEventListener implements SystemEventListener {

 @Override

 public void processEvent(SystemEvent event) throws AbortProcessingException {

  // NOOP

 }

 @Override

 public boolean isListenerForSource(Object source) {

  return source instanceof UIViewRoot;

 }

}

Usage:

Before calling the Events#subscribeToViewEvent() methods, let's have a quick overview of how this methods works. As the post title said, this method allows to subscribe the given callback to the current view that get invoked every time when the given system event type is published on the current view. So, we pass to this method the system event type to be observed, and one of the following OmniFaces Callbacks (this becomes the listener):

- using OmniFaces 2.1 serializable callback

- using OmniFaces 2.0 un-serializable callback

Basically, all we need to do is to subscribe to the desired event(s) with a Callback instance and implement a behavior to the corresponding invoke() method. Behind the scene, OmniFaces takes the provided Callback and creates a custom system event listener for it. When the specified event(s) occurs, OmniFaces will call our invoke() implementation. Knowing this, we can create a simple example:

JSF Page:

<h:form>

 <o:outputLabel for="tomId" value="Tom Enemy:"/>

 <h:inputText id="tomId" value="#{tomBean.enemy}"/>

 <h:commandButton value="Get Advice"/>

</h:form>

<h:outputText rendered="#{facesContext.postback}" value="#{tomBean.advice}"/>

TomBean

import org.omnifaces.util.Callback;

import static org.omnifaces.util.Events.subscribeToViewEvent;

...

@Named

@RequestScoped

public class TomBean {

 private String enemy;

 private String advice;

 @PostConstruct

 public void adviceForTom() {

       

  //the enemy value is not available in @PostConstruct, but we can

  //subscribe with a callback to PreRenderViewEvent, when the enemy is available

  //OmniFaces 2.1
  // serializable void Callback

  subscribeToViewEvent(PreRenderViewEvent.class, new Callback.SerializableVoid() {

   private static final long serialVersionUID = 1L;

   @Override

   public void invoke() {

    if (enemy != null) {

        if (enemy.equals("jerry")) {

            advice = "Tom, watch the dog while you are chasing Jerry!";

        } else if (enemy.equals("dog")) {

            advice = "Tom, be careful to Jerry traps!";

        } else {

            advice = "Tom, you have a new enemy ?!!";

        }

    }

   }

  });

  //OmniFaces 2.0
  // un-serializable void Callback

  subscribeToViewEvent(PreRenderViewEvent.class, new Callback.Void() {

   @Override

   public void invoke() {

    if (enemy != null) {

        if (enemy.equals("jerry")) {

            advice = "Tom, watch the dog while you are chasing Jerry!";

        } else if (enemy.equals("dog")) {

            advice = "Tom, be careful to Jerry traps!";

        } else {

            advice = "Tom, you have a new enemy ?!!";

        }

    }

   }

  });

  //OmniFaces 2.1

  // serializable void Callback with argument

  subscribeToViewEvent(PreRenderViewEvent.class, new Callback.SerializableWithArgument<SystemEvent>() {

   private static final long serialVersionUID = 1L;

   @Override

   public void invoke(SystemEvent event) {

    if (enemy != null) {

        if (enemy.equals("jerry")) {

            advice = "Tom, watch the dog while you are chasing Jerry!";

        } else if (enemy.equals("dog")) {

            advice = "Tom, be careful to Jerry traps!";

        } else {

            advice = "Tom, you have a new enemy ?!!";

        }

    }

   }

  });

  

  //OmniFaces 2.0

  // un-serializable void Callback with argument

  subscribeToViewEvent(PreRenderViewEvent.class, new Callback.WithArgument<SystemEvent>() {  

   @Override

   public void invoke(SystemEvent event) {

    if (enemy != null) {

        if (enemy.equals("jerry")) {

            advice = "Tom, watch the dog while you are chasing Jerry!";

        } else if (enemy.equals("dog")) {

            advice = "Tom, be careful to Jerry traps!";

        } else {

            advice = "Tom, you have a new enemy ?!!";

        }

    }

   }

  });

  ...

When Tom submits the form and the application flow reaches in the @PostConstruct method, the enemy value was not set yet. But, we register a callback as a listener for the PreRenderViewEvent which means that right before the UIViewRoot is about to be rendered the invoke() method is called, and the enemy value is set and can be used to choose the right advice for Tom. Nice!

Further reading: Subscribe With OmniFaces 2.0/2.1 Callback-listeners to JSF System Events

                         JSF-Workingwith @ListenerFor, ComponentSystemEventListener and SystemEventListener

Complete code on GitHub

API 2.1   GH 2.1

API 2.0   GH 2.0