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

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

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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#subscribeToApplicationEvent() methods, let's have a quick overview of how this methods works. As the post title said, this method allows to subscribe the given serializable (in 2.1)/un-serializable (in 2.0) callback to the current application that get invoked every time when the given system event type is published in the current application. 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. Let's suppose that we have a custom component that  needs to know when the view root was just added to the view. For this, the custom component can subscribe with a Callback to the PostAddToViewEvent event, as below:

import org.omnifaces.util.Callback;

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

...
//OmniFaces 2.1

// serializable void Callback with argument

subscribeToApplicationEvent(PostAddToViewEvent.class, new Callback.SerializableWithArgument<SystemEvent>() {

 private static final long serialVersionUID = 1L;

 @Override

 public void invoke(SystemEvent event) {

  System.out.println("..: PostAddToViewEvent event emitted by UIViewRoot :.." + event.getSource());

  //do something ...

 }

});

//OmniFaces 2.0

// un-serializable void Callback with argument

subscribeToApplicationEvent(PostAddToViewEvent.class, new Callback.WithArgument<SystemEvent>() {

 @Override

 public void invoke(SystemEvent event) {

  System.out.println("..: PostAddToViewEvent event emitted by UIViewRoot :.." + event.getSource());

  //do something ...

 }

});

// OmniFaces 2.1

// serializable void Callback

subscribeToApplicationEvent(PostAddToViewEvent.class, new Callback.SerializableVoid() {

 @Override

 private static final long serialVersionUID = 1L;

 public void invoke() {

  System.out.println("..: PostAddToViewEvent event emitted by UIViewRoot :..");

  //do something ...

 }

});

// OmniFaces 2.0

// un-serializable void Callback

subscribeToApplicationEvent(PostAddToViewEvent.class, new Callback.Void() {

 @Override

 public void invoke() {

  System.out.println("..: PostAddToViewEvent event emitted by UIViewRoot :..");

  //do something ...

 }

});

...

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