[OmniFaces utilities 2.1] Subscribe the given callback instance to the given component (request scoped event listener) and unsubscribe the given event listener on the given event from the given component

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[OmniFaces utilities] The subscribeToRequestComponentEvent() method subscribes the given callback instance to the given component that get invoked only in the current request when the given component system event type is published on the given component. The difference with UIComponent#subscribeToEvent(Class, ComponentSystemEventListener) is that this listener is request scoped instead of view scoped as component system event listeners are by default saved in JSF state and thus inherently view scoped.

[OmniFaces utilities] The unsubscribeFromComponentEvent() method unsubscribes the given event listener on the given event from the given component. Normally, you would use UIComponent#unsubscribeFromEvent(Class, ComponentSystemEventListener) for this, but this wouldn't work when executed inside ComponentSystemEventListener#processEvent(javax.faces.event.ComponentSystemEvent), as it would otherwise end up in a ConcurrentModificationException while JSF is iterating over all system event listeners. The trick is to perform the unsubscribe during the after phase of the current request phase #subscribeToRequestAfterPhase(PhaseId, org.omnifaces.util.Callback.Void).

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Methods:

- subscribe the given callback instance to the given component (request scoped event listener)

- unsubscribe the given event listener on the given event from the given component

See also: Events#subscribeToRequestAfterPhase

Note: If the flow is in Render Response phase then there is too late to unsubscribe.

Usage:

Let's suppose that we are in the apply() method of a tag handler and we want to listen (via a callback) the PostValidateEvent emitted by the parent component, without saving this component system event listener in the JSF state. We can easily accomplish this via Events#subscribeToRequestComponentEvent():

@Override

public void apply(FaceletContext context, final UIComponent parent) throws IOException {

 ...

 subscribeToRequestComponentEvent(parent, PostValidateEvent.class, new Callback.WithArgument<ComponentSystemEvent>() {

  @Override

  public void invoke(ComponentSystemEvent event) {

   processTheEvent(event);

  }

 });

}

protected void processTheEvent (ComponentSystemEvent event) {

 UIComponent component = event.getComponent();

 // do something here

 ...

}

Or, maybe you need to use the UIComponent#subscribeToEvent(Class, ComponentSystemEventListener) as below:

@FacesComponent(value = MyComponent.COMPONENT_TYPE, createTag = true)

public class MyComponent extends UIComponentBase implements ComponentSystemEventListener {

 public static final String COMPONENT_FAMILY = "...";

 public static final String COMPONENT_TYPE = "...";

 @PostConstruct

 public void mySubscribeToEvent() {

  subscribeToEvent(PreValidateEvent.class, this);

 }

 @Override

 public void processEvent(ComponentSystemEvent event) throws AbortProcessingException {

  // do something here

 }
 ...
}

Now, in order to unsubscribe from the PreValidateEvent and avoid saving in JSF state this component system event listener, you can explicitly invoke the Events#unsubscribeFromComponentEvent(), as below:

@Override

 public void processEvent(ComponentSystemEvent event) throws AbortProcessingException {

  Events.unsubscribeFromComponentEvent(event.getComponent(), event.getClass(), this);

  // do something here

 }

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

                         JSF-Workingwith @ListenerFor, ComponentSystemEventListener and SystemEventListener

API GH