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vineri, 22 ianuarie 2016

Combining @ListenerFor with SystemEventListener

Conforming to documentation, "if the class to which @ListenerFor annotation is attached implements SystemEventListener and does not implement ComponentSystemEventListener, "target" is the Application instance".

Based on this affirmation, it is possible to believe that the below example will work and will register the TomComponent as a listener for all emitters capable to emit the PostAddToViewEvent event (not just instances of TomComponent):

@FacesComponent(value = TomComponent.COMPONENT_TYPE, createTag = true)
@ListenerFor(systemEventClass = PostAddToViewEvent.class)
public class TomComponent extends UIComponentBase implements SystemEventListener {

 public static final String COMPONENT_FAMILY = "jsf.component";
 public static final String COMPONENT_TYPE = "jsf.component.TomComponent";

 @Override
 public void processEvent(SystemEvent event) throws AbortProcessingException {
  System.out.println("EVENT EMITTED: " + event);
 }

 @Override
 public String getFamily() {
   return COMPONENT_FAMILY;
 }  

 @Override
 public boolean isListenerForSource(Object source) {
  System.out.println("EVENT SOURCE: " + source);
 return true;
 } 
 ...
}

But, remember that an custom component implements the ComponentSystemEventListener interface by inheritance from UIComponent! So, the output of the above case will be:

EVENT SOURCE:
jsf.component.TomComponent

And the processEvent(SystemEvent event) is not called! Since ComponentSystemEventListener is inherited (so implemented), and we have the @ListenerFor, the TomComponent will be registered as the listener for PostAddToViewEvent event emitted only by instances of TomComponent. But, as you notice from the above output, the flow passes through the isListenerForSource(Object source), which means that, in this particular case, we need to return true;, or return source instanceof TomComponent;, otherwise we will block the call of the processEvent(ComponentSystemEventListener event) method.
So, combining @ListenerFor with SystemEventListener and UIComponents is not a good thing, only if you really know what you are doing.

Well, the things changes in case of combining the @ListenerFor with SystemEventListener  and Renderer. Since Renderer doesn't implement the ComponentSystemEventListener, we are in the case from the documentation quoted above. Now, the reasoning is going further and "If "target" is the Application instance, inspect the value of the sourceClass() annotation attribute value. If the value is Void.class, call Application.subscribeToEvent(Class, SystemEventListener), passing the value of systemEventClass() as the first argument and the instance of the class to which this annotation is attached (which must implement SystemEventListener) as the second argument".

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