JSF-Working with @ListenerFor, ComponentSystemEventListener and SystemEventListener-part III

Check first:
JSF-Working with @ListenerFor, ComponentSystemEventListener and SystemEventListener - part I
JSF-Working with @ListenerFor, ComponentSystemEventListener and SystemEventListener - part II

Sometimes you need to install the listener instance referenced by argument listener into the UIViewRoot as a listener for events of type systemEventClass, as in the case of PreRenderViewEvent.

Below you can see the UIViewRoot.subscribeToViewEvent() provided by JSF and some alternatives provided by OmniFaces 2.1 and 2.0:

In such cases, you cannot use @ListenerFor, do not try: @ListenerFor(systemEventClass = PreRenderViewEvent.class), consider this as another bad practice!!!. You need the UIViewRoot.subscribeToViewEvent() method, as below:

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

public class TomComponent extends UIComponentBase implements SystemEventListener {

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

 public static final String COMPONENT_TYPE = "jsf.uicomponentwithsubscribetoevent.TomComponent";

 @PostConstruct

 public void tomSubscribeToViewEvent() {

  FacesContext.getCurrentInstance().getViewRoot().

      subscribeToViewEvent(PreRenderViewEvent.class, this);

 }

 @Override

 public void processEvent(SystemEvent event) throws AbortProcessingException {

  System.out.println("EVENT EMITTED: " + event);       

 }

 @Override

 public void encodeEnd(FacesContext context) throws IOException {

  ResponseWriter responseWriter = context.getResponseWriter();

  responseWriter.write("I'm Tom the cat!");

 }

 @Override

 public String getFamily() {

  return COMPONENT_FAMILY;

 }

 @Override

  public boolean isListenerForSource(Object source) {

   return true;

 }

}

Sometimes, you need to combine an event like PostAddToViewEvent with PreRenderViewEvent. Per example, OmniFaces does this in the OnloadScript component. The next example is inspired by the source code of OnloadScript component:

@ListenerFor and UIViewRoot.subscribeToViewEvent()  used with an UIComponent that implements the ComponentSystemEventListener and SystemEventListener interfaces

§  the UIComponent is annotated with @ListenerFor and listen the PostAddToViewEvent event emitted by instances of this UIComponent

§  if the request (initial/postback) is an AJAX request then the component register as a listener for the PreRenderViewEvent emitted by the UIViewRoot.

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

@ListenerFor(systemEventClass = PostAddToViewEvent.class)

public class TomComponent extends UIComponentBase implements

           ComponentSystemEventListener, SystemEventListener {

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

 public static final String COMPONENT_TYPE = "jsf.uicomponentwithsubscribetoevent.TomComponent";

 @Override

 public void processEvent(ComponentSystemEvent event) throws AbortProcessingException {

  System.out.println("EVENT EMITTED (ComponentSystemEvent): " + event);

  FacesContext context = FacesContext.getCurrentInstance();

  boolean ajaxRequest = context.getPartialViewContext().isAjaxRequest();

  if (ajaxRequest) {

      context.getViewRoot().subscribeToViewEvent(PreRenderViewEvent.class, this);

  }

 }

 @Override

 public void processEvent(SystemEvent event) throws AbortProcessingException {

  System.out.println("EVENT EMITTED (SystemEvent): " + event);

 }

 @Override

 public void encodeEnd(FacesContext context) throws IOException {

  ResponseWriter responseWriter = context.getResponseWriter();

  responseWriter.write("I'm Tom the cat!");

 }

 @Override

 public String getFamily() {

  return COMPONENT_FAMILY;

 }

 @Override

 public boolean isListenerForSource(Object source) {

  return source instanceof TomComponent || source instanceof UIViewRoot;

 }

}

Output at initial request (AJAX reqest):

EVENT EMITTED (ComponentSystemEvent): javax.faces.event.PostAddToViewEvent[source=jsf.listenerforanduicomponent.TomComponent]

Output at postback (AJAX request):

EVENT EMITTED (ComponentSystemEvent): javax.faces.event.PostAddToViewEvent[source=jsf.listenerforanduicomponent.TomComponent]

EVENT EMITTED (ComponentSystemEvent): javax.faces.event.PostRestoreStateEvent[source=jsf.listenerforanduicomponent.TomComponent]

EVENT EMITTED (SystemEvent): javax.faces.event.PreRenderViewEvent[source=javax.faces.component.UIViewRoot]

System events can be configured in faces-config.xml also. If you need to register as listener for a built-in JSF component then this can be the right approach. Per example, we can write a system event for looping through the CSS resources and remove some of them (or, all of them). The code of our listener is pretty simple, as you can see in the following code:

public class ResourcesListener implements SystemEventListener {

 @Override

 public void processEvent(SystemEvent event) throws AbortProcessingException {

  FacesContext context = FacesContext.getCurrentInstance();

     

  int i = context.getViewRoot().getComponentResources(context, "HEAD").size() - 1;

  while (i >= 0) {

   UIComponent resource = context.getViewRoot().getComponentResources(context, "HEAD").get(i);

   String resourceLibrary = (String) resource.getAttributes().get("library");

   String resourceName = (String) resource.getAttributes().get("name");

   if ((resourceLibrary.equals("default")) && (resourceName.equals("css/jerry.css"))) {

        context.getViewRoot().removeComponentResource(context, resource, "HEAD");

   }

   i--;

  }

 }

 @Override

 public boolean isListenerForSource(Object source) {

  return (source instanceof UIViewRoot);

 }

}

The listener should be configured in faces-config.xml, as follows (the tags names reveals their goal):

...

<application>       

 <system-event-listener>

   <system-event-listener-class>

     tom.jerry.ResourcesListener

   </system-event-listener-class>

   <system-event-class>

     javax.faces.event.PreRenderViewEvent

   </system-event-class>

   <source-class>

     javax.faces.component.UIViewRoot

   </source-class>

 </system-event-listener>

</application>

...

So, even if initially we write the following code:

...

<h:head>

 <h:outputStylesheet library="default" name="css/tom.css"/>

 <h:outputStylesheet library="default" name="css/jerry.css"/>

</h:head>

...

JSF will render the following code:

...

<head>

 <link type="text/css" rel="stylesheet"  

       href="/tomandjerry/faces/javax.faces.resource/css/tom.css?ln=default" />

</head>

...

Note: Programmatically, you can unsubscribe from an event using the UIComponent/Application.unsubscribeFromEvent() methods:

//UIComponent

public void unsubscribeFromEvent(java.lang.Class<? extends SystemEvent> eventClass,

                                 ComponentSystemEventListener componentListener)

//Application

public void unsubscribeFromEvent(java.lang.Class<? extends SystemEvent> systemEventClass,

                                 java.lang.Class<?> sourceClass,

                                 SystemEventListener listener)

public void unsubscribeFromEvent(java.lang.Class<? extends SystemEvent> systemEventClass,

                                 SystemEventListener listener)

Useful links: JSF ZEEF

                  OmniFaces Custom Components Approach