View source for Eclipse4/RCP/Modeled UI/Listening to Model Changes

When the model changes and it is not a no-op (for example, setting its label to its current label), an event will be dispatched to all interested event handlers. Eclipse 4 uses a [[Eclipse4/RCP/Event Model|global listener pattern]] so all handler subscriptions are done through the event broker.

==Subscribing to model changes==
Topics for event subscriptions can be generated from the <tt>UIEvents</tt> class.

<source lang="java">
// subscribe to all events generated from MUILabels
eventBroker.subscribe(UIEvents.buildTopic(UIEvents.UILabel.TOPIC),
    labelEventHandler);

// subscribe only to events generated from MItems's selected attribute being switched
eventBroker.subscribe(UIEvents.buildTopic(UIEvents.Item.TOPIC, UIEvents,Item.SELECTED),
    itemSelectionEventHandler);
</source>

==Retrieving information from the events==
<source lang="java">
// subscribe to events generated from MUILabels having their labels changed
eventBroker.subscribe(UIEvents.buildTopic(UIEvents.UILabel.TOPIC, UIEvents.UILabel.LABEL),
    new EventHandler() {
      public void handleEvent(Event event) {
        Object element = event.getProperty(UIEvents.EventTags.ELEMENT);
        if (element instanceof MToolItem) {
          ToolItem item = (ToolItem) ((MToolItem) element).getWidget();
          String value = (String) event.getProperty(UIEvents.EventTags.NEW_VALUE);
          if (value == null) {
            item.setText(""); //$NON-NLS-1$
          } else {
            item.setText(value);
          }
        }
      }
    }
);
</source>