Event Veto

Event Veto
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In this section, you will learn how programmatically veto events.

To veto an event means a listener is able to stop or cancel an event or rather the action which created it. A special listener interface VetoableGraphListener defines an object that is able to receive graph events and subsequently call a veto.

In order to veto an event, a veto listener must throw a new instance of class GraphEventVetoException. A VetoableGraphListener implementation should never process an event like a normal GraphListener does.

Example 8.2. Event Veto Source Listing

void registerVetoListeners()
{
  // register for graph model events
  graphModel.addVetoableGraphListener(new VetoableGraphListener()
  {
   public void vetoableNodePreRemove(GraphEvent graphevent) 
     throws GraphEventVetoException
   {
     logger.logVetoableGraphListener(graphevent);
     
     // now give it a veto
     throw new GraphEventVetoException(
       "Event work off stopped", graphevent);
   }

   public void vetoableEdgePreRemove(GraphEvent graphevent) 
     throws GraphEventVetoException
   {
     logger.logVetoableGraphListener(graphevent);
   }

   public void vetoableEdgePreSplit(GraphEvent graphevent) 
     throws GraphEventVetoException
   {
     logger.logVetoableGraphListener(graphevent);
   }

   public void vetoableNodePreAdd(GraphEvent graphevent) 
     throws GraphEventVetoException
   {
     logger.logVetoableGraphListener(graphevent);
   }

   public void vetoableEdgePreAdd(GraphEvent graphevent) 
     throws GraphEventVetoException
   {
     logger.logVetoableGraphListener(graphevent);
   }

   public void vetoableNodePreDeleteEdgeCascade(GraphEvent graphevent) 
     throws GraphEventVetoException
   {
     logger.logVetoableGraphListener(graphevent);
   }
  });
  
  // it's the same for the view ...
}

The above implementation of a new VetoableGraphListener instance shows the various methods that can throw a veto exception, thereby canceling the atomic operation taking place in the model.

Since there are also some atomic operations that can be made upon the graph view you can find a corresponding interface called VetoableVisualGraphListener. An implementation of this interface is registered at a VisualGraphView. It should work in the same way as the model counterpart does except the it throws a VisualGraphEventVetoException.

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