Notification handling

TStandardCanvasView overrides TCanvasView::RegisterNotificationInterest to register interest in changed canvas graphics, changed selections, and drastic change events. 

      virtual void HandleGraphicsChanged( const TNotification& );
  virtual void HandleSelectionChanged( const TNotification& );
  virtual void HandleDrasticChange( const TNotification& );
A graphics-changed notification occurs when canvas graphics in the model have changed. This includes when graphics have been added to, or deleted from the model.

A selection-changed notification occurs when the current selection has changed in the presenter state.

A drastic change notification normally does not occur. Implement your TStandardCanvasView derived class to broadcast drastic change notification when you want the canvas view to be completely refreshed by overriding the RegisterNotification function.

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