Invalidating the selection
As the command iterates over the selection changing the canvas graphics, it should keep track of the area of the canvas that encloses all of the changed canvas graphics. This area is an instance of TGArea that is passed to the MCanvasSelection::Invalidate function when the command completes. The Invalidate function invalidates the canvas graphics that were changed to notify the application when a command changes any or all of the selection contents.
virtual void Invalidate( const TGArea& ) = 0;
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