= How to detect change in active a2dView =

The active <<Dox(a2dView)>> is changed in all cases via a2dView::Activate( bool ).
This function sends events around which you can intercept. 

   * wxEVT_ACTIVATE_VIEW
   * wxEVT_ACTIVATE_VIEW_SENT_FROM_CHILD

See <<Dox(a2dEventDistributer)>> on how to register a class for the intercepting wxEVT_ACTIVATE_VIEW.
The wxEVT_ACTIVATE_VIEW_SENT_FROM_CHILD is sent to the m_display up to the wxWindow contaning the a2dView, and propagates up to the <<Dox(a2dDocumentFrame)>> (wxFrame derived).  You can intercept it at each level of nested windows staring from a2dView::m_display up till the a2dDocumentFrame. This last event does not require to register a class to the <<Dox(a2dEventDistributer)>> first.