An interface to be executed when an external event is raised. 
   Namespace:   Autodesk.Revit.UI  
  Assembly:   RevitAPIUI  (in RevitAPIUI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2013 
Syntax
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Remarks
 An instance of a class implementing this interface will be registered with Revit first, and every time the corresponding external event is raised, the Execute method of this interface will be invoked.