ENH: allow calling RemoveObserver() from a const itkObject

Modules/Core/Common/include/itkObject.h

@@ -201,7 +201,7 @@ class ITKCommon_EXPORT Object : public LightObject
 
   /** Remove the observer with this tag value. */
   void
-  RemoveObserver(unsigned long tag);
+  RemoveObserver(unsigned long tag) const;
 
   /** Remove all observers . */
   void

Modules/Core/Common/src/itkObject.cxx

@@ -493,7 +493,7 @@ Object::GetCommand(unsigned long tag)
 }
 
 void
-Object::RemoveObserver(unsigned long tag)
+Object::RemoveObserver(unsigned long tag) const
 {
   if (this->m_SubjectImplementation)
   {

Modules/Filtering/ImageGrid/test/itkBasicArchitectureTest.cxx

@@ -21,6 +21,7 @@
 #include "itkShrinkImageFilter.h"
 #include "itkCommand.h"
 #include "itkTextOutput.h"
+#include "itkTestingMacros.h"
 
 // The following three classes are used to support callbacks
 // on the shrink filter in the pipeline that follows later
@@ -142,30 +143,40 @@ itkBasicArchitectureTest(int, char *[])
   command = itk::SimpleMemberCommand<ShowProgressObject>::New();
   command->SetCallbackFunction(&progressWatch, &ShowProgressObject::ShowProgress);
   shrink->AddObserver(itk::ProgressEvent(), command);
+  ITK_TEST_EXPECT_TRUE(shrink->HasObserver(itk::ProgressEvent()));
 
   // Create a command to call StartEndEvent when start event is triggered
   StartEndEvent                                    startEndWatch;
   itk::SimpleMemberCommand<StartEndEvent>::Pointer start;
   start = itk::SimpleMemberCommand<StartEndEvent>::New();
   start->SetCallbackFunction(&startEndWatch, &StartEndEvent::Start);
-  unsigned long tag = shrink->AddObserver(itk::StartEvent(), start);
+  shrink->AddObserver(itk::StartEvent(), start);
+  ITK_TEST_EXPECT_TRUE(shrink->HasObserver(itk::StartEvent()));
 
   // Create a command to call StartEndEvent when end event is triggered
   itk::SimpleMemberCommand<StartEndEvent>::Pointer end;
   end = itk::SimpleMemberCommand<StartEndEvent>::New();
   end->SetCallbackFunction(&startEndWatch, &StartEndEvent::End);
-  shrink->AddObserver(itk::EndEvent(), end);
+  unsigned long endEventObserverTag = shrink->AddObserver(itk::EndEvent(), end);
+  ITK_TEST_EXPECT_TRUE(shrink->HasObserver(itk::EndEvent()));
 
   // Create a command that to call AnyEvent when event is fired
   AllEvents                              allWatch;
   itk::MemberCommand<AllEvents>::Pointer allEvents;
   allEvents = itk::MemberCommand<AllEvents>::New();
   allEvents->SetCallbackFunction(&allWatch, &AllEvents::WatchEvents);
-  shrink->AddObserver(itk::AnyEvent(), allEvents);
+  unsigned long anyEventObserverTag = shrink->AddObserver(itk::AnyEvent(), allEvents);
+  ITK_TEST_EXPECT_TRUE(shrink->HasObserver(itk::AnyEvent()));
   shrink->Update();
 
-  // test RemoveObserver code
-  shrink->RemoveObserver(tag);
+  // Test RemoveObserver code
+  shrink->RemoveObserver(anyEventObserverTag);
+  ITK_TEST_EXPECT_TRUE(!shrink->HasObserver(itk::AnyEvent()));
+
+  // Test RemoveObserver const code
+  const auto & constShrink = *shrink;
+  constShrink.RemoveObserver(endEventObserverTag);
+  ITK_TEST_EXPECT_TRUE(!constShrink.HasObserver(itk::EndEvent()));
 
   return EXIT_SUCCESS;
 }