Some machinery to add more menu handler object classes...

...without adding linkage dependencies to AudacityProject constructor

src/Menus.cpp

@@ -171,8 +171,23 @@ static void AddEffectMenuItemGroup(
 
 constexpr size_t kAlignLabelsCount = 5;
 
+static const AudacityProject::RegisteredAttachedObjectFactory factory{ []{
+   return std::make_unique< MenuCommandHandler >();
+} };
+
+PrefsListener::~PrefsListener()
+{
+}
+
+void PrefsListener::UpdatePrefs()
+{
+}
+
 MenuCommandHandler &GetMenuCommandHandler(AudacityProject &project)
-{ return *project.mMenuCommandHandler; }
+{
+   return static_cast<MenuCommandHandler&>(
+      project.GetAttachedObject( factory ) );
+}
 
 MenuManager &GetMenuManager(AudacityProject &project)
 { return *project.mMenuManager; }

src/Menus.h

@@ -35,8 +35,16 @@ enum EffectType : int;
 typedef wxString PluginID;
 typedef wxArrayString PluginIDList;
 
+class PrefsListener
+{
+public:
+   virtual ~PrefsListener();
+   virtual void UpdatePrefs(); // default is no-op
+};
+
 struct MenuCommandHandler final
    : public CommandHandlerObject // MUST be the first base class!
+   , public PrefsListener
 {
    MenuCommandHandler();
    ~MenuCommandHandler();
@@ -592,7 +600,7 @@ public:
    bool mCircularTrackNavigation{};
    wxLongLong mLastSelectionAdjustment;
 
-   void UpdatePrefs();
+   void UpdatePrefs() override;
 };
 
 class MenuCreator

src/Project.cpp

@@ -98,6 +98,7 @@ scroll information.  It also has some status flags.
 #include "Dependencies.h"
 #include "Diags.h"
 #include "HistoryWindow.h"
+#include "InconsistencyException.h"
 #include "Lyrics.h"
 #include "LyricsWindow.h"
 #include "MixerBoard.h"
@@ -888,6 +889,64 @@ static wxString CreateUniqueName()
           wxString::Format(wxT(" N-%i"), ++count);
 }
 
+namespace {
+
+#if 0
+std::mutex sObjectFactoriesMutex;
+struct ObjectFactorySetLocker : private std::unique_lock< std::mutex >
+{
+   ObjectFactorySetLocker()
+      : std::unique_lock< std::mutex > { sObjectFactoriesMutex }
+   {}
+};
+#else
+struct ObjectFactorySetLocker {};
+#endif
+
+std::vector<AudacityProject::AttachedObjectFactory> &sObjectFactories()
+{
+   // Put this declaration inside a function to avoid problems of undefined
+   // sequence of initialization of file-scope statics in different
+   // compilation units.
+   // Note that mutex locking is not needed for constructing a static object
+   // in C++11:
+   //https://en.cppreference.com/w/cpp/language/storage_duration#Static_local_variables
+   static std::vector<AudacityProject::AttachedObjectFactory> factories;
+   return factories;
+}
+}
+
+AudacityProject::
+RegisteredAttachedObjectFactory::RegisteredAttachedObjectFactory(
+   const AttachedObjectFactory &factory )
+{
+   ObjectFactorySetLocker locker;
+   mIndex = sObjectFactories().size();
+   sObjectFactories().push_back( factory );
+   
+   // In case registration happens while projects exist:
+   for (const auto &pProject : gAudacityProjects) {
+      if (pProject->mAttachedObjects.size() == mIndex) {
+         auto pObject = factory();
+         wxASSERT( pObject );
+         pProject->mAttachedObjects.push_back( std::move( pObject ) );
+      }
+   }
+}
+
+AudacityProject::
+AttachedObject &AudacityProject::GetAttachedObject(
+   const RegisteredAttachedObjectFactory &factory )
+{
+   ObjectFactorySetLocker locker;
+   if ( factory.mIndex >= mAttachedObjects.size() )
+      THROW_INCONSISTENCY_EXCEPTION;
+   auto &pObject = mAttachedObjects[ factory.mIndex ];
+   if ( !pObject )
+      THROW_INCONSISTENCY_EXCEPTION;
+   return *pObject;
+}
+
 enum {
    FirstID = 1000,
 
@@ -982,7 +1041,15 @@ AudacityProject::AudacityProject(wxWindow * parent, wxWindowID id,
    // Initialize view info (shared with TrackPanel)
    //
 
-   mMenuCommandHandler = std::make_unique<MenuCommandHandler>();
+   {
+      ObjectFactorySetLocker locker;
+      for (const auto &factory : sObjectFactories()) {
+         auto pObject = factory();
+         wxASSERT( pObject );
+         mAttachedObjects.push_back( std::move( pObject ) );
+      }
+   }
+
    mMenuManager = std::make_unique<MenuManager>();
 
    UpdatePrefs();
@@ -1364,7 +1431,12 @@ void AudacityProject::UpdatePrefs()
 
    SetProjectTitle();
 
-   GetMenuCommandHandler(*this).UpdatePrefs();
+   {
+      ObjectFactorySetLocker locker;
+      for( const auto &pObject : mAttachedObjects )
+         pObject->UpdatePrefs();
+   }
+
    GetMenuManager(*this).UpdatePrefs();
 
    if (mTrackPanel) {

src/Project.h

@@ -172,6 +172,8 @@ class WaveTrack;
 struct MenuCommandHandler;
 class MenuManager;
 
+class PrefsListener;
+
 class AUDACITY_DLL_API AudacityProject final : public wxFrame,
                                      public TrackPanelListener,
                                      public SelectionBarListener,
@@ -184,6 +186,24 @@ class AUDACITY_DLL_API AudacityProject final : public wxFrame,
                    const wxPoint & pos, const wxSize & size);
    virtual ~AudacityProject();
 
+   using AttachedObject = PrefsListener;
+   using AttachedObjectFactory =
+      std::function< std::unique_ptr<AttachedObject>() >;
+
+   // Typically a static object.  Allows various application code to
+   // attach per-project state, without Project.cpp needing to include a header
+   // file or know the details.
+   class RegisteredAttachedObjectFactory {
+   public:
+      RegisteredAttachedObjectFactory( const AttachedObjectFactory &factory );
+
+   private:
+      friend AudacityProject;
+      size_t mIndex {};
+   };
+   AttachedObject &
+      GetAttachedObject( const RegisteredAttachedObjectFactory& factory );
+
    virtual void ApplyUpdatedTheme();
 
    AudioIOStartStreamOptions GetDefaultPlayOptions();
@@ -807,7 +827,7 @@ private:
 #endif
 
 private:
-   std::unique_ptr<MenuCommandHandler> mMenuCommandHandler;
+   std::vector< std::unique_ptr<AttachedObject> > mAttachedObjects;
    std::unique_ptr<MenuManager> mMenuManager;
 
 public: