static BackgroundCell::Get()...

... not member functions of AudacityProject.

This frees BackgroundCell.cpp from dependency cycles

src/Project.cpp

@@ -161,7 +161,6 @@ scroll information.  It also has some status flags.
 #include "toolbars/ToolsToolBar.h"
 #include "toolbars/TranscriptionToolBar.h"
 
-#include "tracks/ui/BackgroundCell.h"
 #include "tracks/ui/EditCursorOverlay.h"
 #include "tracks/ui/PlayIndicatorOverlay.h"
 #include "tracks/ui/Scrubbing.h"
@@ -1232,8 +1231,6 @@ AudacityProject::AudacityProject(wxWindow * parent, wxWindowID id,
  
    mCursorOverlay = std::make_shared<EditCursorOverlay>(this);
 
-   mBackgroundCell = std::make_shared<BackgroundCell>(this);
-
 #ifdef EXPERIMENTAL_SCRUBBING_BASIC
    mScrubOverlay = std::make_shared<ScrubbingOverlay>(this);
 #endif
@@ -1251,8 +1248,6 @@ AudacityProject::AudacityProject(wxWindow * parent, wxWindowID id,
 
    MenuManager::Get( project ).CreateMenusAndCommands( project );
 
-   mTrackPanel->SetBackgroundCell(mBackgroundCell);
-
    // LLL: When Audacity starts or becomes active after returning from
    //      another application, the first window that can accept focus
    //      will be given the focus even if we try to SetFocus().  By

src/Project.h

@@ -92,7 +92,6 @@ enum class UndoPush : unsigned char;
 
 class Track;
 class WaveClip;
-class BackgroundCell;
 
 
 AudacityProject *CreateNewAudacityProject();
@@ -654,8 +653,6 @@ private:
    std::shared_ptr<Overlay>
       mIndicatorOverlay, mCursorOverlay;
 
-   std::shared_ptr<BackgroundCell> mBackgroundCell;
-
 #ifdef EXPERIMENTAL_SCRUBBING_BASIC
    std::shared_ptr<Overlay> mScrubOverlay;
 private:
@@ -698,8 +695,6 @@ private:
 
 public:
    PlaybackScroller &GetPlaybackScroller() { return *mPlaybackScroller; }
-   std::shared_ptr<BackgroundCell> GetBackgroundCell() const
-      { return mBackgroundCell; }
 
    DECLARE_EVENT_TABLE()
 };

src/tracks/ui/BackgroundCell.cpp

@@ -12,9 +12,11 @@ Paul Licameli split from TrackPanel.cpp
 #include "BackgroundCell.h"
 
 #include "../../HitTestResult.h"
+#include "../../Project.h"
 #include "../../RefreshCode.h"
 #include "../../SelectionState.h"
 #include "../../Track.h"
+#include "../../TrackPanel.h"
 #include "../../TrackPanelMouseEvent.h"
 #include "../../UIHandle.h"
 
@@ -76,6 +78,24 @@ public:
    { return RefreshCode::RefreshNone; }
 };
 
+static const AudacityProject::AttachedObjects::RegisteredFactory key{
+  []( AudacityProject &parent ){
+     auto result = std::make_shared< BackgroundCell >( &parent );
+     parent.GetTrackPanel()->SetBackgroundCell( result );
+     return result;
+   }
+};
+
+BackgroundCell &BackgroundCell::Get( AudacityProject &project )
+{
+   return project.AttachedObjects::Get< BackgroundCell >( key );
+}
+
+const BackgroundCell &BackgroundCell::Get( const AudacityProject &project )
+{
+   return Get( const_cast< AudacityProject & >( project ) );
+}
+
 BackgroundCell::~BackgroundCell()
 {
 }

src/tracks/ui/BackgroundCell.h

@@ -11,6 +11,7 @@ Paul Licameli split from TrackPanel.cpp
 #ifndef __AUDACITY_BACKGROUND_CELL__
 #define __AUDACITY_BACKGROUND_CELL__
 
+#include "ClientData.h"
 #include "CommonTrackPanelCell.h"
 
 class AudacityProject;
@@ -21,9 +22,14 @@ class ZoomHandle;
 
 /// \brief Class representing the background of a Track.  It
 /// provides the hit test function that tells us what was hit.
-class BackgroundCell final : public CommonTrackPanelCell
+class BackgroundCell final
+   : public CommonTrackPanelCell
+   , public ClientData::Base
 {
 public:
+   static BackgroundCell &Get( AudacityProject &project );
+   static const BackgroundCell &Get( const AudacityProject &project );
+
    BackgroundCell(AudacityProject *pProject)
       : mpProject(pProject)
    {}
@@ -44,7 +50,7 @@ private:
 
 public:
    // For want of a better place...
-   std::weak_ptr<ZoomHandle> mZoomHandle;
+   mutable std::weak_ptr<ZoomHandle> mZoomHandle;
 };
 
 #endif

src/tracks/ui/TrackUI.cpp

@@ -40,7 +40,7 @@ std::vector<UIHandlePtr> Track::HitTest
       // Zoom tool is a non-selecting tool that takes precedence in all tracks
       // over all other tools, no matter what detail you point at.
       result = ZoomHandle::HitAnywhere(
-         pProject->GetBackgroundCell()->mZoomHandle);
+         BackgroundCell::Get( *pProject ).mZoomHandle );
       results.push_back(result);
       return results;
    }
@@ -63,7 +63,7 @@ std::vector<UIHandlePtr> Track::HitTest
    // other detailed hits.
    if ( isMultiTool ) {
       result = ZoomHandle::HitTest(
-         pProject->GetBackgroundCell()->mZoomHandle, st.state);
+         BackgroundCell::Get( *pProject ).mZoomHandle, st.state);
       if (result)
          results.push_back(result);
    }