All things with an UpdatePrefs() message listen for an event...

... Still to do, improve the handling of updates of only subsets of the prefs

src/widgets/Meter.cpp

@@ -253,9 +253,6 @@ bool MeterUpdateQueue::Get(MeterUpdateMsg &msg)
 // How many pixels between items?
 const static int gap = 2;
 
-// Event used to notify all meters of preference changes
-wxDEFINE_EVENT(EVT_METER_PREFERENCES_CHANGED, wxCommandEvent);
-
 const static wxChar *PrefStyles[] =
 {
    wxT("AutomaticStereo"),
@@ -348,11 +345,6 @@ MeterPanel::MeterPanel(AudacityProject *project,
    mPeakPeakPen = wxPen(theTheme.Colour( clrMeterPeak),        1, wxPENSTYLE_SOLID);
    mDisabledPen = wxPen(theTheme.Colour( clrMeterDisabledPen), 1, wxPENSTYLE_SOLID);
 
-   // Register for our preference update event
-   wxTheApp->Bind(EVT_METER_PREFERENCES_CHANGED,
-                     &MeterPanel::OnMeterPrefsUpdated,
-                     this);
-
    if (mIsInput) {
       wxTheApp->Bind(EVT_AUDIOIO_MONITOR,
                         &MeterPanel::OnAudioIOStatus,
@@ -457,6 +449,20 @@ void MeterPanel::UpdatePrefs()
    Reset(mRate, false);
 
    mLayoutValid = false;
+
+   Refresh(false);
+}
+
+static int MeterPrefsID()
+{
+   static int value = wxNewId();
+   return value;
+}
+
+void MeterPanel::UpdateSelectedPrefs(int id)
+{
+   if (id == MeterPrefsID())
+      UpdatePrefs();
 }
 
 void MeterPanel::OnErase(wxEraseEvent & WXUNUSED(event))
@@ -1964,15 +1970,6 @@ void MeterPanel::OnMonitor(wxCommandEvent & WXUNUSED(event))
    StartMonitoring();
 }
 
-void MeterPanel::OnMeterPrefsUpdated(wxCommandEvent & evt)
-{
-   evt.Skip();
-
-   UpdatePrefs();
-
-   Refresh(false);
-}
-
 void MeterPanel::OnPreferences(wxCommandEvent & WXUNUSED(event))
 {
    wxTextCtrl *rate;
@@ -2099,9 +2096,8 @@ void MeterPanel::OnPreferences(wxCommandEvent & WXUNUSED(event))
       // Currently, there are 2 playback meters and 2 record meters and any number of 
       // mixerboard meters, so we have to send out an preferences updated message to
       // ensure they all update themselves.
-      wxCommandEvent e(EVT_METER_PREFERENCES_CHANGED);
-      e.SetEventObject(this);
-      GetParent()->GetEventHandler()->ProcessEvent(e);
+      wxTheApp->AddPendingEvent(wxCommandEvent{
+         EVT_PREFS_UPDATE, MeterPrefsID() });
    }
 }
 

src/widgets/Meter.h

@@ -22,14 +22,11 @@
 #include <wx/timer.h> // member variable
 
 #include "../SampleFormat.h"
+#include "../Prefs.h"
 #include "Ruler.h" // member variable
 
 class AudacityProject;
 
-// Event used to notify all meters of preference changes
-wxDECLARE_EXPORTED_EVENT(AUDACITY_DLL_API,
-                         EVT_METER_PREFERENCES_CHANGED, wxCommandEvent);
-
 // Increase this when we add support for multichannel meters
 // (most of the code is already there)
 const int kMaxMeterBars = 2;
@@ -94,7 +91,7 @@ class MeterAx;
 \brief MeterPanel is a panel that paints the meter used for monitoring
 or playback.
 ************************************************************************/
-class MeterPanel final : public wxPanelWrapper
+class MeterPanel final : public wxPanelWrapper, private PrefsListener
 {
    DECLARE_DYNAMIC_CLASS(MeterPanel)
 
@@ -124,7 +121,6 @@ class MeterPanel final : public wxPanelWrapper
 
    void SetFocusFromKbd() override;
 
-   void UpdatePrefs();
    void Clear();
 
    Style GetStyle() const { return mStyle; }
@@ -192,6 +188,9 @@ class MeterPanel final : public wxPanelWrapper
    int GetDBRange() const { return mDB ? mDBRange : -1; }
 
  private:
+   void UpdatePrefs() override;
+   void UpdateSelectedPrefs( int ) override;
+
    static bool s_AcceptsFocus;
    struct Resetter { void operator () (bool *p) const { if(p) *p = false; } };
    using TempAllowFocus = std::unique_ptr<bool, Resetter>;
@@ -232,7 +231,6 @@ class MeterPanel final : public wxPanelWrapper
    void ShowMenu(const wxPoint & pos);
    void OnMonitor(wxCommandEvent &evt);
    void OnPreferences(wxCommandEvent &evt);
-   void OnMeterPrefsUpdated(wxCommandEvent &evt);
 
    wxString Key(const wxString & key) const;