Define registry item class and visitor for Preference pages

src/prefs/PrefsDialog.cpp

@@ -39,6 +39,7 @@
 #include "../AudioIOBase.h"
 #include "../Prefs.h"
 #include "../ShuttleGui.h"
+#include "../commands/CommandManager.h"
 
 #include "PrefsPanel.h"
 
@@ -460,19 +461,21 @@ namespace {
       { return sequenceNumber < other.sequenceNumber; }
    };
    using Entries = std::vector< Entry >;
+   namespace Prefs {
       Entries &Registry()
       {
          static Entries result;
          return result;
       }
    }
+}
 
 PrefsPanel::Registration::Registration( unsigned sequenceNumber,
    const Factory &factory,
    unsigned nChildren,
    bool expanded )
 {
-   auto &registry = Registry();
+   auto &registry = Prefs::Registry();
    Entry entry{ sequenceNumber, { factory, nChildren, expanded } };
    const auto end = registry.end();
    // Find insertion point:
@@ -482,6 +485,53 @@ PrefsPanel::Registration::Registration( unsigned sequenceNumber,
    registry.insert( iter, entry );
 }
 
+namespace {
+
+struct PrefsItem final : Registry::ConcreteGroupItem<false> {
+   PrefsPanel::Factory factory;
+   bool expanded{ false };
+
+   PrefsItem( const wxString &name,
+      const PrefsPanel::Factory &factory_, bool expanded_ )
+         : ConcreteGroupItem<false>{ name }
+         , factory{ factory_ }, expanded{ expanded_ }
+   {}
+};
+
+// Collects registry tree nodes into a vector, in preorder.
+struct PrefsItemVisitor final : Registry::Visitor {
+   PrefsItemVisitor( PrefsDialog::Factories &factories_ )
+      : factories{ factories_ }
+   {
+      childCounts.push_back( 0 );
+   }
+   void BeginGroup( Registry::GroupItem &item, const Path & ) override
+   {
+      auto pItem = dynamic_cast<PrefsItem*>( &item );
+      if (!pItem)
+         return;
+      indices.push_back( factories.size() );
+      factories.emplace_back( pItem->factory, 0, pItem->expanded );
+      ++childCounts.back();
+      childCounts.push_back( 0 );
+   }
+   void EndGroup( Registry::GroupItem &item, const Path & ) override
+   {
+      auto pItem = dynamic_cast<PrefsItem*>( &item );
+      if (!pItem)
+         return;
+      auto &factory = factories[ indices.back() ];
+      factory.nChildren = childCounts.back();
+      childCounts.pop_back();
+      indices.pop_back();
+   }
+
+   PrefsDialog::Factories &factories;
+   std::vector<size_t> childCounts;
+   std::vector<size_t> indices;
+};
+}
+
 PrefsDialog::Factories
 &PrefsDialog::DefaultFactories()
 {
@@ -489,7 +539,7 @@ PrefsDialog::Factories
    static std::once_flag flag;
 
    std::call_once( flag, []{
-      for ( const auto &entry : Registry() ) {
+      for ( const auto &entry : Prefs::Registry() ) {
          factories.push_back( entry.node );
       }
    } );

src/prefs/PrefsDialog.h

@@ -39,8 +39,8 @@ class PrefsDialog /* not final */ : public wxDialogWrapper
          std::function< PrefsPanel * (
             wxWindow *parent, wxWindowID winid, AudacityProject *) >;
        Factory factory;
-       unsigned nChildren;
+       size_t nChildren{ 0 };
-       bool expanded;
+       bool expanded{ false };
 
        PrefsNode(const Factory &factory_,
           unsigned nChildren_ = 0,