Distinguish the quick-to-compute command flags

src/Menus.cpp

@@ -445,22 +445,30 @@ namespace{
       static Predicates thePredicates;
       return thePredicates;
    }
+   std::vector< CommandFlagOptions > &Options()
+   {
+      static std::vector< CommandFlagOptions > options;
+      return options;
+   }
 }
 
-ReservedCommandFlag::ReservedCommandFlag( const Predicate &predicate )
+ReservedCommandFlag::ReservedCommandFlag(
+   const Predicate &predicate, const CommandFlagOptions &options )
 {
    static size_t sNextReservedFlag = 0;
    // This will throw std::out_of_range if the constant NCommandFlags is too
    // small
    set( sNextReservedFlag++ );
    RegisteredPredicates().emplace_back( predicate );
+   Options().emplace_back( options );
 }
 
 const ReservedCommandFlag
    AudioIONotBusyFlag{
       [](const AudacityProject &project ){
          return !AudioIOBusyPred( project );
-      }
+      },
+      CommandFlagOptions{}.QuickTest()
    },
    TimeSelectedFlag{
       TimeSelectedPred
@@ -568,7 +576,8 @@ const ReservedCommandFlag
       }
    },
    AudioIOBusyFlag{
-      AudioIOBusyPred
+      AudioIOBusyPred,
+      CommandFlagOptions{}.QuickTest()
    }, //lll
    PlayRegionLockedFlag{
       [](const AudacityProject &project){
@@ -654,12 +663,14 @@ const ReservedCommandFlag
          return (focus &&
             !static_cast<const wxTopLevelWindow*>(focus)->IsIconized()
          );
-      }
+      },
+      CommandFlagOptions{}.QuickTest()
    }, // prl
    PausedFlag{
       [](const AudacityProject&){
          return AudioIOBase::Get()->IsPaused();
-      }
+      },
+      CommandFlagOptions{}.QuickTest()
    },
    HasWaveDataFlag{
       [](const AudacityProject &project){

src/commands/CommandFlag.h

@@ -13,6 +13,7 @@
 
 #include <bitset>
 #include <functional>
+#include <utility>
 
 class AudacityProject;
 
@@ -32,13 +33,26 @@ constexpr CommandFlag
    AlwaysEnabledFlag{},      // all zeroes
    NoFlagsSpecified{ ~0ULL }; // all ones
 
+struct CommandFlagOptions{
+   CommandFlagOptions() = default;
+   CommandFlagOptions && QuickTest() &&
+   { quickTest = true; return std::move( *this ); }
+
+   // If true, assume this is a cheap test to be done always.  If false, the
+   // test may be skipped and the condition assumed to be unchanged since the
+   // last more comprehensive testing
+   bool quickTest = false;
+};
+
 // Construct one statically to register (and reserve) a bit position in the set
-// an associate it with a test function
+// an associate it with a test function; those with quickTest = true are cheap
+// to compute and always checked
 class ReservedCommandFlag : public CommandFlag
 {
 public:
    using Predicate = std::function< bool( const AudacityProject& ) >;
-   ReservedCommandFlag( const Predicate & );
+   ReservedCommandFlag( const Predicate &predicate,
+      const CommandFlagOptions &options = {} );
 };
 
 // Widely used command flags, but this list need not be exhaustive.  It may be