add ::usable() method to TransportMaster objects to allow GUI to show their usability after backend/engine changes

gtk2_ardour/transport_masters_dialog.cc

@@ -86,6 +86,8 @@ TransportMastersWidget::TransportMastersWidget ()
 	TransportMasterManager::instance().Added.connect (add_connection, invalidator (*this), boost::bind (&TransportMastersWidget::rebuild, this), gui_context());
 	TransportMasterManager::instance().Removed.connect (remove_connection, invalidator (*this), boost::bind (&TransportMastersWidget::rebuild, this), gui_context());
 
+	AudioEngine::instance()->Running.connect (engine_running_connection, invalidator (*this), boost::bind (&TransportMastersWidget::update_usability, this), gui_context());
+
 	rebuild ();
 }
 
@@ -244,6 +246,19 @@ TransportMastersWidget::rebuild ()
 
 		r->prop_change (all_change);
 	}
+
+	update_usability ();
+}
+
+void
+TransportMastersWidget::update_usability ()
+{
+	for (vector<Row*>::iterator r= rows.begin(); r != rows.end(); ++r) {
+		const bool usable = (*r)->tm->usable();
+		(*r)->use_button.set_sensitive (usable);
+		(*r)->collect_button.set_sensitive (usable);
+		(*r)->request_options.set_sensitive (usable);
+	}
 }
 
 TransportMastersWidget::Row::Row (TransportMastersWidget& p)

gtk2_ardour/transport_masters_dialog.h

@@ -143,11 +143,13 @@ class TransportMastersWidget : public Gtk::VBox, public ARDOUR::SessionHandlePtr
 	PBD::ScopedConnection current_connection;
 	PBD::ScopedConnection add_connection;
 	PBD::ScopedConnection remove_connection;
+	PBD::ScopedConnection engine_running_connection;
 
 	void rebuild ();
 	void clear ();
 	void current_changed (boost::shared_ptr<ARDOUR::TransportMaster> old_master, boost::shared_ptr<ARDOUR::TransportMaster> new_master);
 	void add_master ();
+	void update_usability ();
 
   public:
 	bool idle_remove (Row*);

libs/ardour/ardour/transport_master.h

@@ -231,6 +231,16 @@ class LIBARDOUR_API TransportMaster : public PBD::Stateful {
 	 */
 	virtual bool ok() const = 0;
 
+	/**
+	 * reports to ARDOUR whether it is possible to use this slave
+	 *
+	 * @return - true if the slave can be used.
+	 *
+	 * Only the JACK ("Engine") slave is ever likely to return false,
+	 * if JACK is not being used for the Audio/MIDI backend.
+	 */
+	virtual bool usable() const { return true; }
+
 	/**
 	 * reports to ARDOUR whether the slave is in the process of starting
 	 * to roll
@@ -626,6 +636,7 @@ class LIBARDOUR_API Engine_TransportMaster : public TransportMaster
 	void reset (bool with_position);
 	bool locked() const;
 	bool ok() const;
+	bool usable() const;
 	samplecnt_t update_interval () const;
 	samplecnt_t resolution () const { return 1; }
 	bool requires_seekahead () const { return false; }

libs/ardour/engine_slave.cc

@@ -47,6 +47,12 @@ Engine_TransportMaster::init ()
 {
 }
 
+bool
+Engine_TransportMaster::usable () const
+{
+	return AudioEngine::instance()->current_backend_name() == X_("JACK");
+}
+
 void
 Engine_TransportMaster::check_backend()
 {