triggerbox: keep track of, and make available, "empty" status for a triggerbox.

Includes PBD::Signal that notifies when a TriggerBox's empty status changes

libs/ardour/ardour/triggerbox.h

@@ -590,6 +590,9 @@ class LIBARDOUR_API TriggerBox : public Processor
 	bool can_support_io_configuration (const ChanCount& in, ChanCount& out);
 	bool configure_io (ChanCount in, ChanCount out);
 
+	bool empty() const { return _active_slots == 0; }
+	PBD::Signal0<void> EmptyStatusChanged;
+
 	int32_t order() const { return _order; }
 	void set_order(int32_t n);
 
@@ -690,6 +693,7 @@ class LIBARDOUR_API TriggerBox : public Processor
 	Requests                 _requests;
 	bool                     _stop_all;
 	int32_t                  _active_scene;
+	int32_t                  _active_slots;
 
 	boost::shared_ptr<SideChain> _sidechain;
 

libs/ardour/triggerbox.cc

@@ -2124,6 +2124,7 @@ TriggerBox::TriggerBox (Session& s, DataType dt)
 	, _currently_playing (0)
 	, _stop_all (false)
 	, _active_scene (-1)
+	, _active_slots (0)
 	, requests (1024)
 {
 	set_display_to_user (false);
@@ -2220,14 +2221,29 @@ TriggerBox::maybe_swap_pending (uint32_t slot)
 	*/
 
 	Trigger* p = 0;
+	bool empty_changed = false;
 
 	p = all_triggers[slot]->swap_pending (p);
 
 	if (p) {
 
 		if (p == Trigger::MagicClearPointerValue) {
+			if (all_triggers[slot]->region()) {
+				if (_active_slots) {
+					_active_slots--;
+				}
+				if (_active_slots == 0) {
+					empty_changed = true;
+				}
+			}
 			all_triggers[slot]->clear_region ();
 		} else {
+			if (!all_triggers[slot]->region()) {
+				if (_active_slots == 0) {
+					empty_changed = true;
+				}
+				_active_slots++;
+			}
 			/* Note use of a custom delete function. We cannot
 			   delete the old trigger from the RT context where the
 			   trigger swap will happen, so we will ask the trigger
@@ -2237,6 +2253,10 @@ TriggerBox::maybe_swap_pending (uint32_t slot)
 			TriggerSwapped (slot); /* EMIT SIGNAL */
 		}
 	}
+
+	if (empty_changed) {
+		EmptyStatusChanged (); /* EMIT SIGNAL */
+	}
 }
 
 void
@@ -3157,6 +3177,7 @@ TriggerBox::set_state (const XMLNode& node, int version)
 				all_triggers.push_back (trig);
 				trig->set_state (**t, version);
 			}
+			_active_slots++;
 		}
 	}
 
@@ -3172,6 +3193,11 @@ TriggerBox::set_state (const XMLNode& node, int version)
 		}
 	}
 
+	/* Since _active_slots may have changed, we could consider sending
+	 * EmptyStatusChanged, but for now we don't consider ::set_state() to
+	 * be used except at session load.
+	 */
+
 	return 0;
 }