InputCallbackSystem - add an ActiveChanged event handler, the event fires anytime the collection goes from 0 to greater than 0, or greater than 0 to 0

BizHawk.Emulation.Common/Base Implementations/InputCallbackSystem.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 
 namespace BizHawk.Emulation.Common
 {
@@ -15,10 +16,50 @@ namespace BizHawk.Emulation.Common
 
 		public void RemoveAll(IEnumerable<Action> actions)
 		{
+			var hadAny = this.Any();
+
 			foreach (var action in actions)
 			{
 				this.Remove(action);
 			}
+
+			var hasAny = this.Any();
+
+			Changes(hadAny, hasAny);
+		}
+
+		public new void Add(Action item)
+		{
+			var hadAny = this.Any();
+			base.Add(item);
+			var hasAny = this.Any();
+
+			Changes(hadAny, hasAny);
+		}
+
+		public new bool Remove(Action item)
+		{
+			var hadAny = this.Any();
+			return base.Remove(item);
+			var hasAny = this.Any();
+
+			Changes(hadAny, hasAny);
+		}
+
+		// TODO: these just happen to be all the add/remove methods the client uses, to be thorough the others should be overriden as well
+
+		public delegate void ActiveChangedEventHandler();
+		public event ActiveChangedEventHandler ActiveChanged;
+
+		private void Changes(bool hadAny, bool hasAny)
+		{
+			if ((hadAny && !hasAny) || (!hadAny && hasAny))
+			{
+				if (ActiveChanged != null)
+				{
+					ActiveChanged();
+				}
+			}
 		}
 	}
 }