feat: StartStopEventWrapper

src/Uno.UWP/Helpers/StartStopEventWrapper.cs

@@ -0,0 +1,50 @@
+using System;
+
+namespace Uno.Helpers
+{
+	internal class StartStopEventWrapper<TDelegate>
+		where TDelegate : Delegate
+	{
+		private readonly object _syncLock = new object();
+		private readonly Action _onFirst;
+		private readonly Action _onLast;
+
+		public StartStopEventWrapper(
+			Action onFirst,
+			Action onLast)
+		{
+			_onFirst = onFirst;
+			_onLast = onLast;
+		}
+
+		public TDelegate Event { get; private set; } = null;
+
+		public void AddHandler(TDelegate handler)
+		{
+			lock (_syncLock)
+			{
+				var firstSubscriber = Event == null;
+				Event = (TDelegate)Delegate.Combine(Event, handler);
+				if (firstSubscriber)
+				{
+					_onFirst();
+				}
+			}
+		}
+
+		public void RemoveHandler(TDelegate handler)
+		{
+			lock (_syncLock)
+			{
+				if (Event != null)
+				{
+					Event = (TDelegate)Delegate.Remove(Event, handler);
+					if (Event == null)
+					{
+						_onLast();
+					}
+				}
+			}
+		}
+	}
+}