Change SourceChanged event on ImageSource to weak event to allow Images (#268)

referencing application-wide StaticResource ImageSources to be GCed

4 files changed, 174 insertions, 7 deletions

diff --git a/Xamarin.Forms.Core/Image.cs b/Xamarin.Forms.Core/Image.cs
index 4292f952..dd2692ad 100644
--- a/Xamarin.Forms.Core/Image.cs
+++ b/Xamarin.Forms.Core/Image.cs
@@ -1,4 +1,6 @@
 using System;
+using System.Collections.Generic;
+using System.Reflection;
 using Xamarin.Forms.Internals;
 using Xamarin.Forms.Platform;
 
@@ -7,8 +9,8 @@ namespace Xamarin.Forms
 	[RenderWith(typeof(_ImageRenderer))]
 	public class Image : View, IImageController
 	{
-		public static readonly BindableProperty SourceProperty = BindableProperty.Create("Source", typeof(ImageSource), typeof(Image), default(ImageSource), propertyChanging: OnSourcePropertyChanging,
-			propertyChanged: OnSourcePropertyChanged);
+		public static readonly BindableProperty SourceProperty = BindableProperty.Create("Source", typeof(ImageSource), typeof(Image), default(ImageSource), 
+			propertyChanging: OnSourcePropertyChanging, propertyChanged: OnSourcePropertyChanged);
 
 		public static readonly BindableProperty AspectProperty = BindableProperty.Create("Aspect", typeof(Aspect), typeof(Image), Aspect.AspectFit);
 
@@ -126,6 +128,7 @@ namespace Xamarin.Forms
 				newvalue.SourceChanged += OnSourceChanged;
 				SetInheritedBindingContext(newvalue, BindingContext);
 			}
+
 			InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);
 		}
 
@@ -138,7 +141,7 @@ namespace Xamarin.Forms
 		{
 			if (oldvalue == null)
 				return;
-
+			
 			oldvalue.SourceChanged -= OnSourceChanged;
 			try
 			{
diff --git a/Xamarin.Forms.Core/ImageSource.cs b/Xamarin.Forms.Core/ImageSource.cs
index f65d0610..3b59f7fa 100644
--- a/Xamarin.Forms.Core/ImageSource.cs
+++ b/Xamarin.Forms.Core/ImageSource.cs
@@ -14,6 +14,8 @@ namespace Xamarin.Forms
 
 		TaskCompletionSource<bool> _completionSource;
 
+		readonly WeakEventManager _weakEventManager = new WeakEventManager();
+
 		protected ImageSource()
 		{
 		}
@@ -132,11 +134,13 @@ namespace Xamarin.Forms
 
 		protected void OnSourceChanged()
 		{
-			EventHandler eh = SourceChanged;
-			if (eh != null)
-				eh(this, EventArgs.Empty);
+			_weakEventManager.HandleEvent(this, EventArgs.Empty, nameof(SourceChanged));
 		}
 
-		internal event EventHandler SourceChanged;
+		internal event EventHandler SourceChanged
+		{
+			add { _weakEventManager.AddEventHandler(nameof(SourceChanged), value); }
+			remove { _weakEventManager.RemoveEventHandler(nameof(SourceChanged), value);}
+		}
 	}
 }
\ No newline at end of file
diff --git a/Xamarin.Forms.Core/WeakEventManager.cs b/Xamarin.Forms.Core/WeakEventManager.cs
new file mode 100644
index 00000000..226f329a
--- /dev/null
+++ b/Xamarin.Forms.Core/WeakEventManager.cs
@@ -0,0 +1,159 @@
+using System;
+using System.Collections.Generic;
+using System.Reflection;
+
+namespace Xamarin.Forms
+{
+	internal class WeakEventManager
+	{
+		readonly Dictionary<string, List<Subscription>> _eventHandlers =
+			new Dictionary<string, List<Subscription>>();
+
+		public void AddEventHandler<TEventArgs>(string eventName, EventHandler<TEventArgs> handler)
+			where TEventArgs : EventArgs
+		{
+			if (eventName == null)
+			{
+				throw new ArgumentNullException(nameof(eventName));
+			}
+
+			if (handler == null)
+			{
+				throw new ArgumentNullException(nameof(handler));
+			}
+
+			AddEventHandler(eventName, handler.Target, handler.GetMethodInfo());
+		}
+
+		public void AddEventHandler(string eventName, EventHandler handler)
+		{
+			if (eventName == null)
+			{
+				throw new ArgumentNullException(nameof(eventName));
+			}
+
+			if (handler == null)
+			{
+				throw new ArgumentNullException(nameof(handler));
+			}
+
+			AddEventHandler(eventName, handler.Target, handler.GetMethodInfo());
+		}
+
+		public void HandleEvent(object sender, object args, string eventName)
+		{
+			var toRaise = new List<Tuple<object, MethodInfo>>();
+
+			List<Subscription> target;
+			if (_eventHandlers.TryGetValue(eventName, out target))
+			{
+				foreach (Subscription subscription in target)
+				{
+					object subscriber = subscription.Subscriber.Target;
+
+					if (subscriber == null)
+					{
+						// The subscriber was collected, so there's no need to keep this subscription around
+						target.Remove(subscription);
+					}
+					else
+					{
+						toRaise.Add(Tuple.Create(subscriber, subscription.Handler));
+					}
+				}
+			}
+
+			foreach (Tuple<object, MethodInfo> tuple in toRaise)
+			{
+				tuple.Item2.Invoke(tuple.Item1, new[] { sender, args });
+			}
+		}
+
+		public void RemoveEventHandler<TEventArgs>(string eventName, EventHandler<TEventArgs> handler)
+			where TEventArgs : EventArgs
+		{
+			if (eventName == null)
+			{
+				throw new ArgumentNullException(nameof(eventName));
+			}
+
+			if (handler == null)
+			{
+				throw new ArgumentNullException(nameof(handler));
+			}
+
+			RemoveEventHandler(eventName, handler.Target, handler.GetMethodInfo());
+		}
+
+		public void RemoveEventHandler(string eventName, EventHandler handler)
+		{
+			if (eventName == null)
+			{
+				throw new ArgumentNullException(nameof(eventName));
+			}
+
+			if (handler == null)
+			{
+				throw new ArgumentNullException(nameof(handler));
+			}
+
+			RemoveEventHandler(eventName, handler.Target, handler.GetMethodInfo());
+		}
+
+		void AddEventHandler(string eventName, object handlerTarget, MethodInfo methodInfo)
+		{
+			List<Subscription> target;
+			if (!_eventHandlers.TryGetValue(eventName, out target))
+			{
+				target = new List<Subscription>();
+				_eventHandlers.Add(eventName, target);
+			}
+
+			target.Add(new Subscription(new WeakReference(handlerTarget), methodInfo));
+		}
+
+		void RemoveEventHandler(string eventName, object handlerTarget, MemberInfo methodInfo)
+		{
+			List<Subscription> subscriptions;
+			if (!_eventHandlers.TryGetValue(eventName, out subscriptions))
+			{
+				return;
+			}
+
+			for (int n = subscriptions.Count; n > 0; n--)
+			{
+				Subscription current = subscriptions[n - 1];
+
+				if (current.Subscriber != handlerTarget
+				    || current.Handler.Name != methodInfo.Name)
+				{
+					continue;
+				}
+
+				subscriptions.Remove(current);
+			}
+		}
+
+		struct Subscription
+		{
+			public Subscription(WeakReference subscriber, MethodInfo handler)
+			{
+				if (subscriber == null)
+				{
+					throw new ArgumentNullException(nameof(subscriber));
+				}
+
+				if (handler == null)
+				{
+					throw new ArgumentNullException(nameof(handler));
+				}
+
+				Subscriber = subscriber;
+				Handler = handler;
+			}
+
+			public readonly WeakReference Subscriber;
+			public readonly MethodInfo Handler;
+		}
+	}
+}
\ No newline at end of file
diff --git a/Xamarin.Forms.Core/Xamarin.Forms.Core.csproj b/Xamarin.Forms.Core/Xamarin.Forms.Core.csproj
index d0cb293c..0541673f 100644
--- a/Xamarin.Forms.Core/Xamarin.Forms.Core.csproj
+++ b/Xamarin.Forms.Core/Xamarin.Forms.Core.csproj
@@ -331,6 +331,7 @@
     <Compile Include="Internals\ToolbarTracker.cs" />
     <Compile Include="ViewExtensions.cs" />
     <Compile Include="ViewState.cs" />
+    <Compile Include="WeakEventManager.cs" />
     <Compile Include="WeakReferenceExtensions.cs" />
     <Compile Include="WebNavigatedEventArgs.cs" />
     <Compile Include="WebNavigatingEventArgs.cs" />