Allow subscriber to be collected if MessagingCenter is the only reference to it (#617)

* Repro

* Make messaging center callbacks weak references

* Preserve attribute

* Fix test method name

* Watch for collection of actual delegate target instead of wrapper delegate

* Preserve the original platform instance when changing main page

* Better tests for lambda situations

* Update tests, make callback target a weakreference if it's the subscriber

* Ensure old Platform MessagingCenter subs are gone before creating new Platform

2 files changed, 112 insertions, 0 deletions

diff --git a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla45926.cs b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla45926.cs
new file mode 100644
index 00000000..0006d2e8
--- /dev/null
+++ b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla45926.cs
@@ -0,0 +1,111 @@
+using System;
+using System.Threading;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Bugzilla, 45926, "MessagingCenter prevents subscriber from being collected", PlatformAffected.All)]
+	public class Bugzilla45926 : TestNavigationPage
+	{
+		protected override void Init()
+		{
+			Button createPage, sendMessage, doGC;
+
+			Label instanceCount = new Label();
+			Label messageCount = new Label();
+
+			instanceCount.Text = $"Instances: {_45926SecondPage.InstanceCounter.ToString()}";
+			messageCount.Text = $"Messages: {_45926SecondPage.MessageCounter.ToString()}";
+
+			var content = new ContentPage {
+				Title = "Test",
+				Content = new StackLayout {
+					VerticalOptions = LayoutOptions.Center,
+					Children = {
+						(createPage = new Button { Text = "New Page" }),
+						(sendMessage = new Button { Text = "Send Message" }),
+						(doGC = new Button { Text = "Do GC" }),
+						instanceCount, messageCount
+					}
+				}
+			};
+
+			createPage.Clicked += (s, e) => PushAsync (new _45926SecondPage ());
+			sendMessage.Clicked += (s, e) =>
+			{
+				MessagingCenter.Send (this, "Test");
+			};
+
+			doGC.Clicked += (sender, e) => {
+				GC.Collect ();
+				GC.WaitForPendingFinalizers();
+				instanceCount.Text = $"Instances: {_45926SecondPage.InstanceCounter.ToString()}";
+				messageCount.Text = $"Messages: {_45926SecondPage.MessageCounter.ToString()}";
+			};
+
+			PushAsync(content);
+		}
+
+#if UITEST
+		[Test]
+		public void Issue45926Test ()
+		{
+			RunningApp.WaitForElement (q => q.Marked ("New Page"));
+
+			RunningApp.Tap (q => q.Marked ("New Page"));
+			RunningApp.Back();
+			RunningApp.Tap(q => q.Marked("Do GC"));
+			RunningApp.Tap(q => q.Marked("Send Message"));
+			RunningApp.Tap(q => q.Marked("Do GC"));
+
+			RunningApp.WaitForElement (q => q.Marked ("Instances: 0"));
+			RunningApp.WaitForElement (q => q.Marked ("Messages: 0"));
+		}
+#endif
+	}
+
+	[Preserve(AllMembers = true)]
+	public class _45926SecondPage : ContentPage
+	{
+		public static int InstanceCounter = 0;
+		public static int MessageCounter = 0;
+
+		public _45926SecondPage ()
+		{
+			Interlocked.Increment(ref InstanceCounter);
+
+			Content = new Label {
+				HorizontalOptions = LayoutOptions.Center,
+				VerticalOptions = LayoutOptions.Center,
+				Text = "Second Page #" + (InstanceCounter)
+			};
+
+			MessagingCenter.Subscribe<Bugzilla45926> (this, "Test", OnMessage);
+		}
+
+		protected override void OnDisappearing ()
+		{
+			base.OnDisappearing ();
+		}
+
+		void OnMessage (Bugzilla45926 app)
+		{
+			System.Diagnostics.Debug.WriteLine ("Got Test message!");
+			Interlocked.Increment(ref MessageCounter);
+		}
+
+		~_45926SecondPage ()
+		{
+			Interlocked.Decrement(ref InstanceCounter);
+			System.Diagnostics.Debug.WriteLine ("~SecondPage: {0}", GetHashCode ());
+		}
+	}
+}
\ No newline at end of file
diff --git a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
index e0af0ca2..36ed04cb 100644
--- a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
+++ b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
@@ -479,6 +479,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla36802.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla35736.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla48158.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Bugzilla45926.cs" />
   </ItemGroup>
   <ItemGroup>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Bugzilla22229.xaml">