Fix for BindingExpression memory leak (#279)

* Unit test proving a memory leak with Binding

What we were seeing in our app was that Binding objects stay around when
bound to long-lived ViewModels, even when the View is long gone

* BindingExpression - INotifyPropertyChanged should use WeakReference

I had to make a WeakPropertyChangedProxy class for this, I could not
think of a way to get around creating a new object for this

1 files changed, 27 insertions, 0 deletions

diff --git a/Xamarin.Forms.Core.UnitTests/BindingUnitTests.cs b/Xamarin.Forms.Core.UnitTests/BindingUnitTests.cs
index c520e929..c28adda9 100644
--- a/Xamarin.Forms.Core.UnitTests/BindingUnitTests.cs
+++ b/Xamarin.Forms.Core.UnitTests/BindingUnitTests.cs
@@ -5,6 +5,7 @@ using System.Globalization;
 using System.ComponentModel;
 using System.Linq;
 using System.Runtime.CompilerServices;
+using System.Threading.Tasks;
 using NUnit.Framework;
 using CategoryAttribute=NUnit.Framework.CategoryAttribute;
 using DescriptionAttribute=NUnit.Framework.DescriptionAttribute;
@@ -2592,6 +2593,32 @@ namespace Xamarin.Forms.Core.UnitTests
 		}
 
 		[Test]
+		public async Task BindingDoesNotStayAliveForDeadTarget()
+		{
+			TestViewModel viewModel = new TestViewModel();
+			WeakReference bindingRef;
+
+			{ 
+				var binding = new Binding("Foo");
+
+				var button = new Button();
+				button.SetBinding(Button.TextProperty, binding);
+				button.BindingContext = viewModel;
+
+				bindingRef = new WeakReference(binding);
+			}
+
+			Assume.That(viewModel.InvocationListSize(), Is.EqualTo(1));
+
+			//NOTE: this was the only way I could "for sure" get the binding to get GC'd
+			GC.Collect();
+			await Task.Delay(10);
+			GC.Collect();
+
+			Assert.IsFalse(bindingRef.IsAlive, "Binding should not be alive!");
+		}
+
+		[Test]
 		public void BindingCreatesSingleSubscription ()
 		{
 			TestViewModel viewmodel = new TestViewModel();