Merge pull request #1 from jeromelaban/dev/converter-unsetvalue-fix

test: Add unit tests for UnsetValue returning converter

src/Uno.UI.Tests/BinderTests/Given_Binder.Converter.cs

@@ -0,0 +1,70 @@
+#nullable enable
+
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Dynamic;
+using System.Globalization;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Data;
+
+namespace Uno.UI.Tests.BinderTests
+{
+	[TestClass]
+	public partial class Given_Binder_Converter
+	{
+		[TestMethod]
+		public void When_Converter_Returns_UnsetValue()
+		{
+			var SUT = new MyControl();
+
+			var myTestConverter = new MyTestConverter();
+
+			SUT.SetBinding(
+				MyControl.MyPropertyProperty,
+				new Windows.UI.Xaml.Data.Binding()
+				{
+					Path = new PropertyPath("."),
+					Converter = myTestConverter,
+					FallbackValue = "fallback value",
+					TargetNullValue = "null value"
+				}
+			);
+
+			myTestConverter.OutputValue = v => v;
+			SUT.DataContext = "hello";
+			Assert.AreEqual("hello", SUT.MyProperty);
+
+			myTestConverter.OutputValue = v => DependencyProperty.UnsetValue;
+			SUT.DataContext = "hello 3";
+			Assert.AreEqual("fallback value", SUT.MyProperty);
+		}
+
+		internal class MyTestConverter : IValueConverter
+		{
+			public Func<object, object?> OutputValue { get; set; } = o => null;
+
+			public object? Convert(object value, Type targetType, object parameter, string language) => OutputValue(value);
+
+			public object ConvertBack(object value, Type targetType, object parameter, string language) => throw new NotImplementedException();
+		}
+
+		public partial class MyControl : DependencyObject
+		{
+			public string MyProperty
+			{
+				get { return (string)GetValue(MyPropertyProperty); }
+				set { SetValue(MyPropertyProperty, value); }
+			}
+
+			public static readonly DependencyProperty MyPropertyProperty =
+				DependencyProperty.Register("MyProperty", typeof(string), typeof(MyControl), new FrameworkPropertyMetadata(null));
+		}
+	}
+
+}