Tests added

src/Prism.Avalonia/Extensions/ObservableExtensions.cs

@@ -3,7 +3,7 @@
 
 namespace Prism.Extensions
 {
-    internal static class ObservableExtensions
+    public static class ObservableExtensions
     {
         /// <summary>
         /// Subscribes an element handler to an observable sequence.

tests/Avalonia/Prism.Avalonia.Tests/Interactivity/ObservableBehaviorFixture.cs

@@ -0,0 +1,56 @@
+using Avalonia;
+using Xunit;
+using Prism.Extensions;
+
+namespace Prism.Avalonia.Tests.Interactivity
+{
+    public class ObservableBehaviorFixture
+    {
+        [Fact]
+        public void SubscribeWorksOnIObservableWithLambda()
+        {
+            var targetUIElement = new TestAvaloniaObject();
+            targetUIElement.Test = "123";
+
+            Assert.Equal("123", targetUIElement.OutTest);
+        }
+    }
+
+    class TestAvaloniaObject : AvaloniaObject
+    {
+        /// <summary>
+        /// The property to subscribe on.
+        /// </summary>
+        public static readonly StyledProperty<string> TestProperty =
+            AvaloniaProperty.Register<TestAvaloniaObject, string>("Test");
+
+        /// <summary>
+        /// Gets or sets a value to the TestProperty.
+        /// </summary>
+        public string Test
+        {
+            get { return (string)GetValue(TestProperty); }
+            set { SetValue(TestProperty, value); }
+        }
+
+        /// <summary>
+        /// The out test property to check after TestProperty change
+        /// </summary>
+        public string OutTest { get; set; }
+
+        /// <summary>
+        /// Initializes a new instance of <see cref="TestAvaloniaObject"/>.
+        /// </summary>
+#pragma warning disable CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
+        public TestAvaloniaObject()
+#pragma warning restore CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
+        {
+            TestProperty.Changed.Subscribe(args => OnPropertyChanged(args));
+        }
+
+        private void OnPropertyChanged(AvaloniaPropertyChangedEventArgs<string> args)
+        {
+            OutTest = args.NewValue.Value;
+        }
+    }
+}