First pass at observing nested property changes

Added a new method to observe changes in nested properties of INotifyPropertyChanged objects. This allows for more granular observation of property changes, particularly useful when dealing with complex objects. Also updated the test suite to cover this new functionality.

src/R3/Factories/ObserveProperty.cs

@@ -13,7 +13,7 @@ public static Observable<TProperty> ObservePropertyChanged<T, TProperty>(this T
         Func<T, TProperty> propertySelector,
         bool pushCurrentValueOnSubscribe = true,
         CancellationToken cancellationToken = default,
-        [CallerArgumentExpression("propertySelector")] string? expr = null)
+        [CallerArgumentExpression(nameof(propertySelector))] string? expr = null)
         where T : INotifyPropertyChanged
     {
         if (expr == null) throw new ArgumentNullException(expr);
@@ -22,6 +22,37 @@ public static Observable<TProperty> ObservePropertyChanged<T, TProperty>(this T
         return new ObservePropertyChanged<T, TProperty>(value, propertySelector, propertyName, pushCurrentValueOnSubscribe, cancellationToken);
     }
 
+    /// <summary>
+    /// Convert INotifyPropertyChanged to Observable.
+    /// `propertySelector` must be a Func specifying a simple property. For example, it extracts "Foo" from `x => x.Foo`.
+    /// </summary>
+    public static Observable<TProperty2> ObservePropertyChanged<T, TProperty1, TProperty2>(this T value,
+        Func<T, TProperty1> propertySelector1,
+        Func<TProperty1, TProperty2> propertySelector2,
+        bool pushCurrentValueOnSubscribe = true,
+        CancellationToken cancellationToken = default,
+        [CallerArgumentExpression(nameof(propertySelector1))] string? propertySelector1Expr = null,
+        [CallerArgumentExpression(nameof(propertySelector2))] string? propertySelector2Expr = null)
+        where T : INotifyPropertyChanged
+        where TProperty1 : INotifyPropertyChanged
+    {
+        if (propertySelector1Expr == null) throw new ArgumentNullException(propertySelector1Expr);
+        if (propertySelector2Expr == null) throw new ArgumentNullException(propertySelector2Expr);
+
+        var property1Name = propertySelector1Expr!.Substring(propertySelector1Expr.LastIndexOf('.') + 1);
+        var property2Name = propertySelector2Expr!.Substring(propertySelector2Expr.LastIndexOf('.') + 1);
+
+        var firstPropertyChanged = new ObservePropertyChanged<T, TProperty1>(value, propertySelector1, property1Name, pushCurrentValueOnSubscribe, cancellationToken);
+
+        return firstPropertyChanged
+            .Select(
+                (propertySelector2, property2Name, pushCurrentValueOnSubscribe, cancellationToken),
+                (firstPropertyValue, state) =>
+                    (Observable<TProperty2>)new ObservePropertyChanged<TProperty1, TProperty2>(firstPropertyValue, state.propertySelector2, state.property2Name, state.pushCurrentValueOnSubscribe, state.cancellationToken))
+            .Switch();
+    }
+
+
     /// <summary>
     /// Convert INotifyPropertyChanging to Observable.
     /// `propertySelector` must be a Func specifying a simple property. For example, it extracts "Foo" from `x => x.Foo`.
@@ -30,7 +61,7 @@ public static Observable<TProperty> ObservePropertyChanging<T, TProperty>(this T
         Func<T, TProperty> propertySelector,
         bool pushCurrentValueOnSubscribe = true,
         CancellationToken cancellationToken = default,
-        [CallerArgumentExpression("propertySelector")] string? expr = null)
+        [CallerArgumentExpression(nameof(propertySelector))] string? expr = null)
         where T : INotifyPropertyChanging
     {
         if (expr == null) throw new ArgumentNullException(expr);

tests/R3.Tests/FactoryTests/ObservePropertyTest.cs

@@ -21,6 +21,26 @@ public void PropertyChanged()
         liveList.AssertEqual([0, 1]);
     }
 
+    [Fact]
+    public void NestedPropertyChanged()
+    {
+        ChangesProperty propertyChanger = new();
+
+        using var liveList = propertyChanger
+            .ObservePropertyChanged(x => x.InnerPropertyChanged, x => x.Value)
+            .ToLiveList();
+
+        liveList.AssertEqual([]);
+
+        propertyChanger.InnerPropertyChanged = new();
+
+        liveList.AssertEqual([0]);
+
+        propertyChanger.InnerPropertyChanged.Value = 1;
+
+        liveList.AssertEqual([0, 1]);
+    }
+
     [Fact]
     public void PropertyChanging()
     {
@@ -40,6 +60,7 @@ public void PropertyChanging()
     class ChangesProperty : INotifyPropertyChanged, INotifyPropertyChanging
     {
         private int _value;
+        private ChangesProperty _innerPropertyChanged;
 
         public event PropertyChangedEventHandler? PropertyChanged;
         public event PropertyChangingEventHandler? PropertyChanging;
@@ -50,6 +71,12 @@ public int Value
             set => SetField(ref _value, value);
         }
 
+        public ChangesProperty InnerPropertyChanged
+        {
+            get => _innerPropertyChanged;
+            set => SetField(ref _innerPropertyChanged, value);
+        }
+
         private void OnPropertyChanged([CallerMemberName] string? propertyName = null)
         {
             PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));