New reactive core WIP

src/Library/DeclarativeProperty/CombineLatestProperty.cs

@@ -0,0 +1,36 @@
+namespace DeclarativeProperty;
+
+public sealed class CombineLatestProperty<T1, T2, TResult> : DeclarativePropertyBase<TResult>
+{
+    private readonly Func<T1?, T2?, Task<TResult>> _func;
+    private T1? _value1;
+    private T2? _value2;
+
+    public CombineLatestProperty(IDeclarativeProperty<T1> prop1, IDeclarativeProperty<T2> prop2, Func<T1?, T2?, Task<TResult>> func)
+    {
+        ArgumentNullException.ThrowIfNull(prop1);
+        ArgumentNullException.ThrowIfNull(prop2);
+        
+        _func = func;
+
+        _value1 = prop1.Value is null ? default : prop1.Value;
+        _value2 = prop2.Value is null ? default : prop2.Value;
+        
+        prop1.Subscribe(async (value1, token) =>
+        {
+            _value1 = value1;
+            await UpdateAsync(token);
+        });
+        prop2.Subscribe(async (value2, token) =>
+        {
+            _value2 = value2;
+            await UpdateAsync(token);
+        });
+    }
+
+    private async Task UpdateAsync(CancellationToken cancellationToken = default)
+    {
+        var result = await _func(_value1, _value2);
+        await SetNewValueAsync(result, cancellationToken);
+    }
+}