Combine property WIP

src/Library/DeclarativeProperty/CombineProperty.cs

@@ -0,0 +1,44 @@
+namespace DeclarativeProperty;
+
+public sealed class CombineProperty<TFrom, TTo> : DeclarativePropertyBase<TTo>
+{
+    private readonly Func<IReadOnlyList<TFrom?>, Task<TTo>> _combiner;
+    private readonly List<IDeclarativeProperty<TFrom>> _sourceProperties = new();
+
+    public CombineProperty(Func<IReadOnlyList<TFrom?>, Task<TTo>> combiner)
+    {
+        _combiner = combiner;
+    }
+
+    public async Task AddSource(IDeclarativeProperty<TFrom> source)
+    {
+        if (_sourceProperties.Contains(source)) return;
+        _sourceProperties.Add(source);
+        source.Subscribe(OnSourceChanged);
+
+        await Update();
+    }
+
+    public async Task RemoveSource(IDeclarativeProperty<TFrom> source)
+    {
+        _sourceProperties.Remove(source);
+        source.Unsubscribe(OnSourceChanged);
+
+        await Update();
+    }
+
+    private async Task OnSourceChanged(TFrom? _, CancellationToken cancellationToken = default)
+        => await Update(cancellationToken);
+
+    private async Task Update(CancellationToken cancellationToken = default)
+    {
+        var result = await _combiner(
+            _sourceProperties
+                .Select(p => p.Value)
+                .ToList()
+                .AsReadOnly()
+        );
+
+        await SetNewValueAsync(result, cancellationToken);
+    }
+}