Use IDeclarativeProperty instead of IObservable

src/Library/DeclarativeProperty/CombineAllProperty.cs

@@ -0,0 +1,36 @@
+namespace DeclarativeProperty;
+
+public class CombineAllProperty<T, TResult> : DeclarativePropertyBase<TResult>
+{
+    private readonly List<IDeclarativeProperty<T>> _sources;
+    private readonly Func<IEnumerable<T>, Task<TResult?>> _combiner;
+
+    public CombineAllProperty(
+        IEnumerable<IDeclarativeProperty<T>> sources,
+        Func<IEnumerable<T>, Task<TResult?>> combiner,
+        Action<TResult?>? setValueHook = null) : base(setValueHook)
+    {
+        var sourcesList = sources.ToList();
+        _sources = sourcesList;
+        _combiner = combiner;
+
+        var initialValueTask = _combiner(sourcesList.Select(p => p.Value));
+        initialValueTask.Wait();
+        SetNewValueSync(initialValueTask.Result);
+
+        foreach (var declarativeProperty in sourcesList)
+        {
+            AddDisposable(declarativeProperty.Subscribe(OnSourceChanged));
+        }
+    }
+
+    private async Task OnSourceChanged(T? arg1, CancellationToken arg2) => await Update();
+
+    private async Task Update()
+    {
+        var values = _sources.Select(p => p.Value);
+        var result = await _combiner(values);
+
+        await SetNewValueAsync(result);
+    }
+}

src/Library/DeclarativeProperty/DeclarativePropertyExtensions.cs

@@ -93,4 +93,10 @@ public static class DeclarativePropertyExtensions
 
     public static IDeclarativeProperty<T> Switch<T>(this IDeclarativeProperty<IDeclarativeProperty<T>?> from)
         => new SwitchProperty<T>(from);
+    
+    public static IDeclarativeProperty<TResult?> CombineAll<T, TResult>(
+        this IEnumerable<IDeclarativeProperty<T>> sources,
+        Func<IEnumerable<T>, Task<TResult>> combiner,
+        Action<TResult?>? setValueHook = null)
+        => new CombineAllProperty<T,TResult?>(sources, combiner, setValueHook);
 }