New reactive core WIP

src/Library/DeclarativeProperty/DistinctUntilChangedProperty.cs

@@ -0,0 +1,32 @@
+namespace DeclarativeProperty;
+
+public sealed class DistinctUntilChangedProperty<T> : DeclarativePropertyBase<T>
+{
+    private readonly Func<T?, T?, bool>? _comparer;
+    private bool _firstFire = true;
+
+    public DistinctUntilChangedProperty(IDeclarativeProperty<T> from, Func<T?, T?, bool>? comparer = null) : base(from.Value)
+    {
+        _comparer = comparer;
+        AddDisposable(from.Subscribe(Handle));
+    }
+
+    async Task Handle(T? next, CancellationToken cancellationToken = default)
+    {
+        if (_comparer is { } comparer)
+        {
+            if (comparer(Value, next))
+            {
+                return;
+            }
+        }
+        else if ((next is null && Value is null && !_firstFire)
+                 || (Value?.Equals(next) ?? false))
+        {
+            return;
+        }
+
+        _firstFire = false;
+        await SetNewValueAsync(next, cancellationToken);
+    }
+}