Generalized Solution for avoiding Out-of-Order Cache Child ChangeSets

src/DynamicData.Tests/Cache/MergeManyChangeSetsCacheFixture.cs

@@ -811,6 +811,47 @@ public void MergeManyChangeSetsWorksCorrectlyWithValueTypes()
         results.Summary.Overall.Removes.Should().Be(PricesPerMarket);
     }
 
+
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void OrderOfChangesIsPreserved(bool removeFirst)
+    {
+        // Arrange
+        var markets = Enumerable.Range(0, MarketCount).Select(n => new Market(n)).ToArray();
+        AddUniquePrices(markets);
+        _marketCache.AddOrUpdate(markets);
+        var markets2 = Enumerable.Range(0, MarketCount).Select(n => new Market(n)).ToArray();
+        AddUniquePrices(markets2);
+        using var results = _marketCache.Connect().MergeManyChangeSets(m => m.LatestPrices, MarketPrice.EqualityComparer).AsAggregator();
+        (var firstReason, var nextReason, int expectedChanges) = removeFirst 
+            ? (ChangeReason.Remove, ChangeReason.Add, 2 * MarketCount * PricesPerMarket)
+            : (ChangeReason.Add, ChangeReason.Remove, 3 * MarketCount * PricesPerMarket);
+
+        // Act
+        _marketCache.Edit(updater =>
+        {
+            if (removeFirst)
+            {
+                updater.Clear();
+                updater.AddOrUpdate(markets2);
+            }
+            else
+            {
+
+                updater.AddOrUpdate(markets2);
+                updater.Clear();
+            }
+        });
+
+        // Assert
+        results.Messages.Count.Should().Be(2);
+        results.Messages[0].All(change => change.Reason is ChangeReason.Add).Should().BeTrue();
+        results.Messages[1].Count.Should().Be(expectedChanges);
+        results.Messages[1].Take(MarketCount * PricesPerMarket).All(change => change.Reason == firstReason).Should().BeTrue();
+        results.Messages[1].Skip(MarketCount * PricesPerMarket).All(change => change.Reason == nextReason).Should().BeTrue();
+    }
+
     public void Dispose()
     {
         _marketCacheResults.Dispose();

src/DynamicData/Cache/Internal/ChangeSetCache.cs

@@ -16,7 +16,7 @@ internal sealed class ChangeSetCache<TObject, TKey>
     where TKey : notnull
 {
     public ChangeSetCache(IObservable<IChangeSet<TObject, TKey>> source) =>
-        Source = source.IgnoreSameReferenceUpdate().Do(Cache.Clone);
+        Source = source.Do(Cache.Clone);
 
     public Cache<TObject, TKey> Cache { get; } = new();
 

src/DynamicData/Cache/Internal/DynamicGrouper.cs

@@ -44,37 +44,44 @@ public void ProcessChangeSet(IChangeSet<TObject, TKey> changeSet, IObserver<IGro
 
         foreach (var change in changeSet.ToConcreteType())
         {
-            switch (change.Reason)
-            {
-                case ChangeReason.Add when _groupSelector is not null:
-                    PerformAddOrUpdate(change.Key, _groupSelector(change.Current, change.Key), change.Current, suspendTracker);
-                    break;
+            ProcessChange(change, suspendTracker);
+        }
+
+        if (observer != null)
+        {
+            EmitChanges(observer);
+        }
+    }
 
-                case ChangeReason.Remove:
-                    PerformRemove(change.Key, suspendTracker);
-                    break;
+    public void ProcessChange(Change<TObject, TKey> change) => ProcessChange(change, _suspendTracker);
 
-                case ChangeReason.Update when _groupSelector is not null:
-                    PerformAddOrUpdate(change.Key, _groupSelector(change.Current, change.Key), change.Current, suspendTracker);
-                    break;
+    private void ProcessChange(Change<TObject, TKey> change, SuspendTracker? suspendTracker)
+    {
+        switch (change.Reason)
+        {
+            case ChangeReason.Add when _groupSelector is not null:
+                PerformAddOrUpdate(change.Key, _groupSelector(change.Current, change.Key), change.Current, suspendTracker);
+                break;
 
-                case ChangeReason.Update:
-                    PerformUpdate(change.Key, suspendTracker);
-                    break;
+            case ChangeReason.Remove:
+                PerformRemove(change.Key, suspendTracker);
+                break;
 
-                case ChangeReason.Refresh when _groupSelector is not null:
-                    PerformRefresh(change.Key, _groupSelector(change.Current, change.Key), change.Current, suspendTracker);
-                    break;
+            case ChangeReason.Update when _groupSelector is not null:
+                PerformAddOrUpdate(change.Key, _groupSelector(change.Current, change.Key), change.Current, suspendTracker);
+                break;
 
-                case ChangeReason.Refresh:
-                    PerformRefresh(change.Key, suspendTracker);
-                    break;
-            }
-        }
+            case ChangeReason.Update:
+                PerformUpdate(change.Key, suspendTracker);
+                break;
 
-        if (observer != null)
-        {
-            EmitChanges(observer);
+            case ChangeReason.Refresh when _groupSelector is not null:
+                PerformRefresh(change.Key, _groupSelector(change.Current, change.Key), change.Current, suspendTracker);
+                break;
+
+            case ChangeReason.Refresh:
+                PerformRefresh(change.Key, suspendTracker);
+                break;
         }
     }
 

src/DynamicData/Cache/Internal/GroupOnObservable.cs

@@ -2,7 +2,6 @@
 // Roland Pheasant licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
 
-using System.Reactive.Disposables;
 using System.Reactive.Linq;
 using DynamicData.Internal;
 
@@ -13,49 +12,58 @@ internal sealed class GroupOnObservable<TObject, TKey, TGroupKey>(IObservable<IC
     where TKey : notnull
     where TGroupKey : notnull
 {
-    public IObservable<IGroupChangeSet<TObject, TKey, TGroupKey>> Run() => Observable.Create<IGroupChangeSet<TObject, TKey, TGroupKey>>(observer =>
+    public IObservable<IGroupChangeSet<TObject, TKey, TGroupKey>> Run() =>
+        Observable.Create<IGroupChangeSet<TObject, TKey, TGroupKey>>(observer => new Subscription(source, selectGroup, observer));
+
+    // Maintains state for a single subscription
+    private sealed class Subscription : CacheParentSubscription<TObject, TKey, (TGroupKey, TObject), IGroupChangeSet<TObject, TKey, TGroupKey>>
     {
-        var grouper = new DynamicGrouper<TObject, TKey, TGroupKey>();
-        var locker = InternalEx.NewLock();
-        var parentUpdate = false;
-
-        IObservable<TGroupKey> CreateGroupObservable(TObject item, TKey key) =>
-            selectGroup(item, key)
-                .DistinctUntilChanged()
-                .Synchronize(locker!)
-                .Do(
-                    onNext: groupKey => grouper!.AddOrUpdate(key, groupKey, item, !parentUpdate ? observer : null),
-                    onError: observer.OnError);
-
-        // Create a shared connection to the source
-        var shared = source
-            .Synchronize(locker)
-            .Do(_ => parentUpdate = true)
-            .Publish();
-
-        // First process the changesets
-        var subChanges = shared
-            .SubscribeSafe(
-                onNext: changeSet => grouper.ProcessChangeSet(changeSet),
-                onError: observer.OnError);
-
-        // Next process the Grouping observables created for each item
-        var subMergeMany = shared
-            .MergeMany(CreateGroupObservable)
-            .SubscribeSafe(
-                onError: observer.OnError,
-                onCompleted: observer.OnCompleted);
-
-        // Finally, emit the results
-        var subResults = shared
-            .SubscribeSafe(
-                onNext: _ =>
+        private readonly DynamicGrouper<TObject, TKey, TGroupKey> _grouper = new();
+        private readonly Func<TObject, TKey, IObservable<TGroupKey>> _selectGroup;
+
+        public Subscription(IObservable<IChangeSet<TObject, TKey>> source, Func<TObject, TKey, IObservable<TGroupKey>> selectGroup, IObserver<IGroupChangeSet<TObject, TKey, TGroupKey>> observer)
+            : base(observer)
+        {
+            _selectGroup = selectGroup;
+            CreateParentSubscription(source);
+        }
+
+        protected override void ParentOnNext(IChangeSet<TObject, TKey> changes)
+        {
+            // Process all the changes at once to preserve the changeset order
+            foreach (var change in changes.ToConcreteType())
+            {
+                _grouper.ProcessChange(change);
+
+                switch (change.Reason)
                 {
-                    grouper.EmitChanges(observer);
-                    parentUpdate = false;
-                },
-                onError: observer.OnError);
+                    // Shutdown existing sub (if any) and create a new one that
+                    // Will update the group key for the current item
+                    case ChangeReason.Add or ChangeReason.Update:
+                        AddGroupSubscription(change.Current, change.Key);
+                        break;
+
+                    // Shutdown the existing subscription
+                    case ChangeReason.Remove:
+                        RemoveChildSubscription(change.Key);
+                        break;
+                }
+            }
+        }
+
+        protected override void ChildOnNext((TGroupKey, TObject) tuple, TKey parentKey) =>
+            _grouper.AddOrUpdate(parentKey, tuple.Item1, tuple.Item2);
+
+        protected override void EmitChanges(IObserver<IGroupChangeSet<TObject, TKey, TGroupKey>> observer) =>
+            _grouper.EmitChanges(observer);
+
+        protected override void Dispose(bool disposing)
+        {
+            _grouper.Dispose();
+            base.Dispose(disposing);
+        }
 
-        return new CompositeDisposable(shared.Connect(), subMergeMany, subChanges, grouper);
-    });
+        private void AddGroupSubscription(TObject obj, TKey key) =>
+            AddChildSubscription(MakeChildObservable(_selectGroup(obj, key).DistinctUntilChanged().Select(groupKey => (groupKey, obj))), key);
+    }
 }

src/DynamicData/Cache/Internal/MergeChangeSets.cs

@@ -43,14 +43,14 @@ public IObservable<IChangeSet<TObject, TKey>> Run() => Observable.Create<IChange
     // Can optimize for the Add case because that's the only one that applies
 #if NET9_0_OR_GREATER
     private static Change<ChangeSetCache<TObject, TKey>, int> CreateChange(IObservable<IChangeSet<TObject, TKey>> source, int index, Lock locker) =>
-        new(ChangeReason.Add, index, new ChangeSetCache<TObject, TKey>(source.Synchronize(locker)));
+        new(ChangeReason.Add, index, new ChangeSetCache<TObject, TKey>(source.IgnoreSameReferenceUpdate().Synchronize(locker)));
 
     // Create a ChangeSet Observable that produces ChangeSets with a single Add event for each new sub-observable
     private static IObservable<IChangeSet<ChangeSetCache<TObject, TKey>, int>> CreateContainerObservable(IObservable<IObservable<IChangeSet<TObject, TKey>>> source, Lock locker) =>
         source.Select((src, index) => new ChangeSet<ChangeSetCache<TObject, TKey>, int>(new[] { CreateChange(src, index, locker) }));
 #else
     private static Change<ChangeSetCache<TObject, TKey>, int> CreateChange(IObservable<IChangeSet<TObject, TKey>> source, int index, object locker) =>
-        new(ChangeReason.Add, index, new ChangeSetCache<TObject, TKey>(source.Synchronize(locker)));
+        new(ChangeReason.Add, index, new ChangeSetCache<TObject, TKey>(source.IgnoreSameReferenceUpdate().Synchronize(locker)));
 
     // Create a ChangeSet Observable that produces ChangeSets with a single Add event for each new sub-observable
     private static IObservable<IChangeSet<ChangeSetCache<TObject, TKey>, int>> CreateContainerObservable(IObservable<IObservable<IChangeSet<TObject, TKey>>> source, object locker) =>

src/DynamicData/Cache/Internal/MergeManyCacheChangeSets.cs

@@ -2,7 +2,6 @@
 // Roland Pheasant licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
 
-using System.Reactive.Disposables;
 using System.Reactive.Linq;
 using DynamicData.Internal;
 
@@ -11,53 +10,68 @@ namespace DynamicData.Cache.Internal;
 /// <summary>
 /// Operator that is similiar to MergeMany but intelligently handles Cache ChangeSets.
 /// </summary>
-internal sealed class MergeManyCacheChangeSets<TObject, TKey, TDestination, TDestinationKey>(IObservable<IChangeSet<TObject, TKey>> source, Func<TObject, TKey, IObservable<IChangeSet<TDestination, TDestinationKey>>> selector, IEqualityComparer<TDestination>? equalityComparer, IComparer<TDestination>? comparer)
+internal sealed class MergeManyCacheChangeSets<TObject, TKey, TDestination, TDestinationKey>(IObservable<IChangeSet<TObject, TKey>> source, Func<TObject, TKey, IObservable<IChangeSet<TDestination, TDestinationKey>>> changeSetSelector, IEqualityComparer<TDestination>? equalityComparer, IComparer<TDestination>? comparer)
     where TObject : notnull
     where TKey : notnull
     where TDestination : notnull
     where TDestinationKey : notnull
 {
     public IObservable<IChangeSet<TDestination, TDestinationKey>> Run() => Observable.Create<IChangeSet<TDestination, TDestinationKey>>(
-        observer =>
+        observer => new Subscription(source, changeSetSelector, observer, equalityComparer, comparer));
+
+    // Maintains state for a single subscription
+    private sealed class Subscription : CacheParentSubscription<ChangeSetCache<TDestination, TDestinationKey>, TKey, IChangeSet<TDestination, TDestinationKey>, IChangeSet<TDestination, TDestinationKey>>
+    {
+        private readonly Cache<ChangeSetCache<TDestination, TDestinationKey>, TKey> _cache = new();
+        private readonly ChangeSetMergeTracker<TDestination, TDestinationKey> _changeSetMergeTracker;
+
+        public Subscription(
+            IObservable<IChangeSet<TObject, TKey>> source,
+            Func<TObject, TKey, IObservable<IChangeSet<TDestination, TDestinationKey>>> changeSetSelector,
+            IObserver<IChangeSet<TDestination, TDestinationKey>> observer,
+            IEqualityComparer<TDestination>? equalityComparer,
+            IComparer<TDestination>? comparer)
+            : base(observer)
+        {
+            _changeSetMergeTracker = new(() => _cache.Items, comparer, equalityComparer);
+
+            // Child Observable has to go into the ChangeSetCache so the locking protects it
+            CreateParentSubscription(source.Transform((obj, key) =>
+                new ChangeSetCache<TDestination, TDestinationKey>(MakeChildObservable(changeSetSelector(obj, key).IgnoreSameReferenceUpdate()))));
+        }
+
+    protected override void ParentOnNext(IChangeSet<ChangeSetCache<TDestination, TDestinationKey>, TKey> changes)
         {
-            var locker = InternalEx.NewLock();
-            var cache = new Cache<ChangeSetCache<TDestination, TDestinationKey>, TKey>();
-            var parentUpdate = false;
-
-            // This is manages all of the changes
-            var changeTracker = new ChangeSetMergeTracker<TDestination, TDestinationKey>(() => cache.Items, comparer, equalityComparer);
-
-            // Transform to a cache changeset of child caches, synchronize, update the local copy, and publish.
-            var shared = source
-                .Transform((obj, key) => new ChangeSetCache<TDestination, TDestinationKey>(selector(obj, key).Synchronize(locker)))
-                .Synchronize(locker)
-                .Do(changes =>
+            // Process all the changes at once to preserve the changeset order
+            foreach (var change in changes.ToConcreteType())
+            {
+                switch (change.Reason)
                 {
-                    cache.Clone(changes);
-                    parentUpdate = true;
-                })
-                .Publish();
-
-            // Merge the child changeset changes together and apply to the tracker
-            var subMergeMany = shared
-                .MergeMany(cacheChangeSet => cacheChangeSet.Source)
-                .SubscribeSafe(
-                    changes => changeTracker.ProcessChangeSet(changes, !parentUpdate ? observer : null),
-                    observer.OnError,
-                    observer.OnCompleted);
-
-            // When a source item is removed, all of its sub-items need to be removed
-            var subRemove = shared
-                .OnItemRemoved(changeSetCache => changeTracker.RemoveItems(changeSetCache.Cache.KeyValues), invokeOnUnsubscribe: false)
-                .OnItemUpdated((_, prev) => changeTracker.RemoveItems(prev.Cache.KeyValues))
-                .SubscribeSafe(
-                    _ =>
-                    {
-                        changeTracker.EmitChanges(observer);
-                        parentUpdate = false;
-                    },
-                    observer.OnError);
-
-            return new CompositeDisposable(shared.Connect(), subMergeMany, subRemove);
-        });
+                    // Shutdown existing sub (if any) and create a new one that
+                    // Will update the cache and emit the changes
+                    case ChangeReason.Add or ChangeReason.Update:
+                        _cache.AddOrUpdate(change.Current, change.Key);
+                        AddChildSubscription(change.Current.Source, change.Key);
+                        if (change.Previous.HasValue)
+                        {
+                            _changeSetMergeTracker.RemoveItems(change.Previous.Value.Cache.KeyValues);
+                        }
+                        break;
+
+                    // Shutdown the existing subscription and remove from the cache
+                    case ChangeReason.Remove:
+                        _cache.Remove(change.Key);
+                        RemoveChildSubscription(change.Key);
+                        _changeSetMergeTracker.RemoveItems(change.Current.Cache.KeyValues);
+                        break;
+                }
+            }
+        }
+
+        protected override void ChildOnNext(IChangeSet<TDestination, TDestinationKey> changes, TKey parentKey) =>
+            _changeSetMergeTracker.ProcessChangeSet(changes, null);
+
+        protected override void EmitChanges(IObserver<IChangeSet<TDestination, TDestinationKey>> observer) =>
+            _changeSetMergeTracker.EmitChanges(observer);
+    }
 }