Adds optional useReplaceForUpdates to SortedObservableCollectionAdaptor

src/DynamicData.Tests/Binding/ObservableCollectionBindCacheSortedFixture.cs

@@ -196,22 +196,42 @@ public void UpdateToSourceSendsRemoveAndAddIfSortingIsAffected()
     [Fact]
     public void UpdateToSourceSendsReplaceIfSortingIsNotAffected()
     {
-        var person1 = new Person("Adult1", 10);
-        var person2 = new Person("Adult2", 11);
-
-        NotifyCollectionChangedAction action = default;
-        _source.AddOrUpdate(person1);
-        _source.AddOrUpdate(person2);
+        RunTest(true);
+        RunTest(false);
 
-        var person2Updated = new Person("Adult2", 12);
 
-        using (_collection.ObserveCollectionChanges().Select(change => change.EventArgs.Action).Subscribe(act => action = act))
+        void RunTest(bool useReplace)
         {
-            _source.AddOrUpdate(person2Updated);
-        }
+            var collection = new ObservableCollectionExtended<Person>();
+
+            using var source = new SourceCache<Person, string>(p => p.Name);
+            using var binder = source.Connect().Sort(_comparer, resetThreshold: 25).Bind(collection, new ObservableCollectionAdaptor<Person, string>(useReplaceForUpdates: useReplace)).Subscribe();
+
+            var person1 = new Person("Adult1", 10);
+            var person2 = new Person("Adult2", 11);
+
+            NotifyCollectionChangedAction action = default;
+            source.AddOrUpdate(person1);
+            source.AddOrUpdate(person2);
+
+            var person2Updated = new Person("Adult2", 12);
 
-        action.Should().Be(NotifyCollectionChangedAction.Replace, "The notification type should be Replace");
-        _collection.Should().Equal(person1, person2Updated);
+            using (collection.ObserveCollectionChanges().Select(x => x.EventArgs.Action).Subscribe(updateType => action = updateType))
+            {
+                source.AddOrUpdate(person2Updated);
+            }
+
+            if (useReplace)
+            {
+                action.Should().Be(NotifyCollectionChangedAction.Replace, "The notification type should be Replace");
+            }
+            else
+            {
+                action.Should().Be(NotifyCollectionChangedAction.Add, "The notification type should be Add");
+            }
+
+            collection.Should().Equal(person1, person2Updated);
+        }
     }
 
     [Fact]

src/DynamicData/Binding/SortedObservableCollectionAdaptor.cs

@@ -18,14 +18,17 @@ public class SortedObservableCollectionAdaptor<TObject, TKey> : ISortedObservabl
     where TKey : notnull
 {
     private readonly int _refreshThreshold;
+    private readonly bool _useReplaceForUpdates;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="SortedObservableCollectionAdaptor{TObject, TKey}"/> class.
     /// </summary>
     /// <param name="refreshThreshold">The number of changes before a Reset event is used.</param>
-    public SortedObservableCollectionAdaptor(int refreshThreshold = 25)
+    /// <param name="useReplaceForUpdates"> Use replace instead of remove / add for updates. </param>
+    public SortedObservableCollectionAdaptor(int refreshThreshold = 25, bool useReplaceForUpdates = true)
     {
         _refreshThreshold = refreshThreshold;
+        _useReplaceForUpdates = useReplaceForUpdates;
     }
 
     /// <summary>
@@ -89,7 +92,7 @@ public void Adapt(ISortedChangeSet<TObject, TKey> changes, IObservableCollection
         }
     }
 
-    private static void DoUpdate(ISortedChangeSet<TObject, TKey> updates, IObservableCollection<TObject> list)
+    private void DoUpdate(ISortedChangeSet<TObject, TKey> updates, IObservableCollection<TObject> list)
     {
         foreach (var update in updates)
         {
@@ -108,7 +111,7 @@ private static void DoUpdate(ISortedChangeSet<TObject, TKey> updates, IObservabl
                     break;
 
                 case ChangeReason.Update:
-                    if (update.PreviousIndex != update.CurrentIndex)
+                    if (!_useReplaceForUpdates || update.PreviousIndex != update.CurrentIndex)
                     {
                         list.RemoveAt(update.PreviousIndex);
                         list.Insert(update.CurrentIndex, update.Current);

src/DynamicData/Cache/ObservableCacheEx.cs

@@ -745,10 +745,11 @@
     /// <param name="source">The source.</param>
     /// <param name="readOnlyObservableCollection">The resulting read only observable collection.</param>
     /// <param name="resetThreshold">The number of changes before a reset event is called on the observable collection.</param>
+    /// <param name="useReplaceForUpdates"> Use replace instead of remove / add for updates.  NB: Some platforms to not support replace notifications for binding.</param>
     /// <param name="adaptor">Specify an adaptor to change the algorithm to update the target collection.</param>
     /// <returns>An observable which will emit change sets.</returns>
     /// <exception cref="System.ArgumentNullException">source.</exception>
-    public static IObservable<IChangeSet<TObject, TKey>> Bind<TObject, TKey>(this IObservable<ISortedChangeSet<TObject, TKey>> source, out ReadOnlyObservableCollection<TObject> readOnlyObservableCollection, int resetThreshold = 25, ISortedObservableCollectionAdaptor<TObject, TKey>? adaptor = null)
+    public static IObservable<IChangeSet<TObject, TKey>> Bind<TObject, TKey>(this IObservable<ISortedChangeSet<TObject, TKey>> source, out ReadOnlyObservableCollection<TObject> readOnlyObservableCollection, int resetThreshold = 25, bool useReplaceForUpdates = true, ISortedObservableCollectionAdaptor<TObject, TKey>? adaptor = null)
         where TObject : notnull
         where TKey : notnull
     {
@@ -759,7 +760,7 @@
 
         var target = new ObservableCollectionExtended<TObject>();
         var result = new ReadOnlyObservableCollection<TObject>(target);
-        var updater = adaptor ?? new SortedObservableCollectionAdaptor<TObject, TKey>(resetThreshold);
+        var updater = adaptor ?? new SortedObservableCollectionAdaptor<TObject, TKey>(resetThreshold, useReplaceForUpdates);
         readOnlyObservableCollection = result;
         return source.Bind(target, updater);
     }