Ensure realm subscriptions always fire initial callback with null ChangeSet

osu.Game.Tests/Database/RealmSubscriptionRegistrationTests.cs

@@ -71,6 +71,35 @@ void onChanged(IRealmCollection<BeatmapSetInfo> sender, ChangeSet? changes)
  }
  }
 
+ [Test]
+ public void TestSubscriptionInitialChangeSetNull()
+ {
+ ChangeSet? firstChanges = null;
+ int receivedChangesCount = 0;
+
+ RunTestWithRealm((realm, _) =>
+ {
+ var registration = realm.RegisterForNotifications(r => r.All<BeatmapSetInfo>(), onChanged);
+
+ realm.WriteAsync(r => r.Add(TestResources.CreateTestBeatmapSetInfo())).WaitSafely();
+
+ realm.Run(r => r.Refresh());
+
+ Assert.That(receivedChangesCount, Is.EqualTo(1));
+ Assert.That(firstChanges, Is.Null);
+
+ registration.Dispose();
+ });
+
+ void onChanged(IRealmCollection<BeatmapSetInfo> sender, ChangeSet? changes)
+ {
+ if (receivedChangesCount == 0)
+ firstChanges = changes;
+
+ receivedChangesCount++;
+ }
+ }
+
  [Test]
  public void TestSubscriptionWithAsyncWrite()
  {

osu.Game/Database/RealmObjectExtensions.cs

@@ -65,7 +65,8 @@ public static class RealmObjectExtensions
  if (!d.Beatmaps.Contains(existingBeatmap))
  {
  Debug.Fail("Beatmaps should never become detached under normal circumstances. If this ever triggers, it should be investigated further.");
- Logger.Log("WARNING: One of the difficulties in a beatmap was detached from its set. Please save a copy of logs and report this to devs.", LoggingTarget.Database, LogLevel.Important);
+ Logger.Log("WARNING: One of the difficulties in a beatmap was detached from its set. Please save a copy of logs and report this to devs.", LoggingTarget.Database,
+ LogLevel.Important);
  d.Beatmaps.Add(existingBeatmap);
  }
 
@@ -291,7 +292,24 @@ public static IDisposable QueryAsyncWithNotifications<T>(this IRealmCollection<T
  if (!RealmAccess.CurrentThreadSubscriptionsAllowed)
  throw new InvalidOperationException($"Make sure to call {nameof(RealmAccess)}.{nameof(RealmAccess.RegisterForNotifications)}");
 
- return collection.SubscribeForNotifications(callback);
+ bool initial = true;
+ return collection.SubscribeForNotifications(((sender, changes) =>
+ {
+ if (initial)
+ {
+ initial = false;
+
+ // Realm might coalesce the initial callback, meaning we never receive a `ChangeSet` of `null` marking the first callback.
+ // Let's decouple it for simplicity in handling.
+ if (changes != null)
+ {
+ callback(sender, null);
+ return;
+ }
+ }
+
+ callback(sender, changes);
+ }));
  }
 
  /// <summary>