Implement full subscription flow

osu.Game/Database/RealmContextFactory.cs

@@ -2,6 +2,8 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Collections.Generic;
+using System.Diagnostics;
 using System.IO;
 using System.Linq;
 using System.Reflection;
@@ -80,6 +82,10 @@ public Realm Context
                     {
                         context = createContext();
                         Logger.Log(@$"Opened realm ""{context.Config.DatabasePath}"" at version {context.Config.SchemaVersion}");
+
+                        // Resubscribe any subscriptions
+                        foreach (var action in subscriptionActions.Keys)
+                            registerSubscription(action);
                     }
 
                     // creating a context will ensure our schema is up-to-date and migrated.
@@ -226,26 +232,42 @@ public void Write(Action<Realm> action)
             }
         }
 
+        private readonly Dictionary<Func<Realm, IDisposable?>, IDisposable?> subscriptionActions = new Dictionary<Func<Realm, IDisposable?>, IDisposable?>();
+
         /// <summary>
         /// Run work on realm that will be run every time the update thread realm context gets recycled.
         /// </summary>
-        /// <param name="action">The work to run.</param>
-        public void Register(Action<Realm> action)
+        /// <param name="action">The work to run. Return value should be an <see cref="IDisposable"/> from QueryAsyncWithNotifications, or an <see cref="InvokeOnDisposal"/> to clean up any bindings.</param>
+        /// <returns>An <see cref="IDisposable"/> which should be disposed to unsubscribe any inner subscription.</returns>
+        public IDisposable Register(Func<Realm, IDisposable?> action)
         {
-            if (!ThreadSafety.IsUpdateThread && context != null)
-                throw new InvalidOperationException(@$"{nameof(BlockAllOperations)} must be called from the update thread.");
+            if (!ThreadSafety.IsUpdateThread)
+                throw new InvalidOperationException(@$"{nameof(Register)} must be called from the update thread.");
 
-            if (ThreadSafety.IsUpdateThread)
+            subscriptionActions.Add(action, null);
+            registerSubscription(action);
+
+            return new InvokeOnDisposal(() =>
             {
-                current_thread_subscriptions_allowed.Value = true;
-                action(Context);
-                current_thread_subscriptions_allowed.Value = false;
-            }
-            else
+                // TODO: this likely needs to be run on the update thread.
+                if (subscriptionActions.TryGetValue(action, out var unsubscriptionAction))
+                {
+                    unsubscriptionAction?.Dispose();
+                    subscriptionActions.Remove(action);
+                }
+            });
+        }
+
+        private void registerSubscription(Func<Realm, IDisposable?> action)
+        {
+            Debug.Assert(ThreadSafety.IsUpdateThread);
+
+            lock (contextLock)
             {
+                Debug.Assert(context != null);
+
                 current_thread_subscriptions_allowed.Value = true;
-                using (var realm = createContext())
-                    action(realm);
+                subscriptionActions[action] = action(context);
                 current_thread_subscriptions_allowed.Value = false;
             }
         }