Add notification flow for user state changes in room

osu.Game/Online/RealtimeMultiplayer/IMultiplayerServer.cs

@@ -30,5 +30,12 @@ namespace osu.Game.Online.RealtimeMultiplayer
         /// </summary>
         /// <param name="settings">The new settings to apply.</param>
         Task ChangeSettings(MultiplayerRoomSettings settings);
+
+        /// <summary>
+        /// Change the local user state in the currently joined room.
+        /// </summary>
+        /// <param name="newState">The proposed new state.</param>
+        /// <exception cref="InvalidStateChange">If the state change requested is not valid, given the previous state or room state.</exception>
+        Task ChangeState(MultiplayerUserState newState);
     }
 }

osu.Game/Online/RealtimeMultiplayer/ISpectatorClient.cs

@@ -39,5 +39,12 @@ namespace osu.Game.Online.RealtimeMultiplayer
         /// </summary>
         /// <param name="newSettings">The updated room settings.</param>
         Task SettingsChanged(MultiplayerRoomSettings newSettings);
+
+        /// <summary>
+        /// Signals that a user in this room changed their state.
+        /// </summary>
+        /// <param name="userId">The ID of the user performing a state change.</param>
+        /// <param name="state">The new state of the user.</param>
+        Task UserStateChanged(long userId, MultiplayerUserState state);
     }
 }

osu.Game/Online/RealtimeMultiplayer/InvalidStateChange.cs

@@ -0,0 +1,15 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+
+namespace osu.Game.Online.RealtimeMultiplayer
+{
+    public class InvalidStateChange : Exception
+    {
+        public InvalidStateChange(MultiplayerUserState oldState, MultiplayerUserState newState)
+            : base($"Cannot change from {oldState} to {newState}")
+        {
+        }
+    }
+}