Adds shoddy death match system.

It barely even works but oh well.

Content.Server/GameTicking/GameRule.cs

@@ -1,7 +1,18 @@
+using JetBrains.Annotations;
+
 namespace Content.Server.GameTicking
 {
-    public class GameRule
+    [PublicAPI]
+    public abstract class GameRule
     {
-        
+        public virtual void Added()
+        {
+
+        }
+
+        public virtual void Removed()
+        {
+
+        }
     }
 }

Content.Server/GameTicking/GameRules/RuleDeathMatch.cs

@@ -1,7 +1,112 @@
+using System;
+using System.Threading;
+using Content.Server.GameObjects;
+using Content.Server.Interfaces.Chat;
+using Content.Server.Interfaces.GameTicking;
+using Robust.Server.Interfaces.Player;
+using Robust.Server.Player;
+using Robust.Shared.Enums;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Interfaces.GameObjects;
+using Robust.Shared.IoC;
+using Timer = Robust.Shared.Timers.Timer;
+
 namespace Content.Server.GameTicking.GameRules
 {
-    public class DeathMatch
+    /// <summary>
+    ///     Simple GameRule that will do a free-for-all death match.
+    ///     Kill everybody else to win.
+    /// </summary>
+    public sealed class RuleDeathMatch : GameRule, IEntityEventSubscriber
     {
-        
+        private static readonly TimeSpan DeadCheckDelay = TimeSpan.FromSeconds(5);
+
+#pragma warning disable 649
+        [Dependency] private readonly IPlayerManager _playerManager;
+        [Dependency] private readonly IEntityManager _entityManager;
+        [Dependency] private readonly IChatManager _chatManager;
+        [Dependency] private readonly IGameTicker _gameTicker;
+#pragma warning restore 649
+
+        private CancellationTokenSource _checkTimerCancel;
+
+        public override void Added()
+        {
+            _chatManager.DispatchServerAnnouncement("The game is now a death match. Kill everybody else to win!");
+
+            _entityManager.SubscribeEvent<MobDamageStateChangedMessage>(_onMobDamageStateChanged, this);
+            _playerManager.PlayerStatusChanged += PlayerManagerOnPlayerStatusChanged;
+        }
+
+        public override void Removed()
+        {
+            base.Removed();
+
+            _entityManager.UnsubscribeEvent<MobDamageStateChangedMessage>(this);
+            _playerManager.PlayerStatusChanged -= PlayerManagerOnPlayerStatusChanged;
+        }
+
+        private void _onMobDamageStateChanged(object sender, MobDamageStateChangedMessage message)
+        {
+            _runDelayedCheck();
+        }
+
+        private void _checkForWinner()
+        {
+            _checkTimerCancel = null;
+
+            IPlayerSession winner = null;
+            foreach (var playerSession in _playerManager.GetAllPlayers())
+            {
+                if (playerSession.AttachedEntity == null
+                    || !playerSession.AttachedEntity.TryGetComponent(out SpeciesComponent species))
+                {
+                    continue;
+                }
+
+                if (!species.CurrentDamageState.IsConscious)
+                {
+                    continue;
+                }
+
+                if (winner != null)
+                {
+                    // Found a second person alive, nothing decided yet!
+                    return;
+                }
+
+                winner = playerSession;
+            }
+
+            if (winner == null)
+            {
+                _chatManager.DispatchServerAnnouncement("Everybody is dead, it's a stalemate!");
+            }
+            else
+            {
+                // We have a winner!
+                _chatManager.DispatchServerAnnouncement($"{winner} wins the death match!");
+            }
+
+            _chatManager.DispatchServerAnnouncement($"Restarting in 10 seconds.");
+
+            Timer.Spawn(TimeSpan.FromSeconds(10), () => _gameTicker.RestartRound());
+        }
+
+        private void PlayerManagerOnPlayerStatusChanged(object sender, SessionStatusEventArgs e)
+        {
+            if (e.NewStatus == SessionStatus.Disconnected)
+            {
+                _runDelayedCheck();
+            }
+        }
+
+        private void _runDelayedCheck()
+        {
+            _checkTimerCancel?.Cancel();
+            _checkTimerCancel = new CancellationTokenSource();
+
+            Timer.Spawn(DeadCheckDelay, _checkForWinner, _checkTimerCancel.Token);
+        }
     }
 }

Content.Server/GameTicking/GameTicker.cs

@@ -79,6 +79,8 @@ namespace Content.Server.GameTicking
 
         private readonly Random _spawnRandom = new Random();
 
+        [ViewVariables] private readonly List<GameRule> _gameRules = new List<GameRule>();
+
 #pragma warning disable 649
         [Dependency] private IEntityManager _entityManager;
         [Dependency] private IMapManager _mapManager;
@@ -88,6 +90,7 @@ namespace Content.Server.GameTicking
         [Dependency] private IPlayerManager _playerManager;
         [Dependency] private IChatManager _chatManager;
         [Dependency] private IServerNetManager _netManager;
+        [Dependency] private IDynamicTypeFactory _dynamicTypeFactory;
 #pragma warning restore 649
 
         public void Initialize()
@@ -150,7 +153,7 @@ namespace Content.Server.GameTicking
             RunLevel = GameRunLevel.InRound;
 
             // TODO: Allow other presets to be selected.
-            var preset = new PresetTraitor();
+            var preset = _dynamicTypeFactory.CreateInstance<PresetTraitor>();
             preset.Start();
 
             foreach (var (playerSession, ready) in _playersInLobby.ToList())
@@ -219,6 +222,30 @@ namespace Content.Server.GameTicking
             _netManager.ServerSendMessage(_getStatusMsg(player), player.ConnectedClient);
         }
 
+        public T AddGameRule<T>() where T : GameRule, new()
+        {
+            var instance = _dynamicTypeFactory.CreateInstance<T>();
+
+            _gameRules.Add(instance);
+            instance.Added();
+
+            return instance;
+        }
+
+        public void RemoveGameRule(GameRule rule)
+        {
+            if (_gameRules.Contains(rule))
+            {
+                return;
+            }
+
+            rule.Removed();
+
+            _gameRules.Remove(rule);
+        }
+
+        public IEnumerable<GameRule> ActiveGameRules => _gameRules;
+
         private IEntity _spawnPlayerMob()
         {
             var entity = _entityManager.ForceSpawnEntityAt(PlayerPrototypeName, _getLateJoinSpawnPoint());
@@ -290,6 +317,14 @@ namespace Content.Server.GameTicking
             {
                 unCastData.ContentData().WipeMind();
             }
+
+            // Clear up any game rules.
+            foreach (var rule in _gameRules)
+            {
+                rule.Removed();
+            }
+
+            _gameRules.Clear();
         }
 
         private void _preRoundSetup()