Refactor Game Modes (#5857)

Content.Server/GameTicking/Rules/GameRuleSystem.cs

@@ -0,0 +1,61 @@
+using System;
+using JetBrains.Annotations;
+using Robust.Shared.GameObjects;
+using Robust.Shared.IoC;
+
+namespace Content.Server.GameTicking.Rules;
+
+[PublicAPI]
+public abstract class GameRuleSystem : EntitySystem
+{
+    [Dependency] protected GameTicker GameTicker = default!;
+
+    /// <summary>
+    ///     Whether this GameRule is currently enabled or not.
+    ///     Be sure to check this before doing anything rule-specific.
+    /// </summary>
+    public bool Enabled { get; protected set; } = false;
+
+    /// <summary>
+    ///     When the GameRule prototype with this ID is added, this system will be enabled.
+    ///     When it gets removed, this system will be disabled.
+    /// </summary>
+    public abstract string Prototype { get; }
+
+    public override void Initialize()
+    {
+        base.Initialize();
+
+        SubscribeLocalEvent<GameRuleAddedEvent>(OnGameRuleAdded);
+        SubscribeLocalEvent<GameRuleRemovedEvent>(OnGameRuleRemoved);
+
+    }
+
+    private void OnGameRuleAdded(GameRuleAddedEvent ev)
+    {
+        if (ev.Rule.ID != Prototype)
+            return;
+
+        Enabled = true;
+        Added();
+    }
+
+    private void OnGameRuleRemoved(GameRuleRemovedEvent ev)
+    {
+        if (ev.Rule.ID != Prototype)
+            return;
+
+        Enabled = false;
+        Removed();
+    }
+
+    /// <summary>
+    ///     Called when the game rule has been added and this system has been enabled.
+    /// </summary>
+    public abstract void Added();
+
+    /// <summary>
+    ///     Called when the game rule has been removed and this system has been disabled.
+    /// </summary>
+    public abstract void Removed();
+}