RPG world mod: tile maps, grid walking, map events, battles

Maps of square tiles in three layers plus a collision layer, mounted over
the 2D shell when their scene is entered. WASD walking, E/F action events,
touch teleports, story-flag conditions, random encounters, rpg.battle and
rpg.teleport actions, position saving. Integration suite rpg_walk.

src/RpgBattleService.cs

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+using Cthangover.Core.Battle;
+using Cthangover.Core.Events;
+using Cthangover.Core.Scenes;
+using Cthangover.Core.Utils;
+using Godot;
+
+namespace Cthangover.Mods.Rpg
+{
+    /// <summary>
+    /// Starts a battle of the active battle core from the map and brings the
+    /// player back to the same map cell afterwards. It is a thin layer over the
+    /// platform (<see cref="BattleService.Init"/> + the <c>Battle</c> route):
+    /// <list type="bullet">
+    /// <item>the return scene is the current map's scene, and the position is
+    /// already in <see cref="RpgState"/>, so the world restores it;</item>
+    /// <item>the backdrop is the map scene's default background (captured by
+    /// the platform from the scene), so a map chooses its battle background in
+    /// <c>scenes/&lt;map&gt;.json</c>;</item>
+    /// <item>an optional win flag becomes the story flag "1" on victory — how a
+    /// map monster disappears after it is beaten (its event's condition).</item>
+    /// </list>
+    /// </summary>
+    public static class RpgBattleService
+    {
+        private const string BattleRoute = "Battle";
+        private const string VictoryResult = "victory";
+
+        /// <summary>
+        /// Requests the encounter and switches to the battle route on the next
+        /// frame (deferred like <c>switch_scene</c>, so a running dialog finishes
+        /// its current command first). Returns false when there is no scene to come back to.
+        /// </summary>
+        public static bool Start(string enemies, string winFlag)
+        {
+            var sceneManager = FindSceneManager();
+            var returnScene = sceneManager?.CurrentSceneName;
+            if (string.IsNullOrEmpty(returnScene) || string.IsNullOrWhiteSpace(enemies))
+            {
+                GameLogger.Log("RPG", $"battle not started: scene='{returnScene}', enemies='{enemies}'", LogLevel.Error);
+                return false;
+            }
+
+            RpgState.Instance.PendingWinFlag = string.IsNullOrWhiteSpace(winFlag) ? null : winFlag.Trim();
+            BattleService.Init(returnScene, enemies.Replace(" ", string.Empty));
+            GameLogger.Log("RPG", $"battle '{enemies}' from '{returnScene}' (win flag: {RpgState.Instance.PendingWinFlag ?? "none"})");
+            Callable.From(() => sceneManager.SwitchScene(BattleRoute)).CallDeferred();
+            return true;
+        }
+
+        /// <summary>Applies the pending win flag of the battle that just ended.</summary>
+        public static void OnBattleEnded(BattleEndedEvent evt)
+        {
+            var flag = RpgState.Instance.PendingWinFlag;
+            RpgState.Instance.PendingWinFlag = null;
+            if (flag != null && evt?.Result == VictoryResult)
+                ScenarioCondition.SetFlag(flag, "1");
+        }
+
+        private static SceneManager FindSceneManager()
+        {
+            var tree = Engine.GetMainLoop() as SceneTree;
+            return tree?.Root?.GetNodeOrNull<SceneManager>("SceneManager");
+        }
+    }
+}