Fix physics engine error by queueing ItemDropService.Spawn() rather than queueing Die

Author: blake-hu

Entity/AttackRange.cs

@@ -40,6 +40,6 @@ private void OnBodyEntered(Node2D body)
 
         var entityType = predPreyEntity.EntityType();
         if (_predatorTypes.Contains(entityType))
-            _parentEntity.QueueDie();
+            _parentEntity.Die();
     }
 }

Entity/Instance/Cow.cs

@@ -17,6 +17,6 @@ public partial class Cow : PredatorPreyEntity<Cow>, IFreeze
     protected override void DieCustomLogic()
     {
         var beef = ItemProvider.Singleton.Get<Beef>();
-        ItemDropService.Singleton.Spawn(beef, 1, Position);
+        ItemDropService.Singleton.QueueSpawn(beef, 1, Position);
     }
 }

Entity/Instance/Player.cs

@@ -67,7 +67,7 @@ private void DetectWeaponHit()
                 return;
             if (collider is not IKillableEntity entity)
                 throw new System.Exception($"Player attacked collider ({collider}) that was not an IEntity");
-            entity.QueueDie();
+            entity.Die();
         }
     }
 

Entity/Instance/Tree.cs

@@ -13,21 +13,11 @@ public void RegisterEntityContainer(EntityContainer<Tree> container)
         _entityContainer = container;
     }
 
-    public void QueueDie()
-    {
-        _died = true;
-    }
-
-    private void Die()
+    public void Die()
     {
         if (!_entityContainer.TryRemoveEntity(this))
             throw new System.Exception(
                 $"Internal error: Unable to remove entity {Name} from entity container {_entityContainer.GetType()} on death.");
         QueueFree();
     }
-
-    public override void _PhysicsProcess(double delta)
-    {
-        if (_died) Die();
-    }
 }

Entity/PredatorPreyEntity.cs

@@ -63,16 +63,11 @@ public void RegisterEntityContainer(EntityContainer<TEntity> container)
     // - Spawning an ItemDrop involves instantiating an Area2D in the scene tree
     // - The Godot physics engine does not allow modifying an Area2D while in an event handler
     // - Hence, we need to queue the die request and only handle it once we exit the event handler
-    public void QueueDie()
-    {
-        _died = true;
-    }
-
-    private void Die()
+    public void Die()
     {
         DieCustomLogic();
         if (this is not TEntity derivedEntity)
-            throw new KsInvalidCastException(nameof(QueueDie), nameof(PredatorPreyEntity<TEntity>), nameof(TEntity),
+            throw new KsInvalidCastException(nameof(Die), nameof(PredatorPreyEntity<TEntity>), nameof(TEntity),
                 "Likely because entity inheritance hierarchy was not set up correctly.");
         if (!_entityContainer.TryRemoveEntity(derivedEntity))
             throw new System.Exception(
@@ -111,12 +106,6 @@ public override void _Ready()
 
     public override void _PhysicsProcess(double delta)
     {
-        if (_died)
-        {
-            Die();
-            return;
-        }
-
         if (_frozen) return;
 
         var move = Vector2.Zero;

Interface/IKillableEntity.cs

@@ -3,5 +3,5 @@ namespace KentingStation.Interface;
 // Represents in-game characters which can be killed, including friendly and hostile mobs
 public interface IKillableEntity
 {
-    public void QueueDie();
+    public void Die();
 }

Item/ItemDropService.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using Godot;
 using KentingStation.Common.Util;
 using KentingStation.Interface;
@@ -7,6 +8,7 @@ namespace KentingStation.Item;
 
 public partial class ItemDropService : Node2D
 {
+    private readonly List<ItemSpawnInfo> _spawnQueue = [];
     private PackedScene _itemDrop = ResourceLoader.Load<PackedScene>("res://Scene/ItemDrop.tscn");
 
     private ItemDropService()
@@ -24,15 +26,27 @@ public override void _Ready()
     // Called every frame. 'delta' is the elapsed time since the previous frame.
     public override void _Process(double delta)
     {
+        if (_spawnQueue.Count == 0)
+            return;
+        foreach (var itemInfo in _spawnQueue)
+            Spawn(itemInfo);
+        _spawnQueue.Clear();
     }
 
-    public void Spawn(IItem item, int count, Vector2 spawnLocation)
+    public void QueueSpawn(IItem item, int count, Vector2 spawnLocation)
+    {
+        _spawnQueue.Add(new ItemSpawnInfo(item, count, spawnLocation));
+    }
+
+    private void Spawn(ItemSpawnInfo itemInfo)
     {
         var itemDrop = _itemDrop.Instantiate<ItemDrop>();
-        itemDrop.SetItem(item, count);
+        itemDrop.SetItem(itemInfo.Item, itemInfo.Count);
 
-        var boundedSpawnLocation = Rect2Ex.ClosestPointWithin(WorldBoundary.Singleton.Boundary, spawnLocation);
+        var boundedSpawnLocation = Rect2Ex.ClosestPointWithin(WorldBoundary.Singleton.Boundary, itemInfo.SpawnLocation);
         itemDrop.Position = boundedSpawnLocation;
         Singleton.AddChild(itemDrop);
     }
+
+    private record struct ItemSpawnInfo(IItem Item, int Count, Vector2 SpawnLocation);
 }