fix: universe entrance issues where entering behaviours adding more behaviours causing duplicate/missing calls

Engine.Core/Systems/UniverseEntranceManager.cs

@@ -11,14 +11,29 @@ public class UniverseEntranceManager : Internal.BehaviourIndependent
     private readonly ActiveBehaviourCollectorOrdered<int, IEnterUniverse> enterUniverses = new(GetPriority(), SortByAscendingPriority());
     private readonly ActiveBehaviourCollectorOrdered<int, IExitUniverse> exitUniverses = new(GetPriority(), SortByAscendingPriority());
 
+    private bool isInitialCollectionDone = false;
+    private readonly FastListOrdered<int, IEnterUniverse> toCallEnterUniverses = new(GetPriority(), SortByAscendingPriority());
+
     protected override void OnEnteredUniverse(IUniverse universe)
     {
-        // FIXME: This causes an issue when the UniverseEntranceManager is already attached to a UniverseObject then registered into a Universe,
-        // the enter/exit universe collectors call OnUniverseObjectRegistered internally on Assign, but since the Universe calls the OnUniverseObjectRegistered
-        // event it tries to call OnUniverseObjectRegistered again on the same object, causing a duplicate entry error.
-        Debug.Assert.AssertTrue(BehaviourController.Count == 1, $"{nameof(UniverseEntranceManager)} must be in it's own {nameof(IUniverseObject)} with no other {nameof(IBehaviour)}s attached at the moment. Failing to do so might cause instantiation or serialization issues.");
-        enterUniverses.Assign(universe);
         exitUniverses.Assign(universe);
+
+        // FIXME: the isInitialCollectionDone is for the sole reason of some behaviours 
+        // adding more behaviours during entrance calls and the internal workings of 
+        // behaviour collector not being able to tell which behaviour was already called 
+        // (because it just runs a for loop with the behaviour count, and priority ordering doesn't help as well) 
+        // so it sometimes double processes or misses behaviours. A more elegant way of 
+        // handling this would be nice but for now it works good enough.
+        //
+        // SIDE NOTE: This same issue has the potential to occur on exitUniverses as well, but I've yet to run 
+        // into an instance of it actually happening so... I'm not gonna touch it until the edge case happens.  
+        isInitialCollectionDone = false;
+        enterUniverses.Assign(universe);
+        isInitialCollectionDone = true;
+
+        for (int i = toCallEnterUniverses.Count - 1; i >= 0; i--)
+            toCallEnterUniverses[i].EnterUniverse(universe);
+        toCallEnterUniverses.Clear();
     }
 
     protected override void OnExitedUniverse(IUniverse universe)
@@ -26,8 +41,17 @@ public class UniverseEntranceManager : Internal.BehaviourIndependent
         enterUniverses.Unassign();
         exitUniverses.Unassign();
     }
+
     private void OnEnterUniverseCollected(IBehaviourCollector<IEnterUniverse> sender, IBehaviourCollector<IEnterUniverse>.BehaviourCollectedArguments args)
-        => args.BehaviourCollected.EnterUniverse(Universe);
+    {
+        if (!isInitialCollectionDone)
+        {
+            toCallEnterUniverses.Add(args.BehaviourCollected);
+            return;
+        }
+
+        args.BehaviourCollected.EnterUniverse(Universe);
+    }
 
     private void OnExitUniverseRemoved(IBehaviourCollector<IExitUniverse> sender, IBehaviourCollector<IExitUniverse>.BehaviourRemovedArguments args)
         => args.BehaviourRemoved.ExitUniverse(Universe);