Trigger Colliders: adding/removing rigidbody/collider at runtime is unreliable or confusing

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I have a simple script:

   public class TrackingTrigger : MonoBehaviour
   {
   public List<Transform> Inside = new List<Transform>();
   void OnTriggerEnter(Collider other)
   {
      Debug.Log("Enter: ("+this+", "+ other+")");
      Inside.Add(other.transform);
   }
   void OnTriggerExit(Collider other)
   {
      Debug.Log("Exit: ("+this+", " + other+")");
      Inside.Remove(other.transform);
   }
   }

I Create the scene as follows:

   P: Parent GameObject 
   A: Child GameObject Under parent P
   B: Child GameObject Under parent P
   I add TrackingTrigger, SphereCollider, RigidBody in all three (P,A,B) and hit play. It works as expected: each Trigger has inside it all that collide.

1) Test Case: Non Symmetry

   a) Remove B.SphereCollider : OnTriggerExit is not called at P,A,B
   b) Add B.SphereCollider : OnTriggerEnter is called
   Problem: 1a => not symmetric behaviour => Objects are "double added" in lists

2) Test Case: Mindblow

   a) Remove B.RigidBody: causes OnEnter: (A,B) (P,A) (B,A)
   Problem1: "Exit" is never called, instead "Enter" is called
   Problem2: Since B has no rigidbody it attempts to use his parent P rigidbody, but why is A affected ? ie (P,A) makes no sense, it doesn't even cause (A,P) symmetrical callback.
   b) Add B.RigidBody: causes Exit: (A,B) (B,A) Enter: (A,B) (B,A) (P,B) (B,P)
   Final Lists: 
   P={B,A,A,B}
   A={P,B,B}
   B={P,A,A,P}

3) Another issue i had with trigger colliders is that in unity every trigger, checks for all nearby colliders, there is no way to filter "triggers" by layer (without lowering performance). So i cannot use "Collision Layer Matrix" optimizations.

4) To me all this seems confusing. I am testing the system to its limits to understand how the api "works". I was thinking of using this for a system where big triggers get "refined" into smaller triggers when something gets inside them for ultra detailed physics. I don't want to monopolize your time, only answer if it doesn't take much time, I can figure things on my own, if you point me to the right direction. thank you

Answer 1

Trigger Colliders: adding/removing rigidbody/collider at runtime is unreliable or confusing

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