EntityManager.Exists returns true after Entity was destroyed?

Unity Engine
,
1

So I have a ComponentSystem that destroys Entities and one that checks components on Entities. The destroy-System runs shortly before the check-System (but both are ComponentSystems, so both on the Main Thread, right?)

Now when I am checking the Components of an Entity that was destroyed the same frame, EntityManager.Exists returns true, but the Component of the Entity I am trying to access already got removed, so I get an ArgumentException: A component with type:Translation has not been added to the entity.

Is this a bug or are there some multithreaded/other things going on that have to be kept in mind when using EntityManager.Exists?

(Using Entities 0.5.0)

Code:

// Runs first
public class DeathSystem : ComponentSystem
{
    protected override void OnUpdate()
    {
        Entities.ForEach((Entity e, ref Health health) =>
        {
            if (health.Value <= 0)
            {
                Debug.Log("Entity Died: " + e);
                EntityManager.DestroyEntity(e);
                // Same result with PostUpdateCommands.DestroyEntity(e)
            }
        });
    }
}

// Runs second
public class DummyKISystem : ComponentSystem
{
    protected override void OnUpdate()
    {
        Entities.ForEach((ref DummyKIComponent dummy, ref Translation translation, ref Rotation rotation, ref MovementSpeed speed) =>
        {
            if (dummy.CurrentTarget == Entity.Null)
            {
                ...
            }
            else if (!EntityManager.Exists(dummy.CurrentTarget))
                dummy.CurrentTarget = Entity.Null;
            else
            {                
                if (!EntityManager.HasComponent<Translation>(dummy.CurrentTarget))
                    Logger.Log("Blargh! " + dummy.CurrentTarget);
                // ... (Access Translation Component)
            }
        });
    }
}

Result:
5391270--546759--upload_2020-1-20_23-7-50.png

2

I unfortunately don’t have a solution, but did you omit or forget a [UpdateBefore(typeof(DeathSystem))] for DummyKISystem? I don’t know about ForEach but PostUpdateCommands is definitely run on the main thread, sequentially, per the official Command Buffer docs.

To remove the possibility of a race condition, I made just one system:

public class TestComponentSystem : ComponentSystem {

    protected override void OnUpdate()
    {
        int n = 0;
        Entities.ForEach((Entity e) =>
        {
            UnityEngine.Debug.Log(EntityManager.Exists(e));
            PostUpdateCommands.DestroyEntity(e);
            n++;
        });
        UnityEngine.Debug.Log(n);
    }
}

The output is consistent with yours though – the entity is, oddly enough, never destroyed. (The system continues to iterate over the same entities, even after the update function completes and the next frame is run)

3

I am having the same problem, did you find a solution?

4

I would think that the second system would implicitly create a entity query in the lambda so that you don’t have to use the EntiyManager inside the function.

5

Turns out there are types of Components, which can keep an Entity alive after destroying it:
https://docs.unity3d.com/Packages/com.unity.entities@0.0/manual/system_state_components.html

In my case it was a GhostSystemStateComponent from the NetCode Package.
I guess in these cases we have to do a HasComponent check to be completely sure.

2 Likes