Resolved EndFrameParentSystem removes Parent component for children when destroying an entity

I have a hierarchy of entities (A, B, C), and I want to destroy B and reparent C to A. This seems to be a bug in
the EndFrameParentSystem to me. Here's a test I wrote to demonstrate the behaviour.

Code (CSharp):

1.        
2. [Test]
3.         public void ReparentTest()
4.         {
5.             var arch = m_Manager.CreateArchetype(
6.                 typeof(Translation), typeof(Rotation), typeof(Parent), typeof(LocalToParent), typeof(LocalToWorld));
7.  
8.             var bottom = m_Manager.CreateEntity(arch);
9.             var middle = m_Manager.CreateEntity(arch);
10.             var top = m_Manager.CreateEntity(arch);
11.             m_Manager.SetComponentData(top, new Parent { Value = middle });
12.             m_Manager.SetComponentData(middle, new Parent { Value = bottom });
13.  
14.             World.GetOrCreateSystem<EndFrameParentSystem>().Update();
15.  
16.             Assert.IsTrue(m_Manager.HasComponent<Parent>(top)); //This is true
17.  
18.             m_Manager.SetComponentData(top, new Parent{Value = bottom});
19.             m_Manager.DestroyEntity(middle);
20.  
21.             World.GetOrCreateSystem<EndFrameParentSystem>().Update();
22.  
23.             Assert.IsTrue(m_Manager.HasComponent<Parent>(top)); //This is false
24.         }
25.  

 

Does anyone have a workaround for this?

 

I slightly modified and ran your code. I'm using 0.16.0-preview.21 of Unity.Entities. I can confirm that destroying the

middle

(the parent) entity completely removes the parent component from

top

(the child).

At first glance, it may seem that the more intuitive result would be that removing the

middle

entity should cause

top

's parent value to be

Entity.Null

. But! Imagine that were the case. Then, if we later queried some entities with parent components, we'd be irritated about resultant ones containing

Entity.Null

values ("but that one doesn't really have a parent anymore, does it?"). This is a hard design problem in terms of API usability, and I think I prefer the default. At least, I'm assuming it's intentional lol. You could check by reading the code in

ParentSystem

, or one of the associated tests.

Here's a workaround version of your code, @OcarinaBoy2049:

Code (CSharp):

1. [Test]
2. public void ReparentTest()
3. {
4.     var arch = m_Manager.CreateArchetype(typeof(Translation), typeof(Rotation), typeof(Parent), typeof(LocalToParent), typeof(LocalToWorld));
5.  
6.     var bottom = m_Manager.CreateEntity(arch);
7.     var middle = m_Manager.CreateEntity(arch);
8.     var top = m_Manager.CreateEntity(arch);
9.  
10.     m_Manager.SetComponentData(top, new Parent { Value = middle });
11.     m_Manager.SetComponentData(middle, new Parent { Value = bottom });
12.  
13.     World.GetOrCreateSystem<EndFrameParentSystem>().Update();
14.  
15.     Assert.IsTrue(m_Manager.HasComponent<Parent>(top));
16.  
17.     m_Manager.DestroyEntity(middle);
18.  
19.     World.GetOrCreateSystem<EndFrameParentSystem>().Update();
20.  
21.     m_Manager.AddComponentData(top, new Parent { Value = bottom });
22.  
23.     World.GetOrCreateSystem<EndFrameParentSystem>().Update();
24.  
25.     Assert.IsTrue(m_Manager.HasComponent<Parent>(top));
26. }

With this code, both assertions should pass. Note that you can destroy the

middle

entity, then either skip a frame (if using

[UnityTest]

) or run the

EndFrameParentSystem

as you are, and finally add a brand new parent component to

top

—you must use

AddComponentData

there instead of

SetComponentData

.

 

After reading the code in the ParentSystem I discovered that entities get put in the `m_DeletedParentsGroup` if they have a `Child` component and do not have a `LocalToWorld`. So by removing the `Child` component before deleting the entity, the test works as expected.