GlobalTransform not being updated

[yilinwei]

Bevy version

ffecb05

Operating system & version

Linux

What you did

Link to repo here.

The only file is src/main.rs. The bevy submodule is simply to add some extra logging.

What you expected to happen

The TransformPlugin has two systems parent_update and transform_propagate. parent_update is always meant to run before transform_propagate according to the run criteria.

If an Entity has a Parent but no Children, parent_update creates one. The transform_propagate then queries from the root nodes and updates the GlobalTransform.

Whether you insert a Children component shouldn't matter. The Children should be inserted by the parent_update and the transform_propagate should pick up the inserted Children.

What actually happened

Whether you insert the Children does matter, if you comment the line out you get the unexpected matrix: log line, whereas otherwise you don't.

Adding some more println! statements, the root node query doesn't pick up Children at all - it's empty, but you can clearly see the new Children in the later query and adding the println! directly where the Children is added shows that it does hit that part of the code.

Additional information

[alice-i-cecile]

Adding some more println! statements, the root node query doesn't pick up Children at all - it's empty, but you can clearly see the new Children in the later query and adding the println! directly where the Children is added shows that it does hit that part of the code.

The reason for this is that Commands aren't processed until the end of the current stage. The behavior observed here is probably expected, but unfortunate.

[lukors]

I've ran into this twice now, and my solution has been to manually update the GlobalTransform to match the Transform. I've read this is bad practice, are there better workarounds?

[yilinwei]

@lukors

The easiest way to get around this issue is to insert the Children component on the parent entity when spawning it.

[lukors]

@yilinwei I tried adding .insert(Children::with(&[])); when spawning my parent entity. But when I later spawn an entity, set its transform and make it a child of the parent with commands.entity(parent).push_children(&[entity]) in the same frame, the transform does not take.

[yilinwei]

@lukors

The below in a single system should work. It is a little finicky, but it's easiest to check the behaviour by vendoring bevy and debugging the two systems.

let parent = commands.spawn().insert_bundle(
  (Children::with(&[]), Transform::default(), GlobalTransform::default())
).id();

let child = commands.spawn().insert_bundle(
  (Parent(parent), Transform::default(), GlobalTransform::default())
);

[maniwani]

So, internally, we should be applying the commands from parent_update before transform_propagate runs, but we can't do that nicely right now because of all the awkwardness around stages and exclusive system insertion points.

This could be resolved with the changes proposed in bevyengine/rfcs#45.