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Add SpawnableList implementation for adding multiple entities per iteration #21637

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@Ceedrich Ceedrich commented Oct 23, 2025 •
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Objective

When working with complex spawn hierarchies, it can sometimes be useful to spawn multiple for each object in a list. I encountered this, when using bevy_enhanced_input. Consider the following scenario:

struct KeyboardConfig {
  up: [Binding; 3],
  down: [Binding; 3],
  left: [Binding; 3],
  right: [Binding; 3],
}

//Without FlatSpawnVec
impl KeyboardConfig {
  fn x_axis(&self) -> impl SpawnableList<BindingOf> {
    SpawnIter(
      Spawn((self.left.0, Negate::all())),
      Spawn(self.right.0),
      Spawn((self.left.1, Negate::all())),
      Spawn(self.right.1),
      Spawn((self.left.2, Negate::all())),
      Spawn(self.right.2),
    ) 
  }
}


// With FlatSpawnVec
impl KeyboardConfig {
  fn x_axis(&self) -> impl SpawnableList<BindingOf> {
    FlatSpawnVec(self.left.into_iter().zip(self.right)
      .map(|(l, r)| (Spawn((l, Negate::all()), Spawn(r))
      .collect()
    ) 
  }
}

If the bindings didn't have a known size, this would not be possible without a FlatSpawnVec.

Solution

This can be solved by using a FlatSpawnVec implementation that spawns multiple entities per iteration. I could also see the vector being changed in favor of an iterator or similar.

Testing

I added a test bevy_ecs::spawn::test::flat_spawn_vec

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Welcome, new contributor!

Please make sure you've read our contributing guide and we look forward to reviewing your pull request shortly ✨

hukasu
hukasu reviewed Oct 26, 2025
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Overall, not convinced

@hukasu hukasu added C-Feature A new feature, making something new possible A-ECS Entities, components, systems, and events M-Release-Note Work that should be called out in the blog due to impact S-Waiting-on-Author The author needs to make changes or address concerns before this can be merged labels Oct 26, 2025
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It looks like your PR has been selected for a highlight in the next release blog post, but you didn't provide a release note.

Please review the instructions for writing release notes, then expand or revise the content in the release notes directory to showcase your changes.

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Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-21637

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.

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A-ECS Entities, components, systems, and events C-Feature A new feature, making something new possible M-Release-Note Work that should be called out in the blog due to impact S-Waiting-on-Author The author needs to make changes or address concerns before this can be merged

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@Ceedrich @hukasu