Allow entities to be added to a relationship using the new spawn api (remade)

[Freyja-moth]

Objective

Reopens #18961

Solution

Copy the code

Testing

It worked in the previous pull request

Showcase

See #18961

[hukasu]

i would have a #[test] with a simple example, could very well be the code from the old PR with asserts to show that the ChildOf is being added correctly

[Freyja-moth]

Where should I put the test? In src/spawn.rs or are the tests for the Spawn/SpawnIter/SpawnWith that I should place them with.

[Trashtalk217]

Looks good!

[hukasu]

yes, it could be on spawn.rs
on the end of the file do

#[cfg(test)]
mod tests {
   ...
}

[Freyja-moth]

Added the tests, also took the liberty of adding tests for Spawn, SpawnIter, and SpawnWith.

[hukasu]

i would have asserts to check if the ChildOf of child1 and child2 point correctly to parent, just to be pedantic

[Freyja-moth]

I'm not all that familiar with working directly with world and got stuck trying to find a way to do it by validating the Children, rather than ChildOf and eventually gave up. Will try that

[hukasu]

world.entity(child1).get::<ChildOf>() or something similar

[alice-i-cecile]

Reading this I'm not immediately sure what the inferred generic I is, or how that's determined. Can you add a comment?

[Freyja-moth]

Just to check, something like this?

world.spawn((
    Name::new("Root"),
    Children::spawn((
        Spawn(Name::new("Child1")),
        // [`I`] here is [`Iterator<Item = Entity>`]
        WithRelated([child2, child3].into_iter()),
    )),
));

[alice-i-cecile]

No, explaining that WithRelated adds the existing entities as children, inferred due to the Children type above.

[alice-i-cecile]

These docs need to crosslink against WithRelated, explaining the differences.

[alice-i-cecile]

This isn't a particularly useful description by itself, can you add another line explaining why users might to do this?

[alice-i-cecile]

Generally looks like a nice feature, but I found the docs and motivation hard to follow, even after spending a fair bit of time puzzling over it. We need to make sure that this is clear so users know when to use this new API :)