Having an `EntityMeta` SystemParam which gives access to an Entity's metadata

[cBournhonesque]

What problem does this solve or what need does it fill?

I was in a situation where I need to have:

• access to an entity's EntityRef to get some information about its archetype
• mutable access to an entity's component (because the component stores a value that depends on the entity's archetype)

The problem is that EntityRef conflicts with any mutable access of any component (I think) because it has methods get<C> that lets you access the component value.

What solution would you like?

We could have a new SystemParam EntityMeta that provides metadata information about an entity and its archetype without conflicting with any mutable access of other components, since EntityMeta just returns metadata information.

[SkiFire13]

&Entities is already a valid SystemParam which you can use to .get(entity) the entity's metadata, including its ArchetypeId. Given an ArchetypeId you can get the archetype's data using an &Archetypes, which is also a SystemParam.

[alice-i-cecile]

Maybe we could do an EntityMeta implement WorldQuery?

[hymm]

For this use case, why did EntityMut not work? It should have all the methods that EntityRef has and you can get mutable references to components.

[cBournhonesque]

Ah I didn't know about EntityMut; I was able to make it work with EntityMut!

I think it would be nice to improve the documentation on:

• which SystemParams are available. For example I was going on docs to find the list of system-params, but for some reason Archetypes is not listed there
• which SystemParams conflict with which system-params? I tried using EntityMut and another Resource but I was getting a conflict even though that other Resource is not a component so could not possibly have any conflicting access with the EntityMut. That might be a bug?

I can close this PR and open a new one for that specific bug

[hymm]

I see Archetypes there.

There is a list on the system mod page https://docs.rs/bevy/latest/bevy/ecs/system/index.html. But it's hand maintained and easy to get out of date. So PRs are welcome there.

That might be a bug?

There's an issue for that already. https://github.com/bevyengine/bevy/issues/7255

I wouldn't mind leaving this open as I could see just getting EntityMeta in a query useful.

[james7132]

#12398 should implement WorldQuery and QueryData for EntityLocation, which is just missing the generation from the entity, which should be accessible already.