Use traits to make ECS APIs for working with entities more consistent

[MiniaczQ]

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

Parts of ECS API are missing methods, which have no reason to not exist.

Types that this is most visible on are:

1. Commands
2. ChildBuilder
3. EntityCommands
4. World
5. WorldChildBuilder
6. EntityWorldMut

For example, you cannot use trigger from ChildBuilder, but if you spawn an entity and get EntityCommands you can call EntityCommands::commands() and then Commands::trigger_targets().

fn spawn_trigger<E: Event>(&mut self, event: E) -> EntityCommands {
    // Type juggling to get around lifetime downcasting.
    // We cannot recover `entity_commands` if we ever drop it.
    let mut entity_commands = self.spawn_empty();
    let entity = entity_commands.id();
    let mut commands = entity_commands.commands();
    commands.trigger_targets(event, entity);
    entity_commands
}

Proposed solution

This issue proposed the following traits:
a) Spawn with spawn_empty() and spawn() for [1, 2, 4, 5],
b) GetCommands with commands() for [1, 2, 3],
c) GetEntity with entity() for [1, 2, 4, 5],
d) Trigger trait trigger() and trigger_targeted() for [1, 2, 4, 5],
e) TriggerEntity with trigger() for [3, 6],
f) Observe with observe() for [1, 3, 4, 6],
g) ModifyEntity with insert() and remove() for [3, 6],
h) WithChildren with with_children() for [3, 6].

In general pairs of [1, 4], [2, 5], [3, 6] should share traits.

Now, this is just a proposal.
I want this issue to help decide the actual scope.
Ideally, key traits would make it to 0.14 without introducing breaking changes.

Example implementation for Spawn

What alternative(s) have you considered?

The traits can theoretically be a 3rd part crate, but missing methods need to be added in bevy_ecs anyways.

Additional context

This is mostly frustrations collected during development of Bevy Jam Template
Relevant discord discussion can be found here

[alice-i-cecile]

This takes the bevy_color approach to API design. This duplication also extends to EntityRef, EntityMut, and DeferredWorld. We've struggled to keep these in sync before. The downside of course is that core functionality gets moved off of the struct itself, making it a bit less visible in the docs.

[MiniaczQ]

We should consider allowing methods, e.g. spawn to exist both as a struct impl and as a trait impl.
The traits will still ensure that all methods are covered and multiple types share functionality,
while if the type is known the struct impl will be selected and fully documented for that scenario.

[alice-i-cecile]

I'm not worried about migration, so I'd prefer to avoid duplicating the APIs.

[benfrankel]

As part of this, I would also propose considering Spawn::spawn_with as a generalization of Spawn::spawn:

• spawn(bundle) = spawn_empty + add(insert(bundle))
• spawn_with(command) = spawn_empty + add(command)

[ItsDoot]

Old related issue: #5647 (for .spawn() APIs only)

[benfrankel]

I think this could use a working group once there's enough bandwidth, since it'll require design work and has cross-cutting impact.

[alice-i-cecile]

Agreed with that. Lots of changes that need to be made in concert, and high levels of controversy.

[MiniaczQ]

Another family of traits is GetCommands, GetWorld, etc.
Commands and World are usually the core of implementing additional functionality, if access to them is unified across word, subapp, app, etc. it's going to reduce extension trait boilerplate and remove bevy_app requirements

[MiniaczQ]

@alice-i-cecile I think now might be a good time to do this since a lot of work is focused outside of bevy repo, on the editor.
If so, what do you think the next steps should be and who should we bother?
I think we need to decide which traits and what methods ahead of implementation, since the merges and postponing it will be painful.

[alice-i-cecile]

I'd really like to land the BSN work before tackling this, because of the large impact it will have on how users initialize entities.

Fully agree on the design-first approach here; that'll need sign-off from SME-ECS and Cart.