First class support for spawning structures of multiple entities that can be linked together.

[rhedgeco]

Objective

• Adds first class support for structures of entities/components via 'blueprints' (name subject to change)
• Allows spawning 'blueprints' in a way that is consistent with spawning bundles or components.

Solution

• Uses a trait EntityBlueprint that can be implemented for any given struct.
• struct can be used to hold spawning parameters, or settings for the blueprint
• implementing EntityBlueprint requires defining fn build(self, entity: &mut EntityCommands) which is where the structure of the entity and its relations are built.
• Added a function spawn_blueprint into the impl Commands that accepts a EntityBlueprint, similar to spawning a bundle or component.

Notes

All of this is very rudimentary and is mainly intended to be a starting point for discussion about not only the method of implementation, but the necessity for one.

[alice-i-cecile]

Initial discussion of the idea on Discord. This is quite contentious, and we want to have a clear vision for how this fits in with scenes (both on-disk and at compile time).

[Weibye]

I really like this for its simplicity! I'm sure it can be further improved but this is something we could start using and getting benefits from today.

I would love to see an example of how this looks when defining a blueprint using scene assets. Has there been any thoughts on that yet?

[Weibye]

@rhedgeco, I've started exploring this idea in my Bevy Widget exploration: Weibye/bevy-prot-widgets#17

If you take a look at the PR you can see how this can look like a bit fleshed out, but I think I need to get more complicated examples and structures in place before we can really see the implications on whether this is a good design.