Add an example demonstrating how to use default query filtering and entity cloning to create a prefab system

[alice-i-cecile]

Objective

Bevy users have long wished for tools for working with a "prefab"-style workflows: #10949, #3877, #2565, #1446...

The core idea is pretty simple: define an "object" (made up of some collection of entities) that we can save, instantiated, into the world, and then clone and modify them as needed.

Solution

Instead of creating a first-class solution, I've instead chosen to add an example demonstrating how this pattern can be achieved quite simply with a combination of default query filters and recursive entity cloning.

This is useful for teaching users how to create their own prefab-flavored solution, and gives us time to iterate on the ideal solution.

Long-term, I think that Bevy would be well-served by making a first party prefab workflow, complete with a standardized state-powered asset loading solution. It's nice to be able to teach this pattern directly to new users, and more importantly, it allows us to view and building tooling around prefabs for e.g. a GUI-based level editor.

But Bevy itself is a poor place to prove out the architectural details for this: we aren't actually testing it in a large game project, and so the solution isn't subject to the appropriate pressures. Once the community has developed a robust solution, we should upstream it (likely as a standalone crate) and update this example!

To do

This PR is not yet complete!

Before it's merged, we need to:

• merge Allow users to register their own disabling components / default query filters #17768
• figure out why foxes flicker in and out of existence when spawning

It might also be nice to:

• add instruction text
• load more than one model, and actually make use of that hash map
• define and use an asset loading state (please discuss!)
• somehow extract the asset path from the SceneInstanceReady event and cache that

Like always, feel free to PR to my branch if you want to fix these.

Testing

cargo run --example prefabs

Showcase

[ChristopherBiscardi]

video that shows the flickering (and longer non-rendering meshes)

screenshot-2025-02-09-at-17.41.51.mp4

[cart]

I think this deserves a qualifier or a different more specific name less loaded with context. People will see this name and think "this is the Bevy prefab system", which has implications like "an asset format", "inheritance", etc.

[alice-i-cecile]

Good call; I definitely think that's the right call for the current stage of maturity.