Building a Backend

Bevy MotionGfx does exactly one thing: implement SubjectSource for Bevy’s World. That’s the whole seam. This page is what building your own looks like, whether that’s a new backend or just skipping having one at all.

SubjectSource

pub trait SubjectSource<I: SubjectId, S: 'static> {
    fn get_source(&self, id: I) -> Option<&S>;
    fn apply_source<R>(&mut self, id: I, f: impl FnOnce(&mut S) -> R) -> Option<R>;
}

I is however you identify a subject: an index, an Entity, a string, anything Copy + Eq + Hash + Ord. S is the type being animated on that subject.

Implement it on your own world type. Rust’s orphan rule means that type has to be local to your crate; wrap a foreign type in a newtype if you need to.

Registry

Registry::create_builder::<World>() returns a builder typed to your world. The registry is what lets Track and Timeline stay non-generic internally while still animating whatever types you give them. You create one, pass it around, and otherwise don’t touch it.

Interpolation

A subject type needs Interpolation<M> for some marker M:

pub trait Interpolation<M> {
    fn interp(a: &Self, b: &Self, t: f32) -> Self;
}

f32, f64, i32, u32, and u8 already have one built in. For your own types, or foreign ones like Peniko’s Color, implement it under a marker type of your own. That’s what peniko_motiongfx’s Peniko marker is for.

Reference

This page stays conceptual. For exact types and signatures, see docs.rs.