Add support for pre-step and post-step logic

Hi!

Looking for thoughts on adding pre-step and post-step events.

Use cases for this include creating custom character and vehicle controllers, where forces should only be applied once per physics step. When not using timestep="vary", applying forces on each frame isn’t desirable, as not every frame will step the physics world, some will just interpolate.

This could come in the form of hooks, e.g.

import { usePreStep, usePostStep } from "@react-three/rapier"

usePreStep(() => { /* called before stepping the world */ })
usePostStep(() => { /* called after stepping the world */ })

Or via signals/event handlers on the wrapped rapier World, e.g.

world.onPreStep.add(() => { /* called before stepping the world */ })
world.onPostStep.add(() => { /* called after stepping the world */ })

Or maybe a combination of both.

Keen to hear thoughts, and as usual, happy to help implement 🙂

[krispya]

I think both useBeforePhysicsStep and useAfterPhysicsStep should be within the same loop as the physics step - running only once for a rapier physics step.

To get interpolated values, you could already use useFrame with a lower priority than the <Physics /> updatePriority.

(Correct me if you don’t think this is right)

Makes perfect sense to me.

[isaac-mason]

I think both useBeforePhysicsStep and useAfterPhysicsStep should be within the same loop as the physics step - running only once for a rapier physics step.

To get interpolated values, you could already use useFrame with a lower priority than the <Physics /> updatePriority.

(Correct me if you don’t think this is right)