[bug]: Different configs per property not possible in useTransition()

[hatsumatsu]

Which react-spring target are you using?

• @react-spring/web
• @react-spring/three
• @react-spring/native
• @react-spring/konva
• @react-spring/zdog

What version of react-spring are you using?

9.6.0

What's Wrong?

When using useTransition it does not seem to be possible to have different spring configs per property like stated in https://www.react-spring.dev/docs/advanced/config#config-per-springvalue .

To Reproduce

Consider this simple transition:

const transitions = useTransition(data, {
    from: { opacity: 0, y: 2 },
    enter: { opacity: 1, y: 0 },
    leave: { opacity: 0, y: -2 },
    config: (key) => {
        return key === 'y' config.wobbly : { duration: 600 };
    },
});

https://www.react-spring.dev/docs/advanced/config#config-per-springvalue states that a function-based config has access to the key of the animated property. In case of useTransition() the passed variable holds the value of data.

Expected Behaviour

We have access to the property key in function-based configs.

Link to repo

_

[joshuaellis]

Thanks for the report, i've had a quick look to validate this and you're right it's a bug. This is some information so I can solve it later:

Reproduction

// Baseline: useSpring honours the per-key contract — PASSES
useSpring({
  from: { x: 0, y: 0 },
  to: { x: 1, y: 1 },
  config: key => (key === 'y' ? { duration: 600 } : { duration: 100 }),
})
// styles.x.animation.config.duration === 100  ✅
// styles.y.animation.config.duration === 600  ✅

// useTransition: same shape, FAILS
useTransition(true, {
  from: { x: 0, y: 0 },
  enter: { x: 1, y: 1 },
  config: key => (key === 'y' ? { duration: 600 } : { duration: 100 }),
})
// Expected: x → 100, y → 600
// Actual:   x → 100, y → 100   ❌ (function got the item `true`, not the key)

Suspected Root Cause

// packages/core/src/hooks/useTransition.tsx:265
to.config = callProp(config, t.item, i, phase)

useTransition eagerly invokes the user's function with (item, index, phase) and stores the result as a plain object on to.config. By the time it reaches SpringValue.ts:724 — where callProp(props.config, key) would normally pass the key — the function has already been collapsed into a single config object that all keys share.

useSpring/useSprings skip this eager call, which is why they work as documented.