Events

There are a handful of events you can use to react to the state of animation at certain points in time.

Keys vs Springs

Every event function can either be a function on it's own or be an object to which it's keys correlate to the keys of the spring:

useSpring(
  () => ({
    x: 0,
    y: 0,
    // onStart is called when the animation of the spring starts
    onStart: () => console.log('the spring has started'),
  }),
  []
)

useSpring(
  () => ({
    x: 0,
    y: 0,
    onStart: {
      // onStart is called for each key when the animation starts
      x: () => console.log('x key has started'),
      y: () => console.log('y key has started'),
    },
  }),
  []
)

The latter form can be useful if for example you have different configs for different keys. Or you want to do different events based on different keys, e.g. you want to imperatively update an objects rotation based on the x key.

onStart

Called when the animation begins.

type OnStart = (
  result: AnimationResult,
  spring: Controller | SpringValue,
  item?: Item
) => void

onChange

Called on every frame.

type OnChange = (
  result: AnimationResult,
  spring: Controller | SpringValue,
  item?: Item
) => void

onRest

Called when the animation comes to a stand-still.

type OnRest = (
  result: AnimationResult,
  spring: Controller | SpringValue,
  item?: Item
) => void

onPause

Called when the animation is paused.

type OnPause = (
  result: AnimationResult,
  spring: Controller | SpringValue,
  item?: Item
) => void

onResume

Called when the animation is resumed.

type OnResume = (
  result: AnimationResult,
  spring: Controller | SpringValue,
  item?: Item
) => void

onResolve

Called when the promise for the update is resolved.

type OnResolve = (
  result: AnimationResult,
  spring: Controller | SpringValue,
  item?: Item
) => void

onProps

Called after an animation is updated by new props, even if the animation remains idle.

type OnProps = (
  props: {[key: string]: any}
  spring: SpringValue,
) => void

onDestroyed

Called after a transition item is unmounted.

type OnDestroyed = (
  item: Item
  key: string | number
) => void

Typescript

interface AnimationResult {
    // Type inference will solve this for you.
    value: SpringValue | { [keyof SpringValues]: number} | number
    finished: boolean
    cancelled: boolean
}

A note on Item

The Item argument is only present when using the relevant events within the useTransition hook.