react-spring/docs/app/routes/docs.advanced.events.mdx

---
meta:
  title: Events | React Spring
  'og:title': Events | React Spring
  'twitter:title': Events | React Spring
  description: An advanced guide to the event system of Controllers, Components & SpringValues in React Spring.
  'og:description': An advanced guide to the event system of Controllers, Components & SpringValues in React Spring.
  'twitter:description': An advanced guide to the event system of Controllers, Components & SpringValues in React Spring.
  'og:url': https://www.react-spring.dev/docs/advanced/events
  'twitter:url': https://www.react-spring.dev/docs/advanced/events
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---

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# 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:

```jsx line=5-6,15-19
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.

:::warning
`onStart` is called after the first animation tick, this value is therefore considered dirty.
:::

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

## onChange

Called on every frame.

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

## onRest

Called when the animation comes to a stand-still.

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

## onPause

Called when the animation is paused.

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

## onResume

Called when the animation is resumed.

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

## onResolve

Called when the promise for the update is resolved.

```ts
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.

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

## onDestroyed

Called after a transition item is unmounted.

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

## Typescript

```ts
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`](/docs/components/use-transition) hook.