---
category: '@RxJS'
---

# watchExtractedObservable

Watch the values of an RxJS [`Observable`](https://rxjs.dev/guide/observable) as extracted from one or more composables.

Automatically unsubscribe on observable change, and automatically unsubscribe from it when the component is unmounted.

Supports all overloads of [`watch`](https://vuejs.org/guide/essentials/watchers.html#basic-example).

## Usage

<!-- TODO: import rxjs error if enable twoslash -->

```ts no-twoslash
import { watchExtractedObservable } from '@vueuse/rxjs';
import { computed, reactive, useTemplateRef } from 'vue';
import { AudioPlayer } from '../my/libs/AudioPlayer';

// setup()

const audio = useTemplateRef('audio');
const player = computed(() => (audio.value ? new AudioPlayer(audio.value) : null));
const state = reactive({
  progress: 0,
});

watchExtractedObservable(
  player,
  (p) => p.progress$,
  (percentage) => {
    state.progress = percentage * 100;
  },
);
```

If you want to add custom error handling to an `Observable` that might error, you can supply an optional `onError` configuration. Without this, RxJS will treat any error in the supplied `Observable` as an "unhandled error" and it will be thrown in a new call stack and reported to `window.onerror` (or `process.on('error')` if you happen to be in Node).

You can also supply an optional `onComplete` configuration if you need to attach special behavior when the watched observable completes.

```ts no-twoslash
import { watchExtractedObservable } from '@vueuse/rxjs';
import { computed, reactive, useTemplateRef } from 'vue';
import { AudioPlayer } from '../my/libs/AudioPlayer';

// setup()

const audio = useTemplateRef('audio');
const player = computed(() => (audio.value ? new AudioPlayer(audio.value) : null));
const state = reactive({
  progress: 0,
});

watchExtractedObservable(
  player,
  (p) => p.progress$,
  (percentage) => {
    state.progress = percentage * 100;
  },
  {
    onError: (err: unknown) => {
      console.error(err);
    },
    onComplete: () => {
      state.progress = 100; // or 0, or whatever
    },
  },
);
```

If you want, you can also pass `watch` options as the last argument:

```ts no-twoslash
import { watchExtractedObservable } from '@vueuse/rxjs';
import { computed, reactive, useTemplateRef } from 'vue';
import { AudioPlayer } from '../my/libs/AudioPlayer';

// setup()

const audio = useTemplateRef('audio');
const player = computed(() => (audio.value ? new AudioPlayer(audio.value) : null));
const state = reactive({
  progress: 0,
});

watchExtractedObservable(
  player,
  (p) => p.progress$,
  (percentage) => {
    state.progress = percentage * 100;
  },
  {
    onError: (err: unknown) => {
      console.error(err);
    },
  },
  {
    immediate: true,
  },
);
```

## Type Declarations

```ts
export type OnCleanup = (cleanupFn: () => void) => void;
export type WatchExtractedObservableCallback<Value, OldValue, ObservableElement> = (
  value: NonNullable<Value>,
  oldValue: OldValue,
  onCleanup: OnCleanup,
) => Observable<ObservableElement>;
export interface WatchExtractedObservableOptions {
  onError?: (err: unknown) => void;
  onComplete?: () => void;
}
export declare function watchExtractedObservable<
  T extends MultiWatchSources,
  E,
  Immediate extends Readonly<boolean> = false,
>(
  sources: [...T],
  extractor: WatchExtractedObservableCallback<MapSources<T>, MapOldSources<T, Immediate>, E>,
  callback: (snapshot: E) => void,
  subscriptionOptions?: WatchExtractedObservableOptions,
  watchOptions?: WatchOptions<Immediate>,
): WatchHandle;
export declare function watchExtractedObservable<
  T extends Readonly<MultiWatchSources>,
  E,
  Immediate extends Readonly<boolean> = false,
>(
  source: T,
  extractor: WatchExtractedObservableCallback<MapSources<T>, MapOldSources<T, Immediate>, E>,
  callback: (snapshot: E) => void,
  subscriptionOptions?: WatchExtractedObservableOptions,
  watchOptions?: WatchOptions<Immediate>,
): WatchHandle;
export declare function watchExtractedObservable<T, E, Immediate extends Readonly<boolean> = false>(
  source: WatchSource<T>,
  extractor: WatchExtractedObservableCallback<T, Immediate extends true ? T | undefined : T, E>,
  callback: (snapshot: E) => void,
  subscriptionOptions?: WatchExtractedObservableOptions,
  watchOptions?: WatchOptions<Immediate>,
): WatchHandle;
export declare function watchExtractedObservable<
  T extends object,
  E,
  Immediate extends Readonly<boolean> = false,
>(
  source: T,
  extractor: WatchExtractedObservableCallback<T, Immediate extends true ? T | undefined : T, E>,
  callback: (snapshot: E) => void,
  subscriptionOptions?: WatchExtractedObservableOptions,
  watchOptions?: WatchOptions<Immediate>,
): WatchHandle;
```
