Subscription

useSWRSubscription

useSWRSubscription is a React hook that allows subscribing to real-time data sources with SWR.

useSWRSubscription<Data, Error>(key: Key, subscribe: (key: Key, options: { next: (error?: Error | null, data: Data) => void }) => () => void): { data?: Data, error?: Error }

API

This hook subscribes to a real-time data source using the subscribe function provided, and returns the latest data received and any errors encountered. The hook automatically updates the returned data as new events are received.

Parameters

• key: A unique key that identifies the data being subscribed to, same key as useSWR key.
• subscribe: A function that subscribes to the real-time data source. It receives the following arguments:
  • key: same key as above
  • options: an object with the following properties:
    • next: A function that accepts an error and data, and updates the state with the latest data received from the real-time data source.

For instance

function subscribe(key, { next }) {
  const sub = remote.subscribe(key, (err, data) => next(err, data))
  return () => sub.close()
}

You could also pass a updater function as data to next, which will receive the previous data as the first argument and return the new data.

function subscribe(key, { next }) {
  const sub = remote.subscribe(key, (err, data) => next(err, prev => prev.concat(data)))
  return () => sub.close()
}

Return Values

• state: An object with the following properties:
  • data: The latest data received from the real-time data source.
  • error: An Error object if an error occurred while subscribing to the real-time data source, otherwise undefined.

When new data is received, the error will be reset to undefined.

Usage

Using useSWRSubscription to subscribe to a Firestore data source:

import useSWRSubscription from 'swr/subscription'
 
function Post({ id }) {
  const { data } = useSWRSubscription(['views', id], ([_, postId], { next }) => {
    const ref = firebase.database().ref('views/' + postId)
    ref.on('value',
      snapshot => next(null, snapshot.data()),
      err => next(err)
    )
    return () => ref.off()
  })
 
  return <span>Your post has {data} views!</span>
}

Using useSWRSubscription to subscribe to a WebSocket data source:

import useSWRSubscription from 'swr/subscription'
 
function App() {
  const { data, error } = useSWRSubscription('ws://...', (key, { next }) => {
    const socket = new WebSocket(key)
    socket.addEventListener('message', (event) => next(null, event.data))
    socket.addEventListener('error', (event) => next(event.error))
    return () => socket.close()
  })
  if (error) return <div>failed to load</div>
  if (!data) return <div>loading...</div>
  return <div>hello {data}!</div>
}

You could also check TypeScript examples of useSWRSubscription at this page

Deduplication

useSWRSubscription deduplicates the subscription requests with the same key. If there are multiple components using the same key, they will share the same subscription. When the last component using the key unmounts, the subscription will be closed.

This means that if you have multiple components using the same key, they will all receive the same data. And there's only one subscription to the real-time data source per key.