atomWithListeners

atomWithListeners creates an atom and a hook. The hook can be called to add a new listener. The hook takes as an argument a callback, and that callback is called every time the atom's value is set. The hook also returns a function to remove the listener.

This can be useful when you want to create a component that can listen to when an atom's state changes without having to re-render that component with each of those state changes.

import { useEffect } from 'react'
import { atom, useSetAtom, Getter, Setter, SetStateAction } from 'jotai'

type Callback<Value> = (
  get: Getter,
  set: Setter,
  newVal: Value,
  prevVal: Value
) => void

export function atomWithListeners<Value>(initialValue: Value) {
  const baseAtom = atom(initialValue)
  const listenersAtom = atom(<Callback<Value>[]>[])
  const anAtom = atom(
    (get) => get(baseAtom),
    (get, set, arg: SetStateAction<Value>) => {
      const prevVal = get(baseAtom)
      set(baseAtom, arg)
      const newVal = get(baseAtom)
      get(listenersAtom).forEach((callback) => {
        callback(get, set, newVal, prevVal)
      })
    }
  )
  const useListener = (callback: Callback<Value>) => {
    const setListeners = useSetAtom(listenersAtom)
    useEffect(() => {
      setListeners((prev) => [...prev, callback])
      return () =>
        setListeners((prev) => {
          const index = prev.indexOf(callback)
          return [...prev.slice(0, index), ...prev.slice(index + 1)]
        })
    }, [setListeners, callback])
  }
  return [anAtom, useListener] as const
}

In a component:

const [countAtom, useCountListener] = atomWithListeners(0)

function EvenCounter() {
  const [evenCount, setEvenCount] = useState(0)

  useCountListener(
    useCallback(
      (get, set, newVal, prevVal) => {
        // Every time `countAtom`'s value is set, we check if its new value
        // is even, and if it is, we increment `evenCount`.
        if (newVal % 2 === 0) {
          setEvenCount((c) => c + 1)
        }
      },
      [setEvenCount]
    )
  )

  return <>Count was set to an even number {evenCount} times.</>
}