lens

A set of operators to transform actions payload or an atoms state in a FRP style. Simply put, this package is convenient for reactive processing of actions and effects. But some operators could be useful for data processing too, like filter, delay (debounce, throttle) and sample.

included in @reatom/framework

mapState

Simple map utility, which allow you to receive previous dependency state by a second optional argument.

import { mapState } from '@reatom/lens'

// this is a typical code which have a problem with extra updates
// in case when an element of the list changes not `myProp`
export const filteredListAtom = atom((ctx) =>
  ctx.spy(listAtom).map((obj) => obj.myProp),
)
// `mapState` could help to solve this problem, as it pass previous state as a second argument
export const bAtom = listAtom.pipe(
  mapState((ctx, list, prevState) => {
    const newState = list.map((obj) => obj.myProp)
    return isShallowEqual(newState, prevState) ? prevState : newState
  }),
)

filter

Sometimes you already have filteredListAtom from the previous example and it have no internal memoization. So you could use filter operator to prevent extra updates.

Updates filtered by comparator function, which should return true, if new state should continue to propagate. It uses isShallowEqual from utils package by default.

import { filter } from '@reatom/lens'
import { isShallowEqual } from '@reatom/utils'

export const listMemoAtom = filteredListAtom.pipe(filter())
// equals to
export const listMemoAtom = filteredListAtom.pipe(
  filter((ctx, next, prev) => !isShallowEqual(next, prev)),
)

This operator could filter actions too!

import { filter } from '@reatom/lens'

export const linkClicked = onDocumentClick.pipe(
  filter((ctx, event) => event.target.tagName === 'A'),
)

mapPayload

Map payload of each action call. Resulted action is not callable.

import { mapPayload } from '@reatom/lens'

export const changeFullname = changeName.pipe(
  mapPayload((ctx, { firstName, lastName }) => `${firstName} ${lastName}`),
)

You could pass initial state by first argument to create an atom.

import { action } from '@reatom/core'
import { mapPayload } from '@reatom/lens'

export const onInput = action('onInput')
export const inputAtom = onInput.pipe(
  mapPayload('', (ctx, event) => event.currentTarget.value, 'inputAtom'),
)

mapPayloadAwaited

Map fulfilled value of async action call. Resulted action is not callable.

import { mapPayloadAwaited } from '@reatom/lens'

export const newData = fetchData.pipe(mapPayloadAwaited())
// OR pick needed value
export const newData = fetchData.pipe(
  mapPayloadAwaited((ctx, response) => response.data),
)

You could pass initial state by first argument to create an atom.

import { mapPayloadAwaited } from '@reatom/lens'

export const dataAtom = fetchList.pipe(
  [],
  mapPayloadAwaited((ctx, response) => response.data),
  'dataAtom',
)

mapInput

Create action which map input to passed action / atom.

import { atom } from '@reatom/core'
import { mapInput } from '@reatom/lens'

export const inputAtom = atom('', 'inputAtom')
export const changeInput = inputAtom.pipe(
  mapInput((ctx, event) => event.currentTarget.value, 'changeInput'),
)

debounce

Delay updates by timeout.

import { action } from '@reatom/core'
import { debounce, mapPayload } from '@reatom/lens'

export const startAnimation = action()
export const endAnimation = startAnimation.pipe(debounce(250))

sample

Delay updates until other atom update / action call.

This code is taken from this example.

import { mapPayload, sample } from '@reatom/lens'

export const lastRequestTimeAtom = fetchData.pipe(
  mapPayload(0, () => Date.now(), 'fetchStartAtom'),
  sample(fetchData.onSettle),
  mapState((ctx, start) => start && Date.now() - start, 'lastRequestTimeAtom'),
)

toAtom

Convert an action to atom with optional init state.

import { mapPayloadAwaited, toAtom } from '@reatom/lens'

export const dataAtom = fetchData.pipe(mapPayloadAwaited(), toAtom([]))

plain

Removes all extra properties, useful for exports cleaning.

import { plain } from '@reatom/lens'

const _fetchData = reatomFetch('...')

// ... some module logic with `_fetchData.retry` etc

// allow external modules only fetch data and not manage it by other ways
export const fetchData = _fetchData.pipe(plain)

readonly

Removes all callable signature, useful for exports cleaning.

import { readonly } from '@reatom/lens'

const _countAtom = atom(0)
export const changeCount = action((ctx) => {
  // the module extra logic here
  _countAtom(ctx)
})

// disallow atom to be mutated outside the module
export const countAtom = _countAtom.pipe(readonly)

parseAtoms

Recursively unwrap all atoms in an atomized structure. Useful for making snapshots of reactive state. Uses ctx.spy if it’s available.

parseAtoms: persistence example

https://codesandbox.io/s/reatom-react-atomization-k39vrs?file=/src/model.ts

import { action, atom, Action, AtomMut } from '@reatom/core'
import { withLocalStorage } from '@reatom/persist-web-storage'
import { parseAtoms, ParseAtoms } from '@reatom/lens'

export type Field = {
  id: number
  name: string
  value: AtomMut<string>
  remove: Action
}

const getField = (id: number, name: string, value: string): Field => {
  return {
    id,
    name,
    value: atom(value),
    remove: action((ctx) => {
      // ...
    }),
  }
}

export const listAtom = atom<Array<Field>>([], 'listAtom').pipe(
  withLocalStorage({
    toSnapshot: (state) => parseAtoms(state),
    fromSnapshot: (snapshot: any) =>
      getField(snapshot.id, snapshot.name, snapshot.value),
  }),
)

parseAtoms: shortcut example

You can use parseAtoms to reduce the amount of . Let’s suppose you have the following structure:

interface User {
  name: AtomMut<string>
  bio: AtomMut<string>
  website: AtomMut<string>
  address: AtomMut<string>
}

And use it like this:

import { useAtom } from '@reatom/npm-react'

export const User = ({ user }: { user: User }) => {
  const [name] = useAtom(user.name)
  const [bio] = useAtom(user.bio)
  const [website] = useAtom(user.website)
  const [address] = useAtom(user.address)

  return <form>...</form>
}

With parseAtoms you can refactor usage to look like this:

import { parseAtoms } from '@reatom/lens'
import { useAtom, useAction } from '@reatom/npm-react'

export const User = ({ user }: { user: User }) => {
  const [
    {
      name, //
      bio,
      website,
      address,
    },
  ] = useAtom((ctx) => parseAtoms(ctx, user))

  return <form>...</form>
}

match

Creates an atom that depending on some condition, which can be an atom too. Useful for describing UIs with @reatom/jsx.

const num = atom(0, 'num')
const by3 = atom((ctx) => ctx.spy(num) % 3 === 0, 'by3')
const by5 = atom((ctx) => ctx.spy(num) % 5 === 0, 'by5')

const failMessage = atom(
  (ctx) => `${ctx.spy(num)} is not divisible by 3 nor by 5`,
)

const message = match(
  by3,
  'Divisible by 3',
  match(
    by5, //
    'Not divisible by 3 but divisible by 5',
    failMessage,
  ),
)

match JSX example

export function Dashboard({ user }: { user: Atom<User | null> }) {
  return match(
    user,
    atom((ctx) => <div>Dashboard content</div>),
    atom((ctx) => <AuthRedirect />),
  )
}

bind

Bind context to stable function.

import { action, createCtx } from '@reatom/core'
import { bind } from '@reatom/lens'

const doSome = action()
const ctx = createCtx()

export handleSome = bind(ctx, doSome)

handleSome(123)
// 123

bind(ctx, doSome) === bind(ctx, doSome)
// true

withReset

Adds reset action to reset the atom state.

For example, clear state after all dependencies and subscribers are gone.

import { atom } from '@reatom/core'
import { withReset } from '@reatom/lens'
import { onDisconnect } from '@reatom/hooks'

export const dataAtom = atom([], 'dataAtom').pipe(withReset())
onDisconnect(dataAtom, dataAtom.reset)