Create and combine asynchronous data sources that work great with React Concurrent Mode.

Installation

# For npm users...
npm install --save retil-source

# For yarn users...
yarn add retil-source

Sources don't always have values

Why should a data source return a value before it has one?

import { createState, hasSnapshot } from 'retil'

const [stateSource, setState] = createState()
const [getState, subscribe] = stateSource

getState() // throws a promise that resolves to the first value.

setState('yo!')

getState() // now it returns 'yo!'

Examples

Say you have a source that switches between having a value, and not having a value -- e.g. because it maps an id to data on a remote server. But you still want to display the latest value.

import { Source, fuse } from 'retil'

export const fallbackToMostRecent = <T>(source: Source<T>): Source<T> => {
  let fallback: [T] | [] = []
  return fuse(use => {
    const value = use(source, ...fallback)
    fallback = [value]
    return value
  })
}

Maybe as well as remembering the most recent value, you'd also like to set a flag indicating whether the source is missing a current value. To do this, you can use your source twice, with different fallback values. Don't worry, fuse will only subscribe to the source once!

const missingFlag = Symbol('missing')

export function addBusyFlag<T, U>(
  source: Source<T>,
  build: (snapshot: T, flag: boolean) => U,
): Source<U> {
  let fallback: [T] | [] = []
  return fuse((use) => {
    const value = use(source, ...fallback)
    const valueOrFlag = use(source, missingFlag)
    fallback = [value]
    return build(value, valueOrFlag === missingFlag)
  })
}