@coko/server/src/graphql/event-emitter-to-async-iterator.js

Reusable server for use by Coko's projects

/**
 * See comment in PostgresPubSub file
 */

// Based on https://github.com/apollographql/graphql-subscriptions/blob/master/src/event-emitter-to-async-iterator.ts
const { $$asyncIterator } = require('iterall')

function eventEmitterAsyncIterator(
  eventEmitter,
  eventsNames,
  commonMessageHandler = message => message,
) {
  const pullQueue = []
  const pushQueue = []

  const eventsArray =
    typeof eventsNames === 'string' ? [eventsNames] : eventsNames

  let listening = true

  const pushValue = ({ payload: event }) => {
    const value = commonMessageHandler(event)

    if (pullQueue.length !== 0) {
      pullQueue.shift()({ value, done: false })
    } else {
      pushQueue.push(value)
    }
  }

  const pullValue = () => {
    return new Promise(resolve => {
      if (pushQueue.length !== 0) {
        resolve({ value: pushQueue.shift(), done: false })
      } else {
        pullQueue.push(resolve)
      }
    })
  }

  const emptyQueue = () => {
    if (listening) {
      listening = false
      removeEventListeners()
      pullQueue.forEach(resolve => resolve({ value: undefined, done: true }))
      pullQueue.length = 0
      pushQueue.length = 0
    }
  }

  const addEventListeners = () => {
    /* eslint-disable-next-line no-restricted-syntax */
    for (const eventName of eventsArray) {
      eventEmitter.addListener(eventName, pushValue)
    }
  }

  const removeEventListeners = () => {
    /* eslint-disable-next-line no-restricted-syntax */
    for (const eventName of eventsArray) {
      eventEmitter.removeListener(eventName, pushValue)
    }
  }

  addEventListeners()

  return {
    next() {
      return listening ? pullValue() : this.return()
    },
    return() {
      emptyQueue()

      return Promise.resolve({ value: undefined, done: true })
    },
    throw(error) {
      emptyQueue()

      return Promise.reject(error)
    },
    [$$asyncIterator]() {
      return this
    },
  }
}

module.exports = {
  eventEmitterAsyncIterator,
}