FirebaseReplicator.js

import { EventEmitter } from 'events'
import { assign, assignIn, forEach, set, unset } from 'lodash'

export default class FirebaseReplicator extends EventEmitter {
  static all (replicators) {
    const emitter = new EventEmitter()

    const start = () => {
      return Promise.all(replicators.map((rep) => rep.start()))
    }

    const stop = () => {
      replicators.forEach((rep) => rep.stop())
    }

    replicators.forEach((rep) => {
      rep.on('error', (err) => { emitter.emit('error', err) })
    })

    return assignIn({ start, stop }, emitter)
  }

  constructor (firebaseRef, handlers) {
    super()

    this.replicating = false
    this.handlers = {}
    this.ref = firebaseRef

    forEach(handlers, (fn, key) => {
      this.handlers[key] = (snap) => {
        this.onUpdate()
        this.emit(key, snap)
        fn(snap)
      }
    })
  }

  onUpdate () {
    this.emit('update')
  }

  onError (err) {
    this.emit('error', err)
  }

  start () {
    return new Promise((resolve, reject) => {
      if (!this.replicating) {
        this.ref.once('value', (snap) => {
          this.replicating = true

          this.handlers.value(snap) // populate target object with initial data

          // TODO: stop 'child_added' events for objects received in the 'value' event
          this.ref.on('child_added', this.handlers.child_added, this.onError)
          this.ref.on('child_changed', this.handlers.child_changed, this.onError)
          this.ref.on('child_removed', this.handlers.child_removed, this.onError)

          resolve()
        }, (err) => {
          this.replicating = false

          reject(err)
        })
      } else {
        resolve()
      }
    })
  }

  stop () {
    if (this.replicating) {
      this.ref.off('child_added', this.handlers.child_added)
      this.ref.off('child_changed', this.handlers.child_changed)
      this.ref.off('child_removed', this.handlers.child_removed)

      this.replicating = false
    }
  }
}

const FirebaseReplicatorFactory = function (createHandlers) {
  return class extends FirebaseReplicator {
    constructor (firebaseRef, dest) {
      const handlers = createHandlers(dest)
      super(firebaseRef, handlers)
    }
  }
}

export const FirebaseMemoryReplicator = FirebaseReplicatorFactory(function (dest) {
  return {
    value: function (snap) { assign(dest, snap.val()) },
    child_added: function (snap) { set(dest, snap.key(), snap.val()) },
    child_changed: function (snap) { set(dest, snap.key(), snap.val()) },
    child_removed: function (snap) { unset(dest, snap.key()) }
  }
})