observe specific actions

observe-actions.js

import Alt from './'
import { ACTION_KEY } from './lib/symbols/symbols'

const alt = new Alt()

alt.observe = function (action) {
  const chain = []
  const subscriptions = []

  this.dispatcher.register((payload) => {
    if (payload.action !== action[ACTION_KEY]) return
    const state = chain.reduce((state, f) => {
      const value = f(state)
      return state === null ? state : value
    }, payload.data)
    if (state !== null) {
      subscriptions.forEach(subscription => subscription(state))
    }
  })

  return {
    map(f) {
      chain.push(f)
      return this
    },

    filter(f) {
      chain.push((x) => {
        const v = f(x)
        return v ? x : null
      })
      return this
    },

    reduce(f, acc) {
      chain.push(x => f(acc, x))
      return this
    },

    subscribe(onChange) {
      subscriptions.push(onChange)
      const dispose = () => {
        const id = subscriptions.indexOf(onChange)
        if (id >= 0) subscriptions.splice(id, 1)
      }
      return { dispose }
    }
  }
}

const { fire } = alt.generateActions('fire')


const listen = alt.observe(fire).map((data) => {
  return data * 2
})
.filter(x => x !== 12)
.reduce((state, nextState) => {
  return state + nextState
}, 87)

const subscription = listen.subscribe(x => console.log('CHANGED', x))

fire(12)
fire(6)
fire(3)

subscription.dispose()

fire(11)