When building stagetimer.io I decided to use WebSockets to push updates to all connected devices. The app is built on top of Vue using Vuex for state management. For WebSockets, I am using the popular and easy to use socket.io library.

# Contents

1. The desired Result
2. Existing Plugins
3. Instantiating the Socket
4. Using the Socket with Vue
5. Using the Socket with Vuex

# The desired Result

I want to use socket.io inside Vue components as well as in the Vuex store. It must also reuse the same socket connection in both cases.

Inside Vue components I want to have access to the socket instance through this.$socket and use it to listen to and emit messages like this:

export default {
  created () {
    this.$socket.on('message', this.receiveMessage)
  },
  destroyed () {
    this.$socket.removeAllListeners('message')
  }
  methods: {
    sendMessage (payload) {
      this.$socket.emit('message', payload)
    },
    receiveMessage (payload) {
      console.log('received a message', payload)
    },
  }
}

Inside the Vuex store I want to do the same, except that a received event should translate into a store action like so:

return new Vuex.Store({
  state: { ... },
  getters: { ... },
  mutations: { ... },
  actions: {
    sendMessage (context, payload) {
      this.$socket.emit('message', payload)
    },
    receiveMessage ({ commit }, payload) {
      commit('doSomethingWithMessage', payload)
    },
  },
})

# Existing Plugins

There is no lack of socket.io plugins for Vue. vue-socket.io looks like a good solution. However, I try to avoid 3rd party dependencies if I can easily implement their function myself, as in the case of socket.io.

# Instantiating the Socket

To reuse the socket I need to pass the same instance to Vue and the Vuex store. Basically, I want to import the same instance in two different files, kind of like a singleton. Fortunately, the »JavaScript specification guarantees that [I will] receive the same module instance« if I import the same path twice (Source). This means I can create a simple socket.js file that exports an instance of socket.io.

// socket.js
import io from 'socket.io-client'
const socket = io('http://localhost:3000')
export default socket

# Using the Socket with Vue

Now I can import the socket.io instance into the default main.js file where Vue gets instantiated. To have this.$socket available in my Vue single file components I can simply inject the socket into the prototype of Vue.

// main.js
import Vue from 'vue'
import socket from './socket'

Vue.prototype.$socket = socket

new Vue({...})

# Using the Socket with Vuex

With Vuex it’s not quite so simple. I need to write a little plugin to have this.$socket available inside store functions. Additionally, I have to define a mapping between socket events and the store actions they should trigger.

Here is the plugin code:

// websocketStorePlugin.js
export default function createWebSocketPlugin (socket) {
  return store => {
    store.$socket = socket
    socket.on('message', payload => store.dispatch('receiveMessage', payload))
  }
}

Instead of mapping socket events with store actions explicitly, I could also just map every event to a store action of the same name. But in my actual app, I like to keep it specific and I also like to use store modules.

I then import this plugin into my main store file, usually store.js or store/index.js. The createWebSocketPlugin method is called with the socket.io instance and passed to the store as a plugin.

import Vue from 'vue'
import Vuex from 'vuex'
import createWebSocketPlugin from './websocketStorePlugin'
import socket from '../socket'

Vue.use(Vuex)
const websocketPlugin = createWebSocketPlugin(socket)
export default new Vuex.Store({
  // ...
  plugins: [websocketPlugin],
})

And yeah, that’s it!