Add socket.io example (#1766)

* Add socket.io example

* [update] stop handling events before unmount

* [update] add deploy to now button

* Fix linting problems

* Fix last missing linting problem

examples/with-socket.io/README.md

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+[![Deploy to now](https://deploy.now.sh/static/button.svg)](https://deploy.now.sh/?repo=https://github.com/zeit/next.js/tree/master/examples/with-socket.io)
+
+# Socket.io example
+
+## How to use
+
+Download the example [or clone the repo](https://github.com/zeit/next.js):
+
+```bash
+curl https://codeload.github.com/zeit/next.js/tar.gz/master | tar -xz --strip=2 next.js-master/examples/with-socket.io
+cd with-socket.io
+```
+
+Install it and run:
+
+```bash
+npm install
+npm run dev
+```
+
+## The idea behind the example
+
+This example show how to use [socket.io](https://socket.io/) inside a Next.js application. It uses `getInitialProps` to fetch the old messages from a HTTP endpoint as if it was a Rest API. The example combine the WebSocket server with the Next server, in a production application you should split them as different services.
+
+**Example:** [https://next-socket-io.now.sh/](https://next-socket-io.now.sh/)

examples/with-socket.io/package.json

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+{
+  "dependencies": {
+    "express": "^4.15.2",
+    "isomorphic-fetch": "^2.2.1",
+    "next": "latest",
+    "react": "^15.5.4",
+    "react-dom": "^15.5.4",
+    "socket.io": "^1.7.3",
+    "socket.io-client": "^1.7.3"
+  },
+  "scripts": {
+    "dev": "node server.js",
+    "build": "next build",
+    "start": "NODE_ENV=production node server.js"
+  }
+}

examples/with-socket.io/pages/index.js

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+import { Component } from 'react'
+import io from 'socket.io-client'
+import fetch from 'isomorphic-fetch'
+
+class HomePage extends Component {
+  // fetch old messages data from the server
+  static async getInitialProps ({ req }) {
+    const response = await fetch('http://localhost:3000/messages')
+    const messages = await response.json()
+    return { messages }
+  }
+
+  static defaultProps = {
+    messages: []
+  }
+
+  // init state with the prefetched messages
+  state = {
+    field: '',
+    messages: this.props.messages
+  }
+
+  // connect to WS server and listen event
+  componentDidMount () {
+    this.socket = io('http://localhost:3000/')
+    this.socket.on('message', this.handleMessage)
+  }
+
+  // close socket connection
+  componentWillUnmount () {
+    this.socket.off('message', this.handleMessage)
+    this.socket.close()
+  }
+
+  // add messages from server to the state
+  handleMessage = (message) => {
+    this.setState(state => ({ messages: state.messages.concat(message) }))
+  }
+
+  handleChange = event => {
+    this.setState({ field: event.target.value })
+  }
+
+  // send messages to server and add them to the state
+  handleSubmit = event => {
+    event.preventDefault()
+
+    // create message object
+    const message = {
+      id: (new Date()).getTime(),
+      value: this.state.field
+    }
+
+    // send object to WS server
+    this.socket.emit('message', message)
+
+    // add it to state and clean current input value
+    this.setState(state => ({
+      field: '',
+      messages: state.messages.concat(message)
+    }))
+  }
+
+  render () {
+    return (
+      <main>
+        <div>
+          <ul>
+            {this.state.messages.map(message =>
+              <li key={message.id}>{message.value}</li>
+            )}
+          </ul>
+          <form onSubmit={this.handleSubmit}>
+            <input
+              onChange={this.handleChange}
+              type='text'
+              placeholder='Hello world!'
+              value={this.state.field}
+            />
+            <button>Send</button>
+          </form>
+        </div>
+      </main>
+    )
+  }
+}
+
+export default HomePage

examples/with-socket.io/server.js

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+const app = require('express')()
+const server = require('http').Server(app)
+const io = require('socket.io')(server)
+const next = require('next')
+
+const dev = process.env.NODE_ENV !== 'production'
+const nextApp = next({ dev })
+const nextHandler = nextApp.getRequestHandler()
+
+// fake DB
+const messages = []
+
+// socket.io server
+io.on('connection', socket => {
+  socket.on('message', (data) => {
+    messages.push(data)
+    socket.broadcast.emit('message', data)
+  })
+})
+
+nextApp.prepare().then(() => {
+  app.get('/messages', (req, res) => {
+    res.json(messages)
+  })
+
+  app.get('*', (req, res) => {
+    return nextHandler(req, res)
+  })
+
+  server.listen(3000, (err) => {
+    if (err) throw err
+    console.log('> Ready on http://localhost:3000')
+  })
+})