Adds mobx example (#676)

* Adds mobx example

* Fix coding style to match JS Standard

* Removes unecessary link

* Update README.md

* Update README.md

* Change next version from beta to ^2.0.0-beta

* Assigns the store to a local var

* Adds navigation in the example

* Removes unecessary imports

* Fix coding style to match JS Standard

* Fix Clock import

examples/with-mobx/.babelrc

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+{
+  "presets": [
+    "next/babel"
+  ],
+  "plugins": [
+    "transform-decorators-legacy"
+  ]
+}

examples/with-mobx/README.md

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+
+# MobX example
+
+## How to use
+
+Download the example (or clone the repo)[https://github.com/zeit/next.js.git]:
+
+```bash
+curl https://codeload.github.com/zeit/next.js/tar.gz/master | tar -xz --strip=2 next.js-master/examples/with-mobx
+cd with-mobx
+```
+
+Install it and run:
+
+```bash
+npm install
+npm run dev
+```
+
+Deploy it to the cloud with [now](https://zeit.co/now) ([download](https://zeit.co/download))
+
+```bash
+now
+```
+## Notes
+This example is a mobx port of the [with-redux](https://github.com/zeit/next.js/tree/master/examples/with-redux) example. Decorator support is activated by adding a `.babelrc` file at the root of the project:
+
+```json
+{
+  "presets": [
+    "next/babel"
+  ],
+  "plugins": [
+    "transform-decorators-legacy"
+  ]
+}
+```
+
+## The idea behind the example
+
+Usually splitting your app state into `pages` feels natural but sometimes you'll want to have global state for your app. This is an example on how you can use mobx that also works with our universal rendering approach. This is just a way you can do it but it's not the only one.
+
+In this example we are going to display a digital clock that updates every second. The first render is happening in the server and then the browser will take over. To illustrate this, the server rendered clock will have a different background color than the client one.
+
+![](http://i.imgur.com/JCxtWSj.gif)
+
+Our page is located at `pages/index.js` so it will map the route `/`. To get the initial data for rendering we are implementing the static method `getInitialProps`, initializing the mobx store and returning the initial timestamp to be rendered. The root component for the render method is the `mobx-react Provider` that allows us to send the store down to children components so they can access to the state when required.
+
+To pass the initial timestamp from the server to the client we pass it as a prop called `lastUpdate` so then it's available when the client takes over.
+
+The trick here for supporting universal mobx is to separate the cases for the client and the server. When we are on the server we want to create a new store every time, otherwise different users data will be mixed up. If we are in the client we want to use always the same store. That's what we accomplish on `store.js`
+
+The clock, under `components/Clock.js`, has access to the state using the `inject` and `observer` functions from `mobx-react`. In this case Clock is a direct child from the page but it could be deep down the render tree.

examples/with-mobx/components/Clock.js

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+export default (props) => {
+  return (
+    <div className={props.light ? 'light' : ''}>
+      {format(new Date(props.lastUpdate))}
+      <style jsx>{`
+        div {
+          padding: 15px;
+          color: #82FA58;
+          display: inline-block;
+          font: 50px menlo, monaco, monospace;
+          background-color: #000;
+        }
+
+        .light {
+          background-color: #999;
+        }
+      `}</style>
+    </div>
+  )
+}
+
+const format = t => `${pad(t.getHours())}:${pad(t.getMinutes())}:${pad(t.getSeconds())}`
+
+const pad = n => n < 10 ? `0${n}` : n

examples/with-mobx/components/Page.js

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+import React from 'react'
+import Link from 'next/link'
+import { inject, observer } from 'mobx-react'
+import Clock from './Clock'
+
+@inject('store') @observer
+class Page extends React.Component {
+
+  componentDidMount () {
+    this.props.store.start()
+  }
+
+  componentWillUnmount () {
+    this.props.store.stop()
+  }
+
+  render () {
+    return (
+      <div>
+        <h1>{this.props.title}</h1>
+        <Clock lastUpdate={this.props.store.lastUpdate} light={this.props.store.light} />
+        <nav>
+          <Link href={this.props.linkTo}>Navigate</Link>
+        </nav>
+      </div>
+    )
+  }
+}
+
+export default Page

examples/with-mobx/package.json

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+{
+  "name": "with-mobx",
+  "version": "1.0.0",
+  "scripts": {
+    "dev": "next",
+    "build": "next build",
+    "start": "next start"
+  },
+  "dependencies": {
+    "mobx": "^2.7.0",
+    "mobx-react": "^4.0.4",
+    "next": "^2.0.0-beta"
+  },
+  "author": "",
+  "license": "ISC",
+  "devDependencies": {
+    "babel-plugin-transform-decorators-legacy": "^1.3.4"
+  }
+}

examples/with-mobx/pages/index.js

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+import React from 'react'
+import { Provider } from 'mobx-react'
+import { initStore } from '../store'
+import Page from '../components/Page'
+
+export default class Counter extends React.Component {
+  static getInitialProps ({ req }) {
+    const isServer = !!req
+    const store = initStore(isServer)
+    return { lastUpdate: store.lastUpdate, isServer }
+  }
+
+  constructor (props) {
+    super(props)
+    this.store = initStore(props.isServer, props.lastUpdate)
+  }
+
+  render () {
+    return (
+      <Provider store={this.store}>
+        <Page title='Index Page' linkTo='/other' />
+      </Provider>
+    )
+  }
+}

examples/with-mobx/pages/other.js

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+import React from 'react'
+import { Provider } from 'mobx-react'
+import { initStore } from '../store'
+import Page from '../components/Page'
+
+export default class Counter extends React.Component {
+  static getInitialProps ({ req }) {
+    const isServer = !!req
+    const store = initStore(isServer)
+    return { lastUpdate: store.lastUpdate, isServer }
+  }
+
+  constructor (props) {
+    super(props)
+    this.store = initStore(props.isServer, props.lastUpdate)
+  }
+
+  render () {
+    return (
+      <Provider store={this.store}>
+        <Page title='Other Page' linkTo='/' />
+      </Provider>
+    )
+  }
+}

examples/with-mobx/store.js

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+import { action, observable } from 'mobx'
+
+let store = null
+
+class Store {
+  @observable lastUpdate = 0
+  @observable light = false
+
+  constructor (isServer, lastUpdate) {
+    this.lastUpdate = lastUpdate
+  }
+
+  @action start = () => {
+    this.timer = setInterval(() => {
+      this.lastUpdate = Date.now()
+      this.light = true
+    })
+  }
+
+  stop = () => clearInterval(this.timer)
+}
+
+export function initStore (isServer, lastUpdate = Date.now()) {
+  if (isServer && typeof window === 'undefined') {
+    return new Store(isServer, lastUpdate)
+  } else {
+    if (store === null) {
+      store = new Store(isServer, lastUpdate)
+    }
+    return store
+  }
+}