electron + React(create-react-app)でアプリを作り始める。

まずは`create-react-app`で雛形を作ります。

```
$ create-react-app electron_app
$ cd electron_app
$ yarn start
```

でサンプル画面を表示させます。
ここからスタートです。

以下の記事を参考にします。
https://medium.freecodecamp.org/building-an-electron-application-with-create-react-app-97945861647c

Electronをインストールします。

```
$ yarn add electron --dev
```

`package.json`を編集します。

```json:package.json
{
  "name": "electron_app",
  "version": "0.1.0",
  "private": true,
  "main": "src/electron-starter.js",
  "dependencies": {
    "react": "^16.2.0",
    "react-dom": "^16.2.0",
    "react-scripts": "1.1.0"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test --env=jsdom",
    "eject": "react-scripts eject",
    "electron": "electron ."
  },
  "devDependencies": {
    "electron": "^1.7.10"
  }
}
```

`scripts`に`electron`を追加しています。

`src/electron-starter.js`を作成します。

```javascript:src/electron-starter.js
const electron = require('electron');
// Module to control application life.
const app = electron.app;
// Module to create native browser window.
const BrowserWindow = electron.BrowserWindow;

const path = require('path');
const url = require('url');

// Keep a global reference of the window object, if you don't, the window will
// be closed automatically when the JavaScript object is garbage collected.
let mainWindow;

function createWindow() {
    // Create the browser window.
    mainWindow = new BrowserWindow({width: 800, height: 600});

    // and load the index.html of the app.
    /*
    const startUrl = process.env.ELECTRON_START_URL || url.format({
                pathname: path.join(__dirname, '/../build/index.html'),
                protocol: 'file:',
                slashes: true
            });
    mainWindow.loadURL(startUrl);
    */
    mainWindow.loadURL('http://localhost:3000');

    // Open the DevTools.
    mainWindow.webContents.openDevTools();

    // Emitted when the window is closed.
    mainWindow.on('closed', function () {
        // Dereference the window object, usually you would store windows
        // in an array if your app supports multi windows, this is the time
        // when you should delete the corresponding element.
        mainWindow = null
    })
}

// This method will be called when Electron has finished
// initialization and is ready to create browser windows.
// Some APIs can only be used after this event occurs.
app.on('ready', createWindow);

// Quit when all windows are closed.
app.on('window-all-closed', function () {
    // On OS X it is common for applications and their menu bar
    // to stay active until the user quits explicitly with Cmd + Q
    if (process.platform !== 'darwin') {
        app.quit()
    }
});

app.on('activate', function () {
    // On OS X it's common to re-create a window in the app when the
    // dock icon is clicked and there are no other windows open.
    if (mainWindow === null) {
        createWindow()
    }
});

// In this file you can include the rest of your app's specific main process
// code. You can also put them in separate files and require them here.

```

ここまでやって、`yarn start`で`http://localhost:3000`に内容を表示させつつ、`yarn electron`を実行すると、Electron上にReact Appが表示されます。

<img width="1062" alt="2018-01-21-19-16-52.png" src="https://qiita-image-store.s3.amazonaws.com/0/226431/b5cdfce8-bb08-bdcb-0a3f-b23e030737f8.png">

さて、今度は`yarn build`したアプリケーションをelectronで表示させてみます。
create-react-appで作成した`index.html`はデフォルトだと相対パスを利用してJavaScriptを読み込みます。すると、Electron AppからはJavaScriptファイルが見つからず、FILE NOT FOUNDのようなエラーが出ます。

それを防ぐために、`package.json`に`"homepage": "./",`を追加します。

```json:package.json
{
  "name": "electron_app",
  "version": "0.1.0",
  "private": true,
  "main": "src/electron-starter.js",
  "dependencies": {
    "react": "^16.2.0",
    "react-dom": "^16.2.0",
    "react-scripts": "1.1.0"
  },
  "homepage": "./",
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test --env=jsdom",
    "eject": "react-scripts eject",
    "electron": "electron ."
  },
  "devDependencies": {
    "electron": "^1.7.10"
  }
}

```

スターターとなるJavaScriptのlard urlも変更します。

```javascript
    const startUrl = process.env.ELECTRON_START_URL || url.format({
                pathname: path.join(__dirname, '/../build/index.html'),
                protocol: 'file:',
                slashes: true
            });
    mainWindow.loadURL(startUrl);
```

このように`build/index.html`を表示させるように変更しました。

```javascript:src/electron-starter.js
const electron = require('electron');
// Module to control application life.
const app = electron.app;
// Module to create native browser window.
const BrowserWindow = electron.BrowserWindow;

const path = require('path');
const url = require('url');

// Keep a global reference of the window object, if you don't, the window will
// be closed automatically when the JavaScript object is garbage collected.
let mainWindow;

function createWindow() {
    // Create the browser window.
    mainWindow = new BrowserWindow({width: 800, height: 600});

    // and load the index.html of the app.

    const startUrl = process.env.ELECTRON_START_URL || url.format({
                pathname: path.join(__dirname, '/../build/index.html'),
                protocol: 'file:',
                slashes: true
            });
    mainWindow.loadURL(startUrl);

    //mainWindow.loadURL('http://localhost:3000');

    // Open the DevTools.
    mainWindow.webContents.openDevTools();

    // Emitted when the window is closed.
    mainWindow.on('closed', function () {
        // Dereference the window object, usually you would store windows
        // in an array if your app supports multi windows, this is the time
        // when you should delete the corresponding element.
        mainWindow = null
    })
}

// This method will be called when Electron has finished
// initialization and is ready to create browser windows.
// Some APIs can only be used after this event occurs.
app.on('ready', createWindow);

// Quit when all windows are closed.
app.on('window-all-closed', function () {
    // On OS X it is common for applications and their menu bar
    // to stay active until the user quits explicitly with Cmd + Q
    if (process.platform !== 'darwin') {
        app.quit()
    }
});

app.on('activate', function () {
    // On OS X it's common to re-create a window in the app when the
    // dock icon is clicked and there are no other windows open.
    if (mainWindow === null) {
        createWindow()
    }
});

// In this file you can include the rest of your app's specific main process
// code. You can also put them in separate files and require them here.

```

`yarn build`して`yarn electron`を実行すれば、Electron Appが立ち上がり、デフォルトの画面が表示されます。