主进程（Main Process）

正如之前所提，Electron有两大进程：主进程（Main Process）和渲染进程（Renderer Process）。在这个示例程序中，主进程代码就在main.js文件中。

Note
通常将主进程文件命名为main.js，这样表示Node从这里启动应用的主进程。当你的项目代码重构目录结构后，必须在package.json中将主进程文件重新指定。

main.js的基础代码中有许多有用的注释，大致如下：

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.mainWindow.loadURL(url.format({pathname: path.join(__dirname, 'index.html'),protocol: 'file:',slashes: true}))// 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 + Qif (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.

最开始的代码用来获取应用所需模块：

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')

electron正是Electron模块，提供了Electron的API和Node扩展。

app调用Electron的API来控制应用的生命周期，稍后的代码会展示其用法。

BrowserWindow控制Electron的渲染进程。我们将会用它创建一个Chromium实例来生成应用的UI。

path和url是Electron调用的Node API，这些是能供任何Electron程序调用的Node API的一部分。path用来处理文件和目录操作，url用来创建URL。

接下来的代码穿插着注释讲述如下：

// 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

Note
如果你启动应用后窗口却不出现，很可能是因为主进程没有引用mainWindow。这种情况也会导致应用的其他窗口无法创建。

然后是创建主窗口的方法createWindow：

function createWindow() {// Create the browser window.mainWindow = new BrowserWindow({ width: 800, height: 600 })// and load the index.html of the app.mainWindow.loadURL(url.format({pathname: path.join(__dirname, 'index.html'),protocol: 'file:',slashes: true}))// 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})
}

createWindow方法的流程如下：
* 创建一个浏览器窗口（BrowserWindow），传入一个描述窗口尺寸的对象。本例中窗口高800像素，宽600像素。
* 使用path和url连接index.html模块加载窗口。index.html文件是渲染进程的开始点。
* mainWindow.webContents.openDevTools()打开Chromium的开发者工具（Developer Tools）。开发者工具对于调试渲染进程十分有用。
* 监听窗口实例的closed事件。此处注释描述如何管理多个窗口。这个方法仅仅是将你的窗口实例引用置为null，在未来将会加入更多代码。

代码最后的部分是app的监听器（listener），用来控制窗口何时创建、应用何时退出。

// 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 + Qif (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.

app实例监听三个事件：ready、window-all-closed和activate。
当收到ready事件后，调用上面讲过的createWindow方法。

如注释所述，由于在Mac OS X平台下用户可以关闭程序所有的窗口而不退出程序，而在Windows和Linux却并非如此，window-all-closed用来关闭非Max OS X平台的程序。

值得注意的是，我们用了Node API提供的模块process。在本例中process.platform在Max OS X平台会返回darwin，而在Windows平台会返回win32。即时在64位Windows平台下process.platform也会返回win32，用process.arch（表示架构：Architecture）会返回x64。

由于用户能在Max OS X平台下启动一个应用而不打开窗口，activate事件被触发时会创建新窗口。这里没有判断当前系统类型，因为创建窗口的条件为：activate事件被触发，并且主窗口为空（mainWindow === null），这些条件只会在Max OS X平台满足。

在main.js文件的最底端注释提示我们，可以在此为主进程添加其他代码，或者引用外部文件提高代码整洁度。

---