refactor: prefer using app.whenReady() (#21972)

* docs: add references to app.whenReady() in isReady

* refactor: prefer app.whenReady()

In the docs, specs, and lib, replace instances of `app.once('ready')`
(seen occasionally) and `app.on('ready')` (extremely common) with
`app.whenReady()`.

It's better to encourage users to use whenReady():
1. it handles the edge case of registering for 'ready' after it's fired
2. it avoids the minor wart of leaving an active listener alive for
an event that wll never fire again

spec/fixtures/api/command-line/main.js

@@ -1,6 +1,6 @@
 const { app } = require('electron')
 
-app.on('ready', () => {
+app.whenReady().then(() => {
   const payload = {
     hasSwitch: app.commandLine.hasSwitch('foobar'),
     getSwitchValue: app.commandLine.getSwitchValue('foobar')

spec/fixtures/api/cookie-app/main.js

@@ -1,6 +1,6 @@
 const { app, session } = require('electron')
 
-app.on('ready', async function () {
+app.whenReady().then(async function () {
   const url = 'http://foo.bar'
   const persistentSession = session.fromPartition('persist:ence-test')
   const name = 'test'

spec/fixtures/api/default-menu/main.js

@@ -18,7 +18,7 @@ try {
     Menu.setApplicationMenu(null)
   }
 
-  app.on('ready', () => {
+  app.whenReady().then(() => {
     setImmediate(() => {
       try {
         output(Menu.getApplicationMenu() === expectedMenu)

spec/fixtures/api/exit-closes-all-windows-app/main.js

@@ -8,7 +8,7 @@ function createWindow (id) {
   windows.push(window)
 }
 
-app.once('ready', () => {
+app.whenReady().then(() => {
   for (let i = 1; i <= 5; i++) {
     createWindow(i)
   }

spec/fixtures/api/gpu-info.js

@@ -2,7 +2,7 @@ const { app } = require('electron')
 
 app.commandLine.appendSwitch('--disable-software-rasterizer')
 
-app.on('ready', () => {
+app.whenReady().then(() => {
   const infoType = process.argv.pop()
   app.getGPUInfo(infoType).then(
     (gpuInfo) => {

spec/fixtures/api/leak-exit-webcontents.js

@@ -1,5 +1,5 @@
 const { app, webContents } = require('electron')
-app.on('ready', function () {
+app.whenReady().then(function () {
   webContents.create({})
 
   app.quit()

spec/fixtures/api/locale-check/main.js

@@ -1,6 +1,6 @@
 const { app } = require('electron')
 
-app.on('ready', () => {
+app.whenReady().then(() => {
   process.stdout.write(app.getLocale())
   process.stdout.end()
 

spec/fixtures/api/mixed-sandbox-app/main.js

@@ -12,7 +12,7 @@ if (process.argv.includes('--app-enable-sandbox')) {
 
 let currentWindowSandboxed = false
 
-app.once('ready', () => {
+app.whenReady().then(() => {
   function testWindow (isSandboxed, callback) {
     currentWindowSandboxed = isSandboxed
     const currentWindow = new BrowserWindow({

spec/fixtures/api/quit-app/main.js

@@ -1,6 +1,6 @@
 const { app } = require('electron')
 
-app.on('ready', function () {
+app.whenReady().then(function () {
   // This setImmediate call gets the spec passing on Linux
   setImmediate(function () {
     app.exit(123)

spec/fixtures/api/relaunch/main.js

@@ -7,7 +7,7 @@ process.on('uncaughtException', () => {
   app.exit(1)
 })
 
-app.once('ready', () => {
+app.whenReady().then(() => {
   const lastArg = process.argv[process.argv.length - 1]
   const client = net.connect(socketPath)
   client.once('connect', () => {

spec/fixtures/api/singleton/main.js

@@ -1,6 +1,6 @@
 const { app } = require('electron')
 
-app.once('ready', () => {
+app.whenReady().then(() => {
   console.log('started') // ping parent
 })
 

spec/fixtures/api/window-all-closed/main.js

@@ -14,7 +14,7 @@ app.on('quit', () => {
   process.stdout.end()
 })
 
-app.on('ready', () => {
+app.whenReady().then(() => {
   const win = new BrowserWindow()
   win.close()
 })