App生命周期

应用状态

App主要有五种状态，分别是：

未运行——Not running

应用程序没启动

未激活——Inactive

程序在前台运行，不过没有接收到事件。
一般每当应用要从一个状态切换到另一个不同的状态时，中途过渡会短暂停留在此状态。唯一在此状态停留时间比较长的情况是：当用户锁屏时，或者系统提示用户去响应某些（诸如电话来电、有未读短信等）事件的时候。

激活——Active

程序在前台运行而且接收到了事件，这也是前台的一个正常的模式。

后台——Backgroud

程序在后台而且能执行代码，大多数程序进入这个状态后会在在这个状态上停留一会，时间到之后会进入挂起状态(Suspended)，有的程序经过特殊的请求后可以长期处于Backgroud状态。

挂起——Suspended

程序在后台不能执行代码。系统会自动把程序变成这个状态而且不会发出通知。当挂起时，程序还是停留在内存中的，当系统内存低时，系统就把挂起的程序清除掉，为前台程序提供更多的内存。

关系图

生命周期方法

APP的生命周期就是UIApplicationDelegate中的回调方法，这些方法是根据状态变化进行响应的地方，其中最常用的就是以下7个方法：

application:willFinishLaunchingWithOptions:
在App启动时调用表示应用加载进程已经开始,常用来处理应用状态的存储和恢复application:didFinishLaunchingWithOptions:
表示App将从未运行状态进入运行状态,用于对App的初始化操作applicationDidBecomeActive:
当应用即将进入前台运行时调用applicationWillResignActive:
当应用即将进从前台退出时调用applicationDidEnterBackground:
当应用开始在后台运行的时候调用applicationWillEnterForeground:
当程序从后台将要重新回到前台（但是还没变成Active状态）时候调用applicationWillTerminate:
当前应用即将被终止，在终止前调用的函数。通常是用来保存数据和一些退出前的清理工作。如果应用当前处在suspended，此方法不会被调用。 该方法最长运行时限为5秒，过期应用即被kill掉并且移除内存。

相关方法

AppDelegate实现了UIApplicationDelegate，有以下方法可供测试：

	//在App启动时调用表示应用加载进程已经开始,常用来处理应用状态的存储和恢复func application(_ application: UIApplication, willFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {print("willFinishLaunchingWithOptions")print("——应用开始加载——")return true}//表示App将从未运行状态进入运行状态,用于对App的初始化操作func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {// Override point for customization after application launch.print("调用：didFinishLaunchingWithOptions")print("——应用开始运行——")return true}//当应用即将进入前台运行时调用func applicationDidBecomeActive(_ application: UIApplication) {print("调用：applicationDidBecomeActive")print("——应用即将进入前台——")}//当应用即将进从前台退出时调用func applicationWillResignActive(_ application: UIApplication) {print("调用：applicationWillResignActive")print("——应用即将从前台退出——")}//当程序从后台将要重新回到前台（但是还没变成Active状态）时候调用func applicationWillEnterForeground(_ application: UIApplication) {print("调用：applicationWillEnterForeground")print("——应用即将从后台进入前台——")}//当应用开始在后台运行的时候调用func applicationDidEnterBackground(_ application: UIApplication) {print("调用：applicationDidEnterBackground")print("——应用开始在后台运行——")}//当前应用即将被终止，在终止前调用的函数。通常是用来保存数据和一些退出前的清理工作。如果应用当前处在suspended，此方法不会被调用。 该方法最长运行时限为5秒，过期应用即被kill掉并且移除内存func applicationWillTerminate(_ application: UIApplication) {print("调用：applicationWillTerminate")print("——应用即将终止——")}

注意

虽然都有这些方法，但是我实测下来，在AppDelegate种有的方法并没有打印信息，也就说明没有被调用。

applicationDidBecomeActiveapplicationWillResignActiveapplicationWillEnterForegroundapplicationDidEnterBackground

这几个方法并没有打印信息，不管我如何在前台或者后台。因此，我又在SceneDelegate中找到了对应的方法：

func sceneDidBecomeActive(_ scene: UIScene) {// Called when the scene has moved from an inactive state to an active state.// Use this method to restart any tasks that were paused (or not yet started) when the scene was inactive.print("调用：applicationDidBecomeActive")print("——应用即将进入前台——")}func sceneWillResignActive(_ scene: UIScene) {// Called when the scene will move from an active state to an inactive state.// This may occur due to temporary interruptions (ex. an incoming phone call).print("调用：applicationWillResignActive")print("——应用即将从前台退出——")}func sceneWillEnterForeground(_ scene: UIScene) {// Called as the scene transitions from the background to the foreground.// Use this method to undo the changes made on entering the background.print("调用：applicationWillEnterForeground")print("——应用即将从后台进入前台——")}func sceneDidEnterBackground(_ scene: UIScene) {// Called as the scene transitions from the foreground to the background.// Use this method to save data, release shared resources, and store enough scene-specific state information// to restore the scene back to its current state.print("调用：applicationDidEnterBackground")print("——应用开始在后台运行——")}

这次就完整的打印了生命周期方法，操作依次是：运行、按一次Home键返回桌面、重新进入App、双击Home键然后Kill掉应用。

在其他地方监听

除了在AppDelegate和SceneDelegate中可以监听生命周期之外，还可以在ViewController等地方监听，代码如下：

	func applicationNotification() {//应用开始在后台运行的时候通知NotificationCenter.default.addObserver(self, selector: #selector(applicationDidEnterBackground), name: UIApplication.didEnterBackgroundNotification, object: nil)//当程序从后台将要重新回到前台（但是还没变成Active状态）时候通知NotificationCenter.default.addObserver(self, selector: #selector(applicationWillEnterForeground), name: UIApplication.willEnterForegroundNotification, object: nil)//应用进入运行状态通知NotificationCenter.default.addObserver(self, selector: #selector(applicationDidFinishLaunching), name: UIApplication.didFinishLaunchingNotification, object: nil)//应用即将进入前台运行时通知NotificationCenter.default.addObserver(self, selector: #selector(applicationDidBecomeActive), name: UIApplication.didBecomeActiveNotification, object: nil)//应用即将进从前台退出时通知NotificationCenter.default.addObserver(self, selector: #selector(applicationWillResignActive), name: UIApplication.willResignActiveNotification, object: nil)//应用因内存警告退出通知NotificationCenter.default.addObserver(self, selector: #selector(applicationDidReceiveMemoryWarning), name: UIApplication.didReceiveMemoryWarningNotification, object: nil)//双击Home键杀掉应用通知NotificationCenter.default.addObserver(self, selector: #selector(applicationWillTerminate), name: UIApplication.willTerminateNotification, object: nil)}@objc func applicationDidEnterBackground() {print("app status : applicationDidEnterBackground——APP开始在后台运行")}@objc func applicationWillEnterForeground() {print("app status : applicationWillEnterForeground——APP从后台重新进入前台")}@objc func applicationDidFinishLaunching() {print("app status : applicationDidFinishLaunching——APP进入运行状态")}@objc func applicationDidBecomeActive() {print("app status : applicationDidBecomeActive——APP即将进入前台")}@objc func applicationWillResignActive() {print("app status : applicationWillResignActive——APP即将从前台退出")}@objc func applicationDidReceiveMemoryWarning() {print("app status : applicationDidReceiveMemoryWarning——APP因内存警告退出")}@objc func applicationWillTerminate() {print("app status : applicationWillTerminate——APP应用将被终止")}

结果如图：

ViewController生命周期

常用的控制器生命周期方法如下：

	override func viewDidLoad() {super.viewDidLoad()print("viewDidLoad-当视图控制器的视图加载完成后调用")}override func viewWillAppear(_ animated: Bool) {print("viewWillAppear-当视图即将显示在屏幕上时调用")}override func viewWillLayoutSubviews() {print("viewWillLayoutSubviews-将要对子视图进行调整")}override func viewDidLayoutSubviews() {print("viewDidLayoutSubviews-对子视图调整完毕")}override func viewDidAppear(_ animated: Bool) {print("viewDidAppear-当视图已经显示在屏幕上时调用")}override func viewWillDisappear(_ animated: Bool) {print("viewWillDisappear-当视图即将从屏幕上消失时调用")}override func viewDidDisappear(_ animated: Bool) {print("viewDidDisappear-当视图已经从屏幕上消失时调用")}override func didReceiveMemoryWarning() {print("didReceiveMemoryWarning-当系统内存不足时调用")}

结果如图：

UIView生命周期

UIView常用的生命周期方法如下：

import Foundation
import UIKitclass Button:UIButton{override func willRemoveSubview(_ subview: UIView) {super.willRemoveSubview(subview)print("willRemoveSubview-当子视图从本视图移除时调用")}override func willMove(toSuperview newSuperview: UIView?) {super.willMove(toSuperview: newSuperview)print("willMove-当视图即将加入父视图时 / 当视图即将从父视图移除时调用")}override func didMoveToSuperview() {super.didMoveToSuperview()print("didMoveToSuperview-当视图加入父视图时 / 当视图从父视图移除时调用")}override func willMove(toWindow newWindow: UIWindow?) {super.willMove(toWindow: newWindow)print("willMove-当视图即将加入window视图时 / 当视图即将从window视图移除时调用")}override func didMoveToWindow() {super.didMoveToWindow()print("didMoveToWindow-当视图加入window视图时 / 当视图从window视图移除时调用")}override func didAddSubview(_ subview: UIView) {super.didAddSubview(subview)print("didAddSubview-当视图添加子视图时调用")}
}

这里是通过创建一个自定义的按钮监听，然后使用：

    var button:Button!override func viewDidLoad() {super.viewDidLoad()button=Button(frame: CGRect(x: 100, y: 100, width: 100, height: 80))button.backgroundColor=UIColor.orangebutton.setTitle("按钮", for: .normal)self.view.addSubview(button)button.addTarget(self, action: #selector(click), for: UIControl.Event.touchUpInside)}@objc func click(){button.removeFromSuperview()}

点击按钮然后将其从父布局中移除，结果如下：

参考链接：iOS 生命周期 (最新最完整)