applicationwillenter前台被称为:

当应用程序重新启动(从后台到前台) 当应用程序第一次启动时，即当applicationDidFinishLaunch被调用时，该方法不会被调用，而只在来自后台时调用 applicationDidBecomeActive

applicationDidBecomeActive被调用

当应用程序在didFinishLaunching后第一次启动时 在applicationwillenter前台之后，如果没有URL要处理。 调用handleOpenURL。 在applicationWillResignActive后，如果用户忽略中断，如电话或短信。 在alertView从应用程序的任何地方消失后

applicationwillenter前台被称为:

当应用程序重新启动(从后台到前台) 当应用程序第一次启动时，即当applicationDidFinishLaunch被调用时，该方法不会被调用，而只在来自后台时调用 applicationDidBecomeActive

applicationDidBecomeActive被调用

当应用程序在didFinishLaunching后第一次启动时 在applicationwillenter前台之后，如果没有URL要处理。 调用handleOpenURL。 在applicationWillResignActive后，如果用户忽略中断，如电话或短信。 在alertView从应用程序的任何地方消失后

applicationWillResignActive在系统请求权限时被调用。(在iOS 10中)。以防有人遇到和我一样的麻烦……

管理应用的生命周期有助于解决你的问题。对于快速概念，您可以查看该文档中的图。 你也可以从XCode向导生成的代码中读取注释。列出如下:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
    return YES;
}

- (void)applicationWillResignActive:(UIApplication *)application
{
    /*
     Sent when the application is about to move from active to inactive state. 
     This can occur for certain types of temporary interruptions (such as an 
     incoming phone call or SMS message) or when the user quits the application 
     and it begins the transition to the background state.
     Use this method to pause ongoing tasks, disable timers, and throttle down 
     OpenGL ES frame rates. Games should use this method to pause the game.
     */
}

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    /*
     Use this method to release shared resources, save user data, invalidate 
     timers, and store enough application state information to restore your 
     application to its current state in case it is terminated later. 
     If your application supports background execution, this method is called 
     instead of applicationWillTerminate: when the user quits.
     */
}

- (void)applicationWillEnterForeground:(UIApplication *)application
{
    /*
     Called as part of the transition from the background to the active state; 
     here you can undo many of the changes made on entering the background.
     */
}

- (void)applicationDidBecomeActive:(UIApplication *)application
{
    /*
     Restart any tasks that were paused (or not yet started) while the 
     application was inactive. If the application was previously in the 
     background, optionally refresh the user interface.
     */
}

- (void)applicationWillTerminate:(UIApplication *)application
{
    /*
     Called when the application is about to terminate.
     Save data if appropriate.
     See also applicationDidEnterBackground:.
     */
}

更详细的解释请参考UIApplicationDelegate的官方文档

当唤醒，即重新启动一个应用程序(通过跳板，应用程序切换或URL) applicationwillenter前台:被调用。它只在应用准备好使用时执行一次，在被放入后台后，而applicationDidBecomeActive:可能在启动后被调用多次。这使得applicationWillEnterForeground:非常适合重新启动后只需要发生一次的设置。

applicationwillenter前台:被称为:

当应用程序重新启动 applicationDidBecomeActive之前:

applicationDidBecomeActive:被调用:

当应用程序第一次启动后应用程序:didFinishLaunchingWithOptions: after applicationwillenter前台:如果没有URL要处理。 调用handleOpenURL。 after applicationWillResignActive:如果用户忽略中断，如电话或短信。

applicationWillResignActive:被调用:

当有电话之类的干扰时。 如果用户进行呼叫，则调用applicationDidEnterBackground。 如果用户忽略调用，则调用applicationDidBecomeActive。 当用户按下home键或切换应用程序时。 医生说你应该 暂停正在进行的任务 关闭定时器 暂停游戏 降低OpenGL帧速率

applicationDidEnterBackground:被调用:

applicationWillResignActive之后: 医生说你应该: 释放共享资源 保存用户数据 无效计时器 保存应用程序状态，以便在应用程序终止时恢复它。 禁用UI更新 您有5秒钟的时间来做需要做的事情并返回方法 如果你在5秒内没有返回，应用将被终止。 你可以用beginBackgroundTaskWithExpirationHandler请求更多的时间:

官方文件。

对于iOS 13+，将执行以下方法:

- (void)sceneWillEnterForeground:(UIScene *)scene
- (void)sceneDidBecomeActive:(UIScene *)scene