我有一个View Controller，其中我的值为0（标签），当我从另一个视图控制器中打开该View Controller时，ViewController我已设置viewDidAppear为label 设置为20。它可以正常工作，但是当我关闭我的应用程序并再次打开我的应用程序时，该值不会更改，因为viewDidLoad，viewDidAppear并且viewWillAppear没有任何调用。打开我的应用程序时该怎么打电话。我需要做些什么applicationDidBecomeActive吗？

我对事件的确切顺序感到好奇，所以我对应用程序进行了如下检测：（@ Zohaib，您可以使用下面的NSNotificationCenter代码来回答您的问题）。

// AppDelegate.m

- (void)applicationWillEnterForeground:(UIApplication *)application
{
    NSLog(@"app will enter foreground");
}

- (void)applicationDidBecomeActive:(UIApplication *)application
{
    NSLog(@"app did become active");
}

// ViewController.m

- (void)viewDidLoad
{
    [super viewDidLoad];
    NSLog(@"view did load");

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appDidBecomeActive:) name:UIApplicationDidBecomeActiveNotification object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appWillEnterForeground:) name:UIApplicationWillEnterForegroundNotification object:nil];
}

- (void)appDidBecomeActive:(NSNotification *)notification {
    NSLog(@"did become active notification");
}

- (void)appWillEnterForeground:(NSNotification *)notification {
    NSLog(@"will enter foreground notification");
}

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    NSLog(@"view will appear");
}

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    NSLog(@"view did appear");
}

在启动时，输出如下所示：

2013-04-07 09:31:06.505 myapp[15459:11303] view did load
2013-04-07 09:31:06.507 myapp[15459:11303] view will appear
2013-04-07 09:31:06.511 myapp[15459:11303] app did become active
2013-04-07 09:31:06.512 myapp[15459:11303] did become active notification
2013-04-07 09:31:06.517 myapp[15459:11303] view did appear

输入背景，然后重新输入前景：

2013-04-07 09:32:05.923 myapp[15459:11303] app will enter foreground
2013-04-07 09:32:05.924 myapp[15459:11303] will enter foreground notification
2013-04-07 09:32:05.925 myapp[15459:11303] app did become active
2013-04-07 09:32:05.926 myapp[15459:11303] did become active notification

使用Objective-C

您应该UIApplicationWillEnterForegroundNotification在ViewController的viewDidLoad方法中注册一个，只要应用从后台返回，您就可以在注册通知的方法中做任何您想做的事情。当应用程序从后台返回到前景时，不会调用ViewController的viewWillAppear或viewDidAppear。

-(void)viewDidLoad{

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(doYourStuff)

  name:UIApplicationWillEnterForegroundNotification object:nil];
}

-(void)doYourStuff{

   // do whatever you want to do when app comes back from background.
}

不要忘记取消注册您的注册通知。

-(void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

请注意，如果您为注册viewController，UIApplicationDidBecomeActiveNotification则每次您的应用程序激活时都会调用您的方法，不建议注册viewController此通知。

使用Swift

要添加观察者，您可以使用以下代码

 override func viewDidLoad() {
    super.viewDidLoad()

     NotificationCenter.default.addObserver(self, selector: "doYourStuff", name: UIApplication.willEnterForegroundNotification, object: nil)
 }

 func doYourStuff(){
     // your code
 }

要删除观察者，您可以使用swift的deinit功能。

deinit {
    NotificationCenter.default.removeObserver(self)
}

Swift 3.0 ++版本

在您的中viewDidLoad，在通知中心注册以收听此后台操作打开的操作

NotificationCenter.default.addObserver(self, selector:#selector(doSomething), name: NSNotification.Name.UIApplicationWillEnterForeground, object: nil)
        

然后添加此功能并执行所需的操作

func doSomething(){
    //...
}

最后，在销毁视图控制器时添加此功能以清理通知观察器。

deinit {
    NotificationCenter.default.removeObserver(self)
}

斯威夫特4.2。版

向NotificationCenter注册以在viewDidLoad应用程序从后台返回时收到通知

NotificationCenter.default.addObserver(self, selector: #selector(doSomething), name: UIApplication.willEnterForegroundNotification, object: nil)

实现应调用的方法。

@objc private func doSomething() {
    // Do whatever you want, for example update your view.
}

ViewController摧毁后，您可以删除观察者。仅在iOS9和macOS 10.11以下才需要

deinit {
    NotificationCenter.default.removeObserver(self)
}

只需让您的视图控制器注册该UIApplicationWillEnterForegroundNotification通知并做出相应反应即可。

我认为注册UIApplicationWillEnterForegroundNotification是冒险的，因为您可能最终会遇到多个控制器对该通知做出反应。当收到通知时，这些控制器仍然可见。

这是我的工作：我直接从应用程序的委托didBecomeActive方法强制在活动控制器上调用viewDidAppear：

将以下代码添加到 - (void)applicationDidBecomeActive:(UIApplication *)application

UIViewController *activeController = window.rootViewController;
if ([activeController isKindOfClass:[UINavigationController class]]) {
    activeController = [(UINavigationController*)window.rootViewController topViewController];
}
[activeController viewDidAppear:NO];

尝试将其添加到AppDelegate applicationWillEnterForeground中。

func applicationWillEnterForeground(_ application: UIApplication) {        
    // makes viewWillAppear run
    self.window?.rootViewController?.beginAppearanceTransition(true, animated: false)
    self.window?.rootViewController?.endAppearanceTransition()
}

根据Apple的文档：

(void)beginAppearanceTransition:(BOOL)isAppearing animated:(BOOL)animated;

说明：
告诉子控制器其外观即将更改。如果要实现自定义容器控制器，请使用此方法告诉子项其视图即将出现或消失。不要调用viewWillAppear:，viewWillDisappear:，viewDidAppear:，或viewDidDisappear:直接。

(void)endAppearanceTransition;

描述：

告诉子控制器其外观已更改。如果要实现自定义容器控制器，请使用此方法告诉子视图转换已完成。

样例代码：

(void)applicationDidEnterBackground:(UIApplication *)application
{

    [self.window.rootViewController beginAppearanceTransition: NO animated: NO];  // I commented this line

    [self.window.rootViewController endAppearanceTransition]; // I commented this line

}

问题：我如何解决？

答：我发现这条线在应用中。这些行使我的应用程序未收到任何ViewWillAppear通知。当我评论这些行时，它工作正常。