iOS NSNotificationCenter检查应用程序是否从后台返回前台

Question

我有一个情况，在后台返回前台时需要每次初始化一个对象，而且应该使用NSNotificationCenter而不是AppDelegate，因为我正在构建一个静态库，因此没有AppDelegate，请在此方面帮助我。

- user3115014

Answer 1

你尝试过UIApplicationWillEnterForegroundNotification吗？

在调用applicationWillEnterForeground:之前，该应用还会发布一个 UIApplicationWillEnterForegroundNotification 通知，以便给感兴趣的对象一个响应转换的机会。

订阅通知：

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(yourUpdateMethodGoesHere:)
                                             name:UIApplicationWillEnterForegroundNotification
                                           object:nil];

实现一个需要被调用的代码：

- (void) yourUpdateMethodGoesHere:(NSNotification *) note {
// code
}

别忘了取消订阅：

[[NSNotificationCenter defaultCenter] removeObserver:self];

- Nikita Took

---

请查看此链接：https://dev59.com/pnI95IYBdhLWcg3wsQP9 - Vinod Singh

Answer 2

Swift 4.2

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

- Andrea Miotto

Answer 3

Swift 5

订阅通知 -

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)

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

取消订阅 -

override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)

        NotificationCenter.default.removeObserver(self)
} 

需要调用的函数 -

@objc func applicationWillEnterForeground(_ notification: NSNotification) {
       self.volumeSlider.value = AVAudioSession.sharedInstance().outputVolume
    }

- Kiran Jasvanee

Answer 4

Swift 3版本

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    NotificationCenter.default.addObserver(self,
                                           selector:#selector(applicationWillEnterForeground(_:)),
                                           name:NSNotification.Name.UIApplicationWillEnterForeground,
                                           object: nil)
}

override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
    NotificationCenter.default.removeObserver(self)
}

func applicationWillEnterForeground(_ notification: NSNotification) {
   ....
}

您还可以使用NSNotification.Name.UIApplicationDidBecomeActive

- mt81

Answer 5

Swift 5使用：

UIApplication.willEnterForegroundNotification

- SmileBot

Answer 6

Swift 5

 override func viewDidAppear(_ animated: Bool) {
    NotificationCenter.default.addObserver(self, selector: #selector(appMovedToForeground), name: UIApplication.willEnterForegroundNotification, object: nil)
    }
    
    override func viewDidDisappear(_ animated: Bool) {
    NotificationCenter.default.removeObserver(self, name: NSNotification.Name(rawValue: UIApplication.willEnterForegroundNotification.rawValue), object: nil)
    }

    @objc func appMovedToForeground() {
       // Do stuff
   }

- Gurjinder Singh

Answer 7

Swift 3和4版本

NotificationCenter.default.addObserver(forName: NSNotification.Name.UIApplicationWillEnterForeground, object: nil, queue: nil) { notification in
        ...
}

- omerx