Question

我的主AppDelegate.m类中有UIApplicationDelegate协议，并定义了applicationDidBecomeActive方法。

我想在应用程序从后台返回时调用方法，但该方法在另一个视图控制器中。如何检查当前在applicationDidBecomeActive方法中显示哪个视图控制器，然后调用该控制器中的方法？

Answer 1

应用程序中的任何类都可以成为应用程序中不同通知的“观察者”。当您创建（或加载）视图控制器时，您需要将其注册为UIApplicationDidBecomeActiveNotification的观察者，并指定在将通知发送到您的应用程序时要调用的方法。

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(someMethod:)
                                             name:UIApplicationDidBecomeActiveNotification object:nil];

别忘了自己清理！当你的观点消失时，请记得以观察者身份移除自己：

[[NSNotificationCenter defaultCenter] removeObserver:self 
                                                name:UIApplicationDidBecomeActiveNotification
                                              object:nil];

有关Notification Center。

的更多信息

Answer 2

Swift 3,4等效：

添加观察者

NotificationCenter.default.addObserver(self,
    selector: #selector(applicationDidBecomeActive),
    name: .UIApplicationDidBecomeActive, // UIApplication.didBecomeActiveNotification for swift 4.2+
    object: nil)

删除观察者

NotificationCenter.default.removeObserver(self,
    name: .UIApplicationDidBecomeActive, // UIApplication.didBecomeActiveNotification for swift 4.2+
    object: nil)

回调

@objc func applicationDidBecomeActive() {
    // handle event
}

Answer 3

Swift 2等效：

let notificationCenter = NSNotificationCenter.defaultCenter()

// Add observer:
notificationCenter.addObserver(self,
  selector:Selector("applicationWillResignActiveNotification"),
  name:UIApplicationWillResignActiveNotification,
  object:nil)

// Remove observer:
notificationCenter.removeObserver(self,
  name:UIApplicationWillResignActiveNotification,
  object:nil)

// Remove all observer for all notifications:
notificationCenter.removeObserver(self)

// Callback:
func applicationWillResignActiveNotification() {
  // Handle application will resign notification event.
}

Answer 4

Swift 4.2

添加观察者-

NotificationCenter.default.addObserver(self, selector: #selector(handleEvent), name: UIApplication.didBecomeActiveNotification, object: nil)

删除观察者-

NotificationCenter.default.removeObserver(self, name: UIApplication.didBecomeActiveNotification, object: nil)

处理事件-

@objc func handleEvent() {
}

Answer 5

使用Swift 4，Apple建议通过新的编译器警告我们避免在这种情况下使用#selector。以下是实现此目的的更安全的方法：

首先，创建一个可供通知使用的惰性var：

lazy var didBecomeActive: (Notification) -> Void = { [weak self] _ in
    // Do stuff
}

如果您需要包含实际通知，只需将_替换为notification。

接下来，我们设置通知以观察应用程序是否处于活动状态。

func setupObserver() {
    _ = NotificationCenter.default.addObserver(forName: .UIApplicationDidBecomeActive,
                                               object: nil,
                                               queue:.main,
                                               using: didBecomeActive)
}

这里的重大变化是，我们现在调用上面创建的var，而不是调用#selector。这可以消除导致选择器崩溃的情况。

最后，我们删除了观察者。

func removeObserver() {
    NotificationCenter.default.removeObserver(self, name: .UIApplicationDidBecomeActive, object: nil)
}

Answer 6

快速5

fileprivate  func addObservers() {
      NotificationCenter.default.addObserver(self,
                                             selector: #selector(applicationDidBecomeActive),
                                             name: UIApplication.didBecomeActiveNotification,
                                             object: nil)
    }

fileprivate  func removeObservers() {
        NotificationCenter.default.removeObserver(self, name: UIApplication.didBecomeActiveNotification, object: nil)
    }

@objc fileprivate func applicationDidBecomeActive() {
// here do your work
    }

Answer 7

组合方式：

import Combine

var cancellables = Set<AnyCancellable>()
NotificationCenter.default.publisher(for: UIApplication.didBecomeActiveNotification)
    .sink { notification in
            // do stuff
    }.store(in: &cancellables)

Answer 8

Swift 5 版本：

 NotificationCenter.default.addObserver(self,
                                               selector: #selector(loadData),
                                               name: UIApplication.didBecomeActiveNotification,
                                               object: nil)

在iOS 9及更高版本中删除观察者is no longer required。

Answer 9

在 Swift 5 中

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

         NotificationCenter.default.addObserver(self, selector: #selector(applicationWillResignActive), name: UIApplication.willResignActiveNotification, object: nil)
    
         NotificationCenter.default.addObserver(self, selector: #selector(applicationDidBecomeActive), name: UIApplication.didBecomeActiveNotification, object: nil)
        }
    
    override func viewWillDisappear(_ animated: Bool) { 
        super.viewWillDisappear(animated)

        NotificationCenter.default.removeObserver(self, name: UIApplication.willResignActiveNotification, object: nil)

        NotificationCenter.default.removeObserver(self, name: UIApplication.didBecomeActiveNotification, object: nil)
    }


@objc private func applicationWillResignActive() {
    }

    @objc private func applicationDidBecomeActive() {
    }

Answer 10

如果您使用的是SwiftUI：

.onReceive(NotificationCenter.default.publisher(
    for: UIApplication.didBecomeActiveNotification)) { _ in
        print("DID BECOME ACTIVE")
    }
)

Answer 11

对于 Swift5 MacOS，您需要使用 NSApplication 而不是 UIApplication。

NotificationCenter.default.addObserver(self,
                                       selector: #selector(applicationDidBecomeActive),
                                       name: (NSApplication.didBecomeActiveNotification),
                                       object: nil)
    }

处理applicationDidBecomeActive - “视图控制器如何响应应用程序变为活动状态？”

11 个答案: