Swift-通知观察者被多次调用

Question

I have viewController and inside in viewDidLoad I have

NSNotificationCenter.defaultCenter().addObserver(self, selector: "showNextQuestions", name: "showNextQuestionsID", object: nil)

In another controller I have

NSNotificationCenter.defaultCenter().postNotificationName("showNextQuestionsID", object: nil)

If I go home from app and launch it again function showNextQuestionID fires two times.

I tried to use

func applicationDidEnterBackground(application: UIApplication) {
    NSNotificationCenter.defaultCenter().removeObserver(self, name: "showNextQuestionsID", object: nil)
}

But this doesn't help,

and in viewController

deinit {
    NSNotificationCenter.defaultCenter().removeObserver(self)
}

How can I fix this ?

Answer 1

You are not removing your notification observer in the right place. You register the observer in your view controller subclass, and you need to remove it in the same class. A logical place is to override the viewWillDisappear method. Place the following code in your view controller subclass:

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

    NSNotificationCenter.defaultCenter().removeObserver(self)
}

Also remove

 NSNotificationCenter.defaultCenter().removeObserver(self, name: "showNextQuestionsID", object: nil)

From your AppDelegate. When you supply the 'self' argument in the AppDelegate, it is referring to the AppDelegate class, not your view controller. When you call to remove your notification observer in the view controller sublcass, self is your view controller, which is what you want.

Last, when you call simply removeObserver(self) with no other arguments, it will unregister all the observers for that object. That way you don't have to go through and list each observer by name.

Answer 2

将观察者置于AppDelegate或单例中，以便您可以在应用程序状态期间轻松添加和删除观察者。

Answer 3

applicationDidEnterBackground and deinit should be ok.

The problem is the way you are trying to remove the observer in applicationDidEnterBackground . You are trying to remove the observer from AppDelegate and you need to remove the observer from your ViewController.

To fix the problem:

1) Listen for UIApplicationDidEnterBackgroundNotification in your view controller:

func init() {
  super.init()

  NSNotificationCenter.defaultCenter().addObserver(self, selector: "myAppDidEnterBackground", name: UIApplicationDidEnterBackgroundNotification, object: nil) 
}

2) Implement the method that listen to UIApplicationDidEnterBackgroundNotification

func myAppDidEnterBackground() {
NSNotificationCenter.defaultCenter().removeObserver(self, name: "showNextQuestionsID", object: nil)
}

3) Extra. You could also listen for UIApplicationWillEnterForegroundNotification in order to add again your custom notification