如何避免添加多个 NSNotification 观察者？

Question

目前，API 似乎没有提供一种方法来检测是否已为特定 NSNotification 添加了观察者。除了在端维护一个标志来跟踪之外，避免添加多个 NSNotification 观察者的最佳方法是什么？有人已经创建了一个类别来促进这一点吗？

Answer 1

防止添加重复观察者的一种方法是在再次添加目标/选择器之前显式调用removeObserver。我想您可以将其添加为类别方法：

@interface NSNotificationCenter (UniqueNotif)

- (void)addUniqueObserver:(id)observer selector:(SEL)selector name:(NSString *)name object:(id)object {

        [[NSNotificationCenter defaultCenter] removeObserver:observer name:name object:object];
        [[NSNotificationCenter defaultCenter] addObserver:observer selector:selector name:name object:object];

}

@end

这假设您只会为任何通知名称的每个目标添加一个唯一的观察者，因为它将删除该通知名称的任何现有观察者。

Answer 2

Swift 5：

import Foundation

extension NotificationCenter {
  func setObserver(_ observer: Any, selector: Selector, name: Notification.Name, object: Any?) {
    removeObserver(observer, name: name, object: object)
    addObserver(observer, selector: selector, name: name, object: object)
  }
}

---

Swift 3-4：

import Foundation

extension NotificationCenter {
  func setObserver(_ observer: AnyObject, selector: Selector, name: NSNotification.Name, object: AnyObject?) {
    removeObserver(observer, name: name, object: object)
    addObserver(observer, selector: selector, name: name, object: object)
  }
}

---

Swift 2：

import Foundation

extension NSNotificationCenter {
  func setObserver(observer: AnyObject, selector: Selector, name: String?, object: AnyObject?) {
    removeObserver(observer, name: name, object: object)
    addObserver(observer, selector: selector, name: name, object: object)
  }
}

Answer 3

带有 extension NotificationCenter { ... } 的 Upvoted 答案对我不起作用，因为每次发布通知时我的应用程序都会创建一个 viewController 的新实例（这有一个通知观察者），所以删除 viewController 的新实例上的观察者显然不起作用。
先前具有通知观察者的 viewController 实例正在被调用。

下面的内容对我有用，因为一旦视图消失，就会删除通知观察者。

// Notification observer added 

override func viewWillAppear(_ animated: Bool) {

    NotificationCenter.default.addObserver(self, selector: #selector(self.someFunc(notification:)), name: Notification.Name("myNotification"), object: nil)


}


// Notification observer removed 

override func viewWillDisappear(_ animated: Bool) {

    NotificationCenter.default.removeObserver(self, name: Notification.Name("myNotification"), object: nil)


}

Answer 4

好吧，应该检查一下苹果文档。
https://developer.apple.com/documentation/foundation/notificationcenter/1411723- addobserver

let center = NSNotificationCenter.defaultCenter
var tokenOpt: NSObjectProtocol?
tokenOpt = center.addObserverForName("OneTimeNotification", object: nil, queue: nil) { (note) in
    print("Received the notification!")
    center.removeObserver(token!)
}

因此，要确保通知已存在则不会添加

if let token = tokenOpt{
  center.removeObserver(token)
}
tokenOpt = center.addObserverForName("OneTimeNotification", object: nil, queue: mainQueue) { (note) in
    print("Received the notification!")
}

Answer 5

Swift 5：

基于 @futureelite7 和 @dimpiax 之前的答案对 Swift 5 进行了一些调整

extension NotificationCenter {
    func setObserver(_ observer: AnyObject, selector: Selector, name: NSNotification.Name, object: AnyObject?) {
        NotificationCenter.default.removeObserver(observer,
                                                  name: name,
                                                  object: object)
        NotificationCenter.default.addObserver(observer,
                                               selector: selector,
                                               name: name,
                                               object: object)
    }
}