如何在Swift中将观察者添加到默认通知中心？我正在尝试移植此行代码，以便在电池电量变化时发送通知。

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(batteryLevelChanged:) name:UIDeviceBatteryLevelDidChangeNotification object:nil];

与Objective-C API相同，但是使用Swift的语法。

Swift 4.2和Swift 5：

NotificationCenter.default.addObserver(
    self,
    selector: #selector(self.batteryLevelChanged),
    name: UIDevice.batteryLevelDidChangeNotification,
    object: nil)

如果您的观察者未继承自Objective-C对象，则必须在方法前加上前缀@objc，才能将其用作选择器。

@objc private func batteryLevelChanged(notification: NSNotification){     
    //do stuff using the userInfo property of the notification object
}

请参阅NSNotificationCenter类参考，与Objective-C API交互

Swift 4.0和Xcode 9.0+：

发送（发布）通知：

NotificationCenter.default.post(name: Notification.Name("NotificationIdentifier"), object: nil)

要么

NotificationCenter.default.post(name: Notification.Name("NotificationIdentifier"), object: nil, userInfo: ["Renish":"Dadhaniya"])

接收（获取）通知：

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

收到通知的函数方法处理程序：

@objc func methodOfReceivedNotification(notification: Notification) {}

Swift 3.0和Xcode 8.0+：

发送（发布）通知：

NotificationCenter.default.post(name: Notification.Name("NotificationIdentifier"), object: nil)

接收（获取）通知：

NotificationCenter.default.addObserver(self, selector: #selector(YourClassName.methodOfReceivedNotification(notification:)), name: Notification.Name("NotificationIdentifier"), object: nil)

收到通知的方法处理程序：

func methodOfReceivedNotification(notification: Notification) {
  // Take Action on Notification
}

删除通知：

deinit {
  NotificationCenter.default.removeObserver(self, name: Notification.Name("NotificationIdentifier"), object: nil)
}

Swift 2.3和Xcode 7：

发送（发布）通知

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

接收（获取）通知

NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(YourClassName.methodOfReceivedNotification(_:)), name:"NotificationIdentifier", object: nil)

收到通知的方法处理程序

func methodOfReceivedNotification(notification: NSNotification){
  // Take Action on Notification
}

对于历史Xcode版本...

发送（发布）通知

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

接收（获取）通知

NSNotificationCenter.defaultCenter().addObserver(self, selector: "methodOfReceivedNotification:", name:"NotificationIdentifier", object: nil)

删除通知

NSNotificationCenter.defaultCenter().removeObserver(self, name: "NotificationIdentifier", object: nil)
NSNotificationCenter.defaultCenter().removeObserver(self) // Remove from all notifications being observed

收到通知的方法处理程序

func methodOfReceivedNotification(notification: NSNotification) {
  // Take Action on Notification
}

用@objc注释类或目标方法

@objc private func methodOfReceivedNotification(notification: NSNotification) {
  // Take Action on Notification
}

// Or

dynamic private func methodOfReceivedNotification(notification: NSNotification) {
  // Take Action on Notification
}

这样做的一种好方法是使用该addObserver(forName:object:queue:using:)方法，而不是addObserver(_:selector:name:object:)使用Objective-C代码中经常使用的方法。第一个变体的优点是您不必@objc在方法中使用属性：

    func batteryLevelChanged(notification: Notification) {
        // do something useful with this information
    }

    let observer = NotificationCenter.default.addObserver(
        forName: NSNotification.Name.UIDeviceBatteryLevelDidChange,
        object: nil, queue: nil,
        using: batteryLevelChanged)

如果需要，甚至可以使用闭包代替方法：

    let observer = NotificationCenter.default.addObserver(
        forName: NSNotification.Name.UIDeviceBatteryLevelDidChange,
        object: nil, queue: nil) { _ in print("🔋") }

您可以使用返回的值稍后停止监听通知：

    NotificationCenter.default.removeObserver(observer)

使用此方法以前还有另一个优势，那就是它不需要您使用编译器无法静态检查的选择器字符串，因此，如果重命名该方法，则很容易出错，但是Swift 2.2和稍后包含可解决该问题的#selector表达式。

Xcode 8中的Swift 3.0

Swift 3.0已将许多“字符串型” API替换为struct“包装类型”，如NotificationCenter一样。通知现在由struct Notfication.Name而不是标识String。请参阅《迁移到Swift 3》指南。

先前的用法：

// Define identifier
let notificationIdentifier: String = "NotificationIdentifier"

// Register to receive notification
NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(YourClassName.methodOfReceivedNotification(_:)), name: notificationIdentifier, object: nil)

// Post a notification
NSNotificationCenter.defaultCenter().postNotificationName(notificationIdentifier, object: nil)

Swift 3.0的新用法：

// Define identifier
let notificationName = Notification.Name("NotificationIdentifier")

// Register to receive notification
NotificationCenter.default.addObserver(self, selector: #selector(YourClassName.methodOfReceivedNotification), name: notificationName, object: nil)

// Post notification
NotificationCenter.default.post(name: notificationName, object: nil)

现在，所有系统通知类型都被定义为on上的静态常量Notification.Name。即.UIDeviceBatteryLevelDidChange，.UIApplicationDidFinishLaunching，.UITextFieldTextDidChange等等。

您可以扩展Notification.Name自己的自定义通知，以与系统通知保持一致：

// Definition:
extension Notification.Name {
    static let yourCustomNotificationName = Notification.Name("yourCustomNotificationName")
}

// Usage:
NotificationCenter.default.post(name: .yourCustomNotificationName, object: nil)

1. 声明通知名称

   extension Notification.Name {
       static let purchaseDidFinish = Notification.Name("purchaseDidFinish")
   }

2. 您可以通过两种方式添加观察者：

   使用 Selector

   NotificationCenter.default.addObserver(self, selector: #selector(myFunction), name: .purchaseDidFinish, object: nil)
   
   @objc func myFunction(notification: Notification) {
       print(notification.object ?? "") //myObject
       print(notification.userInfo ?? "") //[AnyHashable("key"): "Value"]
   }

   或使用 block

   NotificationCenter.default.addObserver(forName: .purchaseDidFinish, object: nil, queue: nil) { [weak self] (notification) in
       guard let strongSelf = self else {
           return
       }
   
       strongSelf.myFunction(notification: notification)
   }
   
   func myFunction(notification: Notification) {
       print(notification.object ?? "") //myObject
       print(notification.userInfo ?? "") //[AnyHashable("key"): "Value"]
   }

3. 发布您的通知

   NotificationCenter.default.post(name: .purchaseDidFinish, object: "myObject", userInfo: ["key": "Value"])

从iOS 9和OS X 10.11开始。取消分配时，NSNotificationCenter观察者不再需要注销自身。更多信息

对于block基于基础的实现，如果要self在块内使用，则需要做一个弱强共舞。更多信息

需要删除基于块的观察者的更多信息

let center = NSNotificationCenter.defaultCenter()
center.removeObserver(self.localeChangeObserver)

使用NSNotificationCenter传递数据

您还可以使用swift 3.0中的NotificationCentre和swift 2.0中的NSNotificationCenter传递数据。

Swift 2.0版本

使用userInfo传递信息，它是[NSObject：AnyObject]类型的可选词典？

let imageDataDict:[String: UIImage] = ["image": image]

// Post a notification
 NSNotificationCenter.defaultCenter().postNotificationName(notificationName, object: nil, userInfo: imageDataDict)

// Register to receive notification in your class
NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(self.showSpinningWheel(_:)), name: notificationName, object: nil)

// handle notification
func showSpinningWheel(notification: NSNotification) {
  if let image = notification.userInfo?["image"] as? UIImage {
  // do something with your image   
  }
}

Swift 3.0版本

现在，userInfo需要[AnyHashable：Any]吗？作为参数，我们在Swift中将其作为字典文字提供

let imageDataDict:[String: UIImage] = ["image": image]

// post a notification
 NotificationCenter.default.post(name: NSNotification.Name(rawValue: "notificationName"), object: nil, userInfo: imageDataDict) 
// `default` is now a property, not a method call

// Register to receive notification in your class
NotificationCenter.default.addObserver(self, selector: #selector(self.showSpinningWheel(_:)), name: NSNotification.Name(rawValue: "notificationName"), object: nil)

// handle notification
func showSpinningWheel(_ notification: NSNotification) {

  if let image = notification.userInfo?["image"] as? UIImage {
  // do something with your image   
  }
}

使用NotificationCentre（swift 3.0）和NSNotificationCenter（swift 2.0）的源传递数据

在Swift 5中

假设要从ViewControllerB接收数据到ViewControllerA

ViewControllerA（接收器）

import UIKit

class ViewControllerA: UIViewController  {

    override func viewDidLoad() {
        super.viewDidLoad()

        //MARK: - - - - - Code for Passing Data through Notification Observer - - - - -
        // add observer in controller(s) where you want to receive data
        NotificationCenter.default.addObserver(self, selector: #selector(self.methodOfReceivedNotification(notification:)), name: Notification.Name("NotificationIdentifier"), object: nil)
    }

    //MARK: - - - - - Method for receiving Data through Post Notificaiton - - - - -
    @objc func methodOfReceivedNotification(notification: Notification) {
        print("Value of notification : ", notification.object ?? "")
    }
}

ViewControllerB（发送方）

import UIKit

class ViewControllerB: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        //MARK: - - - - - Set data for Passing Data Post Notification - - - - -
        let objToBeSent = "Test Message from Notification"
        NotificationCenter.default.post(name: Notification.Name("NotificationIdentifier"), object: objToBeSent)
    }

}

我能够执行以下操作之一以成功使用选择器- 无需使用@objc 进行注释：

NSNotificationCenter.defaultCenter().addObserver(self,
    selector:"batteryLevelChanged:" as Selector,
    name:"UIDeviceBatteryLevelDidChangeNotification",
    object:nil)    

要么

let notificationSelector: Selector = "batteryLevelChanged:"

NSNotificationCenter.defaultCenter().addObserver(self,
    selector: notificationSelector,
    name:"UIDeviceBatteryLevelDidChangeNotification",
    object:nil)    

我的xcrun版本显示了Swift 1.2，并且可以在Xcode 6.4和Xcode 7 beta 2（我认为应该使用Swift 2.0）上运行：

$xcrun swift --version

Apple Swift version 1.2 (swiftlang-602.0.53.1 clang-602.0.53)

在Swift 2.2-XCode 7.3中，我们#selector用于NSNotificationCenter

 NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(rotate), name: UIDeviceOrientationDidChangeNotification, object: nil)

我们也应该删除通知。

例如

deinit 
{
  NotificationCenter.default.removeObserver(self, name:NSNotification.Name(rawValue: "notify"), object: nil)

}

在Swift 3中，Xcode 8.2：-检查电池状态级别

//Add observer
NotificationCenter.default.addObserver(self, selector: #selector(batteryStateDidChange), name: NSNotification.Name.UIDeviceBatteryStateDidChange, object: nil)


 //Fired when battery level changes

 func batteryStateDidChange(notification: NSNotification){
        //perform manipulation here
    }

NSNotificationCenter在iOS 4.0的Swift 4.0中添加观察者语法

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

这是用于keyboardWillShow通知名称类型。可以从可用选项中选择其他类型

选择器的类型为@objc func，用于处理键盘的显示方式（这是您的用户功能）

Swift 5和Xcode 10.2：

NotificationCenter.default.addObserver(
            self,
            selector: #selector(batteryLevelDidChangeNotification),
            name: UIDevice.batteryLevelDidChangeNotification,
            object: nil)

Swift 5通知观察者

override func viewDidLoad() {
    super.viewDidLoad() 
    NotificationCenter.default.addObserver(self, selector: #selector(batteryLevelChanged), name: UIDevice.batteryLevelDidChangeNotification, object: nil)
}

@objc func batteryLevelChanged(notification : NSNotification){
    //do here code
}

override func viewWillDisappear(_ animated: Bool) {
    NotificationCenter.default.removeObserver(self, name: UIDevice.batteryLevelDidChangeNotification, object: nil)

}