Swift iOS : NotificationCenter

类NotificationCenter提供了一种轻耦合的消息传递机制。可以发起一个通知，在多处监听此通知。比如说一个App的主题样式被修改，就可以通过此类来通知多个相关UI，做响应的处理。

如下案例展示了这种可能：

import UIKit
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        self.window = UIWindow();
        self.window?.frame=UIScreen.main.bounds;
        self.window?.makeKeyAndVisible();
        NotificationCenter.default.addObserver(self, selector: #selector(themeChange), name: Notification.Name("themeChange"), object: nil)
        self.window?.rootViewController = Page()
        return true
    }
    func themeChange(){
        print("themeChange2")
    }
}
class Page: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .blue
        NotificationCenter.default.addObserver(self, selector: #selector(themeChange), name: Notification.Name("themeChange"), object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(themeChange1), name: Notification.Name("themeChange"), object: nil)
        NotificationCenter.default.post(name: Notification.Name("themeChange"), object: nil)
    }
    func themeChange(){
        print("themeChange")
    }
    func themeChange1(){
        print("themeChange1")
    }
}

执行此代码，输出应该是：

themeChange2
themeChange
themeChange1

通过 NotificationCenter.default.addObserver在类Page1做了两处对“themeChange”通知的监听，在类AppDelegate做了一处对此通知的监听。然后当：

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

执行时，三处监听函数都会被调用。

NotificationCenter还可以监听系统通知，比如App进入前景和背景，按如下方法监听即可：

import UIKit
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        self.window = UIWindow();
        self.window?.frame=UIScreen.main.bounds;
        self.window?.makeKeyAndVisible();
        self.window?.rootViewController = Page()
        return true
    }
}
class Page: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .blue
        NotificationCenter.default.addObserver(self, selector: #selector(applicationWillEnterForeground), name: NSNotification.Name.UIApplicationWillEnterForeground, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(applicationDidEnterBackground), name: NSNotification.Name.UIApplicationDidEnterBackground, object: nil)
    }
    func applicationWillEnterForeground(){
        print("applicationWillEnterForeground")
    }
    func applicationDidEnterBackground(){
        print("applicationDidEnterBackground")
    }
}

应用执行后，按HOME按钮，可以看到输出：

applicationDidEnterBackground

再度执行App可以看到输出：

applicationWillEnterForeground