Swift iOS : NotificationCenter

类NotificationCenter提供了一种轻耦合的消息传递机制。能够发起一个通知，在多处监听此通知。好比说一个App的主题样式被修改，就能够经过此类来通知多个相关UI，作响应的处理。

以下案例展现了这种可能：

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")
    }
}复制代码

执行此代码，输出应该是：

themeChange2
themeChange
themeChange1复制代码

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

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

执行时，三处监听函数都会被调用。
NotificationCenter还能够监听系统通知，好比App进入前景和背景，按以下方法监听便可：

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")
    }
}复制代码

应用执行后，按HOME按钮，能够看到输出：

applicationDidEnterBackground复制代码

再度执行App能够看到输出：

applicationWillEnterForeground复制代码

能够传递和接受对象做为参数，像是这样传递：

let cd = {(_ a : String) in print(a)}
NotificationCenter.default.post(name: Notification.Name("dive2"), object: [1,"2",cd])复制代码

像是这样接收：

func dive2(_ obj : Any){
        nav?.pushViewController(Level2(), animated: true)
        print(obj)
//        {name = dive2; object = (
//        1,
//        2,
//        "(Function)"
//        )}

    }复制代码