Swift iOS : NotificationCenter

類NotificationCenter提供了一種輕耦合的消息傳遞機制。能夠發起一個通知，在多處監聽此通知。好比說一個App的主題樣式被修改，就能夠經過此類來通知多個相關UI，作響應的處理。

以下案例展現了這種可能：

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")
    }
}複製代碼

執行此代碼，輸出應該是：

themeChange2
themeChange
themeChange1複製代碼

經過 NotificationCenter.default.addObserver在類Page1作了兩處對「themeChange」通知的監聽，在類AppDelegate作了一處對此通知的監聽。而後當：

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

執行時，三處監聽函數都會被調用。
NotificationCenter還能夠監聽系統通知，好比App進入前景和背景，按以下方法監聽便可：

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")
    }
}複製代碼

應用執行後，按HOME按鈕，能夠看到輸出：

applicationDidEnterBackground複製代碼

再度執行App能夠看到輸出：

applicationWillEnterForeground複製代碼

能夠傳遞和接受對象做爲參數，像是這樣傳遞：

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

像是這樣接收：

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

    }複製代碼