有的时候，当App从任务管理器被强制关闭时，我们可能需要和服务器通信，或者写Log文件等等，这时候可以在applicationWillTerminate中做这个事情。可以在很短暂的时间，做些事情。

import UIKit

@main
class AppDelegate: UIResponder, UIApplicationDelegate {

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        UNUserNotificationCenter.current().delegate = self
        return true
    }

    // MARK: UISceneSession Lifecycle

    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        // Called when a new scene session is being created.
        // Use this method to select a configuration to create the new scene with.
        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
    }

    func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
        // Called when the user discards a scene session.
        // If any sessions were discarded while the application was not running, this will be called shortly after application:didFinishLaunchingWithOptions.
        // Use this method to release any resources that were specific to the discarded scenes, as they will not return.
    }

    func applicationWillTerminate(_ application: UIApplication) {
        // 服务器通信或者写文件。
        HttpService.xxxx() { _ in }
        // ※Sleep4秒 不加这个不好用
        sleep(4)
    }

}