class UserManager {
 static let shared = UserManager()
 
 private init() {
 // 單例模式，防止出現(xiàn)多個實例
 }
 
 ....
}

extension UserManager {
 func logOut( ) {
 ...
 }
 
 func logIn( ) {
 ...
 }
}

class ProfileViewController: UIViewController {
 private lazy var nameLabel = UILabel()

 override func viewDidLoad() {
 super.viewDidLoad()
 nameLabel.text = UserManager.shared.currentUser?.name
 }

 private func handleLogOutButtonTap() {
 UserManager.shared.logOut()
 }
}

class ProfileViewController: UIViewController {
 private let user: User
 private let logOutService: LogOutService
 private lazy var nameLabel = UILabel()

 init(user: User, logOutService: LogOutService) {
 self.user = user
 self.logOutService = logOutService
 super.init(nibName: nil, bundle: nil)
 }

 override func viewDidLoad() {
 super.viewDidLoad()
 nameLabel.text = user.name
 }

 private func handleLogOutButtonTap() {
 logOutService.logOut()
 }
}

class LogOutService {
 private let user: User
 private let networkService: NetworkService
 private let navigationService: NavigationService

 init(user: User,
 networkService: NetworkService,
 navigationService: NavigationService) {
 self.user = user
 self.networkService = networkService
 self.navigationService = navigationService
 }

 func logOut() {
 networkService.request(.logout(user)) { [weak self] in
 self?.navigationService.showLoginScreen()
 }
 }
}

protocol LogOutService {
 func logOut()
}

protocol NetworkService {
 func request(_ endpoint: Endpoint, completionHandler: @escaping () -> Void)
}

protocol NavigationService {
 func showLoginScreen()
 func showProfile(for user: User)
 ...
}

extension UserManager: LoginService, LogOutService {}

extension AppDelegate: NavigationService {
 func showLoginScreen() {
 navigationController.viewControllers = [
 LoginViewController(
 loginService: UserManager.shared,
 navigationService: self
 )
 ]
 }

 func showProfile(for user: User) {
 let viewController = ProfileViewController(
 user: user,
 logOutService: UserManager.shared
 )

 navigationController.pushViewController(viewController, animated: true)
 }
}

//MARK - : 單例:方法1
 static let shareSingleOne = Single()

let single = Single()
class Single: NSObject {
 //-MARK: 單例:方法2
 class var sharedInstance2 : Single {
  return single
 }
}

 //-MARK: 單例:方法3
 static var shareInstance3:Single{
 struct MyStatic{
  static var instance :Single = Single()
 }
 return MyStatic.instance;
 }

public extension DispatchQueue { 
 
 private static var _onceTracker = [String]() 
 
 /** 
 Executes a block of code, associated with a unique token, only once. The code is thread safe and will 
 only execute the code once even in the presence of multithreaded calls. 
 
 - parameter token: A unique reverse DNS style name such as com.vectorform.<name> or a GUID 
 - parameter block: Block to execute once 
 */ 
 public class func once(token: String, block:()->Void) { 
 objc_sync_enter(self) 
 defer { objc_sync_exit(self) } 
 
 if _onceTracker.contains(token) { 
  return 
 } 
 
 _onceTracker.append(token) 
 block() 
 } 
} 

DispatchQueue.once(token: "com.vectorform.test") { 
 print( "Do This Once!" ) 
} 

private let _onceToken = NSUUID().uuidString 
 
DispatchQueue.once(token: _onceToken) { 
 print( "Do This Once!" ) 
}