• 將常用的功能，寫成一個方便呼叫的 function，並以 extension 的方式加在專案底下

  xtension MainViewController {
  
  / MARK: - Alert
  
  /// 確認、取消按鈕的 Alert
  /// - Parameters:
  ///   - title: Alert 的標題
  ///   - message: Alert 的訊息
  ///   - vc: 要在哪個畫面跳出來
  ///   - confirmTitle: 確認按鈕的文字
  ///   - cancelTitle: 取消按鈕的文字
  ///   - confirm: 按下確認按鈕後要做的事
  ///   - cancel: 按下取消按鈕後要做的事
  public func showAlertWith(title: String?, message: String?, vc: UIViewController, confirmTitle: String, cancelTitle: String, confirm: (() -> Void)?, cancel: (() -> Void)?) {
  	DispatchQueue.main.async {
  		let alertController = UIAlertController(title: title, message: message, preferredStyle: .alert)
  		let confirmAction = UIAlertAction(title: confirmTitle, style: .default) { action in
  			confirm?()
  		}
  		let cancelAction = UIAlertAction(title: cancelTitle, style: .cancel) { action in
  			cancel?()
  		}
  		alertController.addAction(confirmAction)
  		alertController.addAction(cancelAction)
  		vc.present(alertController, animated: true, completion: nil)
  	}
  }
  
  
  / MARK: - NavigationController.push
  
  /// NavigationController.pushViewController 跳頁 (不帶 Closure)
  /// - Parameters:
  ///   - viewController: 要跳頁到的 UIViewController
  ///   - animated: 是否要換頁動畫，預設為 true
  public func pushViewController(_ viewController: UIViewController, animated: Bool = true) {
  	if let navigationController = UIApplication.shared.windows.first(where: { $0.isKeyWindow })?.rootViewController as? UINavigationController {
  		navigationController.pushViewController(viewController, animated: animated)
  	}
  }
  
  /// NavigationController.pushViewController 跳頁 (帶 Closure)
  /// - Parameters:
  ///   - viewController: 要跳頁到的 UIViewController
  ///   - animated: 是否要換頁動畫
  ///   - completion: 換頁過程中，要做的事
  public func pushViewController(_ viewController: UIViewController, animated: Bool, completion: @escaping () -> Void) {
  	self.navigationController?.pushViewController(viewController, animated: animated)
  	guard animated, let coordinator = transitionCoordinator else {
  		DispatchQueue.main.async { completion() }
  		return
  	}
  	coordinator.animate(alongsideTransition: nil) { _ in completion() }
  }
  
  // MARK: - NavigationController.pop
  
  /// NavigationController.popViewController 回上一頁 (不帶 Closure)
  /// - Parameters:
  ///   - animated: 是否要換頁動畫，預設為 true
  public func popViewController(_ animated: Bool = true) {
  	self.navigationController?.popViewController(animated: animated)
  }
  
  /// NavigationController.popViewController 回上一頁 (帶 Closure)
  /// - Parameters:
  ///   - animated: 是否要換頁動畫
  ///   - completion: 換頁過程中，要做的事
  public func popViewController(animated: Bool, completion: @escaping () -> Void) {
  	self.navigationController?.popViewController(animated: animated)
  	guard animated, let coordinator = transitionCoordinator else {
  		DispatchQueue.main.async { completion() }
  		return
  	}
  	coordinator.animate(alongsideTransition: nil) { _ in completion() }
  }
  }
  
  ``
  
  

這樣就大功告成啦！

可以從 GitHub - AVCaptureVideoPreviewLayerDemo 看完整的程式碼