Navigation-FlowController.swift

internal class OnboardingViewController: UIViewController {
 
	...
	private func pushRegistrationPage() {
		let registerViewController = RegisterViewController()
		navigationController?.pushViewController(registerViewController, animated: true)
	}
	
	private func pushLoginPage() {
		let loginViewController = LoginViewController()
		navigationController?.pushViewController(loginViewController, animated: true)
	}
	...

}

class LoginViewController: UIViewController {
	
	...
	var onShowNextScreen: (Screen -> Void)?
	
	@IBAction func signUpButtonTapped(sender: UIButton) {
		onShowNextScreen?(.Register)
	}
	
	func backToPreviousScreen() {
		onShowPreviousScreen?()
	}
	...
	
}

internal class RegisterViewController: UIViewController {
	
	...
	@IBAction func pushLoginPage(sender: AnyObject) {
		onShowNextScreen?(.Login)
	}
	
	func backToPreviousScreen() {
		onShowPreviousScreen?()
	}
	...

}

class OnboardingFlowController {
	...
	private func createLoginScreen() -> UIViewController {
		let viewController = LoginViewController()
		
		viewController.onShowNextScreen = { [weak self] screen in
			if case .Register = screen,
				let firstViewController = self?.navigationController.viewControllers.first,
				let registerViewController = self?.createRegisterScreen() {
				self?.navigationController.setViewControllers([ firstViewController, registerViewController ], animated: true)
				return
			}
			self?.onShowNextScreen?(screen)
		}
		return viewController
	}
	
	private func configureOnShowPreviousScreen() {
		onShowPreviousScreen = { [weak self] in
			self?.navigationController.popViewControllerAnimated(true)
		}
	}
	...
}

internal class SettingFlowController {
	...
	private func createLoginScreen() -> UIViewController {
		let viewController = LoginViewController()
		
		viewController.onShowNextScreen = { [weak self] screen in
			if case .Register = screen,
				let firstViewController = self?.navigationController.viewControllers.first,
				let registerViewController = self?.createRegisterScreen() {
				
				self?.navigationController.setViewControllers([ firstViewController, registerViewController ], animated: true)
				return
			}
			self?.onShowNextScreen?(screen)
		}
		return viewController
	}
	
	private func configureOnShowPreviousScreen() {
		onShowPreviousScreen = { [weak self] in
			self?.navigationController.popViewControllerAnimated(true)
		}
	}
	...
}