利用 前天的 UserPreferences 再加上昨天的 客製化 NavigationBar 實作出的成品：

1. 宣告 UserDefaults 的部分

   import Foundation
   
   class UserPreferences {
   
   	static let shared = UserPreferences()
   
   	private let userPreference: UserDefaults
   
   	private init() {
   		// 設定要儲存的值(value) 及 key
   		userPreference = UserDefaults.standard
   	}
   
   	enum UserPreference: String {
   		case numIndex   // 顯示在 SecondViewController 的數值
   	}
   
   	var numIndex: String {
   		get { return userPreference.string(forKey: UserPreference.numIndex.rawValue) ?? "0" }
   		set { userPreference.set(newValue, forKey: UserPreference.numIndex.rawValue) }
   	}
   }
   

2. 寫一個 BaseViewController，裡面可以寫常用的 function

   import Foundation
   import UIKit
   class BaseViewController: UIViewController {
   
   	// 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() }
   	}
   }
   

3. 個別新增第一頁、第二頁

   • 第一頁（MainViewController）：

     

     mport UIKit
     
     / 將 MainViewController 的型態設為 BaseViewController
     lass MainViewController: BaseViewController {
     
     override func viewDidLoad() {
     	super.viewDidLoad()
     }
     
     @IBAction func touchUpInside(_ sender: UIButton) {
     	// 宣告下一頁為 SecondViewController
     	let nextVC = SecondViewController()
     	self.pushViewController(nextVC, animated: false)
     }
     
     ``
     
     

   • 第二頁（SecondViewController）：

     

     mport UIKit
     
     / 將 SecondViewController 的型態設為 BaseViewController
     lass SecondViewController: BaseViewController {
     
     @IBOutlet weak var numLabel: UILabel!
     
     var index = UserPreferences.shared.numIndex
     
     // MARK: - LifeCycle
     
     override func viewDidLoad() {
     	super.viewDidLoad()
     	setupUI()
     }
     
     // MARK: - UI Initial Setup
     
     func setupUI() {
     	setupLeftAndRightNavigationBarItems()
     	numLabel.text = index
     }
     
     // MARK: - 客製化 NavigationBar
     
     /// CustomizationNavigationBarItems
     /// - Parameters:
     ///   - LeftNavigationBarItems: 返回鍵
     ///   - RightNavigationBarItems: 加1、減1、歸零
     private func setupLeftAndRightNavigationBarItems() {
     
     	// MARK: - Left NavigationBarItems 由左至右
     
     	let backButton = UIButton(type: .system)
     
     	backButton.setImage(UIImage(systemName: "chevron.left"), for: .normal)
     
     	backButton.setTitle("返回鍵", for: .normal)
     
     	// 新增返回鍵被點擊後的觸發事件
     	backButton.addTarget(self, action: #selector(self.backButtonAction), for: .touchUpInside)
     
     	self.navigationItem.leftBarButtonItem = UIBarButtonItem(customView: backButton)
     
     	// MARK: - Right NavigationBarItems 由右至左
     
     	let valuePlus = UIBarButtonItem(barButtonSystemItem: .add, target: self, action: #selector(valuePlusOneObjc))
     
     	let valueMinus = UIBarButtonItem(image: UIImage(systemName: "minus"), style: .done, target: self, action: #selector(valueMinusOneObjc))
     
     	let valueZero = UIBarButtonItem(image: UIImage(systemName: "clear"), style: .done, target: self, action: #selector(valueToZeroObjc))
     
     	self.navigationItem.rightBarButtonItems = [valuePlus, valueMinus, valueZero]
     }
     
     /// 數值運算
     private func valueChange(calculation: String){
     
     	switch calculation{
     
     	/// 數值加一
     	case "plus":
     		index = "\(Int(index)! + 1)"
     		numLabel.text = index
     		UserPreferences.shared.numIndex = index
     
     	/// 數值減一
     	case "minus":
     		index = "\(Int(index)! - 1)"
     		numLabel.text = index
     		UserPreferences.shared.numIndex = index
     
     	/// 數值歸零
     	case "clear":
     		index = "0"
     		numLabel.text = index
     		UserPreferences.shared.numIndex = index
     
     	default:
     		break
     	}
     }
     
     // MARK: - Right NavigationBarItems Function
     
     @objc func backButtonAction() {
     	popViewController(false)
     }
     
     @objc func valuePlusOneObjc() {
     	valueChange(calculation: "plus")
     }
     
     @objc func valueMinusOneObjc() {
     	valueChange(calculation: "minus")
     }
     
     @objc func valueToZeroObjc() {
     	valueChange(calculation: "clear")
     }
     
     ``
     
     

大功告成！可以參考 GitHub - SetupNavigationBarItemsDemo