无法让状态恢复与编程导航控制器一起使用

Question

我无法使用导航控制器进行状态恢复。我正在使用 Swift，不想使用 Storyboards (Programmatic)。我在网上找到的几乎所有帮助都不是 Swift 或 Storyboards。

在下面的演示代码中，ViewController 包含一个简单的 PickerView，一个 selection 变量跟踪 Picker 的选择。 AppDelegate 提供了 2 个选项。使用 Option 1，不使用 Navigation Controller，并且 Picker 的状态恢复正常，但使用 Option 2 中的 Navigation Controller 无法恢复。 （在下面的代码中，选项 1 被注释掉，选项 2 处于活动状态）。

您可以将下面的代码复制并粘贴到新的 singleView 应用程序中，它应该会重现我所描述的内容。 （我测试过）

AppDelegate 代码：

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

  var window: UIWindow?

  func application(_ application: UIApplication, shouldSaveApplicationState coder: NSCoder) -> Bool {
    return true
  }
  func application(_ application: UIApplication, shouldRestoreApplicationState coder: NSCoder) -> Bool {
    return true
  }

  func application(_ application: UIApplication, willFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey : Any]? = nil) -> Bool {
    window = UIWindow(frame: UIScreen.main.bounds)
    window?.makeKeyAndVisible()
    let pickerVC = ViewController()
    //Option 1: No NavC used
    //window?.rootViewController = pickerVC

    //Option 2: NavC used
    let navC = UINavigationController(rootViewController: pickerVC)
    navC.restorationIdentifier = "PickerNav"
    window?.rootViewController = navC

    return true
  }

//  func application(_ application: UIApplication, viewControllerWithRestorationIdentifierPath identifierComponents: [Any], coder: NSCoder) -> UIViewController? {
//    let storyboard = UIStoryboard(name: "Main", bundle: nil)
//      if let lastItem = identifierComponents.last as? String {
//        return storyboard.instantiateViewController(withIdentifier: lastItem)
//      }
//    return nil
//  }
}

ViewController 代码：

import UIKit

class ViewController: UIViewController, UIPickerViewDelegate, UIPickerViewDataSource {

  var pickerView: UIPickerView!
  var selection = 0
  let group = ["Fruit","Vegetable","Meat","Bread"]

  override func viewDidLoad() {
    super.viewDidLoad()
    restorationIdentifier = "PickerVC"
    setupPicker()
  }

  override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    pickerView.selectRow(selection, inComponent: 0, animated: false)
  }

  override func encodeRestorableState(with coder: NSCoder) {
    coder.encode(selection, forKey: "selection")
    super.encodeRestorableState(with: coder)
  }

  override func decodeRestorableState(with coder: NSCoder) {
    selection = coder.decodeInteger(forKey: "selection")
    super.decodeRestorableState(with: coder)
  }

  func setupPicker() {
    pickerView = UIPickerView()
    pickerView.delegate = self
    pickerView.dataSource = self
    view.addSubview(pickerView)

    pickerView.translatesAutoresizingMaskIntoConstraints = false
    pickerView.leadingAnchor.constraint(equalTo: view.layoutMarginsGuide.leadingAnchor).isActive = true
    pickerView.trailingAnchor.constraint(equalTo: view.layoutMarginsGuide.trailingAnchor).isActive = true
    pickerView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    pickerView.heightAnchor.constraint(equalToConstant: 300).isActive = true
  }

  func numberOfComponents(in pickerView: UIPickerView) -> Int {
    return 1
  }

  func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
    return group.count
  }

  func pickerView(_ pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int) {
    selection = pickerView.selectedRow(inComponent: 0)
  }

  func pickerView(_ pickerView: UIPickerView, viewForRow row: Int, forComponent component: Int, reusing view: UIView?) -> UIView {
    let pickerLabel = UILabel()
    pickerLabel.font = UIFont.systemFont(ofSize: 26)
    pickerLabel.textAlignment = .center
    pickerLabel.text = group[row]
    pickerLabel.textColor = .white
    return pickerLabel
  }
}

我的测试详情： 使用选项 1，更改 Picker 后跟 cmd-Shift-H 会导致 selection 变量保存在 encodeRestorableState 中。然后我点击 Xcode Stop 按钮，然后再次运行，selection 变量在decodeRestorableState 中恢复。相比之下，使用选项 2 状态恢复不起作用，因为从未调用过decodeRestorableState，因此不会恢复selection 变量。但是，viewDidAppear 处的断点显示 navigationController?.restorationIdentifier = "PickerNav" 和 restorationIdentifier = "PickerVC"

根据我的阅读，我怀疑我可能需要在 AppDelegate 中使用viewControllerWithRestorationIdentifierPath，但我不知道如何正确使用它。我在 AppDelegate 底部的尝试（代码被注释掉）导致应用程序崩溃。

Answer 1

您可以为您的 ViewController 添加一个扩展，使其符合协议UIViewControllerRestoration 并实现viewControllerWithRestorationIdentifierPath 方法。 在ViewController的viewDidLoad函数中添加restorationClass就像restorationIdentifer一样

    override func viewDidLoad() {
        super.viewDidLoad()
        restorationIdentifier = "PickerVC"
        restorationClass = ViewController.self
        setupPicker()
    }


extension ViewController: UIViewControllerRestoration {

    static func viewController(withRestorationIdentifierPath identifierComponents: [Any], coder: NSCoder) -> UIViewController? {
        let vc = ViewController()
        return vc
    }

}

在您的 ViewController 类中添加此代码并删除您在 AppDelegate 中添加的 viewControllerWithRestorationIdentifierPath 方法