如何在Swift中管理ViewController之间的状态？

Question

我正在努力解决ViewControllers之间的状态问题。

导航路径如下；

1。用户从VenueDetailsVC导航到DiscoverVC，并像这样传递Venue对象：

func presentDiscoverVC(){

    guard let discoverVC = storyboard?.instantiateViewController(withIdentifier: "DiscoverVC") as? DiscoverVC else { return }

    discoverVC.venue = self.venue!
    discoverVC.showBackButton = true

    DispatchQueue.main.async {
        self.presentDetail(discoverVC)
    }

}

2。然后，用户可以从DiscoverVC导航到PreferencesVC并传递相同的Venue对象，同时清除本地Venue对象，如下所示：

class DiscoverVC: UIViewController{

    var venue: Venue?{
        didSet{
            print("DiscoverVC venue name: \(venue?.name ?? "venue name")")
        }
    }

    @IBAction func preferencesBtnTapped(_ sender: Any) {

        guard let preferencesVC = storyboard?.instantiateViewController(withIdentifier: "PreferencesVC") as? PreferencesVC else { return }

        preferencesVC.selectedVenue = self.venue!

        presentDetail(preferencesVC)

    }

    override func viewWillAppear(_ animated: Bool) {

            print("inside viewWillAppear venue: \(venue?.name ?? "venue name")")

    }


    override func viewWillDisappear(_ animated: Bool) {

        venue = nil

    }
}

3。然后，用户从DiscoverVC导航回到PreferencesVC，并传递相同的Venue对象

@IBAction func doneBtnTapped(_ sender: Any) {

    guard let discoverVC = storyboard?.instantiateViewController(withIdentifier: "DiscoverVC") as? DiscoverVC else { return }

    discoverVC.venue = self.selectedVenue!

    DataService.run.updateUserDiscoveryPreferences(forUID: Auth.auth().currentUser!.uid, discoverableBool: discoverable!, preferences: preferences) { (success) in

        self.dismissDetail()
    }

}

问题：Venue内的DiscoverVC对象是否为零，即使它已在上述第三步中从PreferencesVC传递回来了？ < / p>

这是控制台输出：

Answer 1

执行此操作时：

@IBAction func doneBtnTapped(_ sender: Any) {

    guard let discoverVC = storyboard?.instantiateViewController(withIdentifier: "DiscoverVC") as? DiscoverVC else { return }

    discoverVC.venue = self.selectedVenue!

您没有设置提供首选项VC的Discover VC的venue。您正在设置全新 Discover VC的venue。

您可以使用委托模式将venue传递回去。

创建一个PreferencesVCDelegate：

protocol PreferencesVCDelegate: class {
    func preferencesVC(_ vc: PreferencesVC, didDismissWithVenue venue: Venue)
}

在delegate中添加一个PreferencesVC属性：

weak var delegate: PreferencesVCDelegate?

用对委托方法的调用替换设置venue的行：

@IBAction func doneBtnTapped(_ sender: Any) {

    guard let discoverVC = storyboard?.instantiateViewController(withIdentifier: "DiscoverVC") as? DiscoverVC else { return }

    delegate?.preferencesVC(self, didDismissWithVenue: self.venue)

出示PreferencesVC时，将self设置为代表：

@IBAction func preferencesBtnTapped(_ sender: Any) {

    guard let preferencesVC = storyboard?.instantiateViewController(withIdentifier: "PreferencesVC") as? PreferencesVC else { return }

    preferencesVC.selectedVenue = self.venue!
    preferencesVC.delegate = self

    presentDetail(preferencesVC)

}

并使DiscoverVC符合PreferencesVCDelegate。在此处设置发现VC的venue

extension DiscoverVC: PreferencesVCDelegate {
    func preferencesVC(_ vc: PreferencesVC, didDismissWithVenue venue: Venue) {
        self.venue = venue
    }
}