如何在 SwiftUI NavigationView 中重新初始化类

Question

I have two views - a MasterView and DetailView. When opening the DetailView, I initialise a new class that tracks data about the view (in the real implementation, the detail view involves a game).

However, when I press the back button from the DetailView to return to the MasterView, and then press the button to return to the DetailView, my class is unchanged. However, I would like to re-initialise a new copy of this class (in my case to re-start the game) whenever I move from the MasterView to the DetailView.

I have condensed the problem to this code:

import SwiftUI
import Combine

class Model: ObservableObject {
    @Published var mytext: String = "mytext"
}

struct MasterView: View {
    var body: some View {
        NavigationView {
            NavigationLink(destination: DetailView(model: Model())) {
                Text("press me")
            }
        }
    }
}

struct DetailView: View {
    @ObservedObject var model: Model = Model()

    var body: some View {
        TextField("Enter here", text: $model.mytext)
    }
}


struct MasterView_Previews: PreviewProvider {
    static var previews: some View {
        MasterView()
    }
}

I would like to create a new instance of Model every time I click the NavigationLink to the detail view, but it seems like it always refers back to the same original instance - I can see this by typing a change into the text field of the DetailView, which persists if I go back and forward again.

Is there any way of doing this?

Answer 1

Based on your comments - and correct me where wrong - here's how I'd set things up.

Your needs are:

• A "base" class. Call it MasterView , "settings", "view state", whatever. This is where everything starts.
• A "current game".... well, it could be a struct , a class , even properties in an ObservableObject .

I think that's about it. Hierarchically, your model could be:

ViewState

...Player

......Properties, including ID and history

...Current Game

...... Properties, including difficulty

Please note, I've changed some names and am being very vague on properties. The point is, you can encapsulate all of this in an ObservableObject , create an `EnvironmentObject of it, and have all your SwiftUI views "react" to changes in it.

Leaving out views, hopefully you can see where this "model" can contain just about all the Swift code you wish to do everything - now all you need is to tie in your views.

(1) Create your ObservableObject . It needs to (a) be a class object and (b) conform to the ObservableObject protocol. Here's a simple one:

class ViewState : ObservableObject {
    var objectWillChange = PassthroughSubject<Void, Never>()
    @Published var playerID = ""  {
        willSet {
            objectWillChange.send()
        }
    }
}

You can create more structs/classes and instantiate them as needed in your model.

(2) Instantiate ViewState once min your environment. In SceneDelegate :

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
    if let windowScene = scene as? UIWindowScene {
        let window = UIWindow(windowScene: windowScene)
        window.rootViewController = UIHostingController(rootView: ContentView()
            .environmentObject(ViewState())
        )
        self.window = window
        window.makeKeyAndVisible()
    }
}

Note that there's a single line added here and that ViewState is instantiated a single time.

(3) Finally, in any SwiftUI view that needs to know your view state, bind it by adding one line of code:

@EnvironmentObject var model: ViewState

If you want, you can do virtually anything in your model ( ViewState ) from instantiating a new game, flag something to result in a modal popup, add a player to an array, whatever.

The main thing I hope I'm explaining is there's no need to instantiate a second view state - rather instantiate a second game instance inside your single view state.

Again, if I'm way off from your needs, let me know - I'll gladly delete my answer. Good luck!