Question 292219 in IOS Swift Question posted in
The official documentation can be found here.

Ios swift – Adding view model causes view updates to fail

I’m attempting to separate my model and view by adding a view model as a bridge. When I do this in the following code, clicking on the ‘Increase Score’ button no longer triggers a view update. It works if I connect the view directly to the model. Any suggestions on how to get this to work?

import SwiftUI

@main
struct MVVM_Test_Watch_AppApp: App {
    @StateObject var viewModel = ViewModel()
    
    var body: some Scene {
        WindowGroup {
            ContentView(viewModel: viewModel)
        }
    }
}

class Model {
    var score = 0
}

class ViewModel: ObservableObject {
    @Published private var model: Model
    
    init() {
        model = Model()
    }
    var score: Int {
        model.score
    }
    func incScore() {
        model.score += 1
    }
}

struct ContentView: View {
    @ObservedObject var viewModel: ViewModel
    
    var body: some View {
        VStack {
            Text("Your score is (viewModel.score)")
            Button("Increase Score") {
                viewModel.incScore()
            }
        }
    }
}
Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    According to this migration page, there's a new way to handle observation. I updated and tested my code.

    import SwiftUI

@main
struct MVVM_Test_Watch_AppApp: App {
    @State var viewModel = ViewModel()
    
    var body: some Scene {
        WindowGroup {
            ContentView()
                .environment(viewModel)
        }
    }
}

@Observable
class Model {
    var score = 0
}

@Observable
class ViewModel {
    private var model: Model
    
    init() {
        model = Model()
    }
    var score: Int {
        model.score
    }
    func incScore() {
        model.score += 1
    }

}

struct ContentView: View {
    @Environment(ViewModel.self) private var viewModel

    
    var body: some View {
        VStack {
            Text("Your score is (viewModel.score)")
            Button("Increase Score") {
                viewModel.incScore()
            }
        }
    }
}

  2. 0

    Typically, in models within SwiftUI, struct is used, and while this might not be the exact cause of the issue you’re encountering.

    In your code, a minimal change to detect updates and refresh the view would be to add objectWillChange.send() to the function. Here’s a sample code for reference:

    import SwiftUI

@main
struct MVVM_Test_Watch_AppApp: App {
    @StateObject var viewModel = ViewModel()
    
    var body: some Scene {
        WindowGroup {
            ContentView(viewModel: viewModel)
        }
    }
}

class Model {
    var score = 0
}

class ViewModel: ObservableObject {
    @Published private var model: Model
    
    init() {
        model = Model()
    }
    var score: Int {
        model.score
    }
    func incScore() {
        model.score += 1

        objectWillChange.send()

    }
}

struct ContentView: View {
    @ObservedObject var viewModel: ViewModel
    
    var body: some View {
        VStack {
            Text("Your score is (viewModel.score)")
            Button("Increase Score") {
                viewModel.incScore()
            }
        }
    }
}

    Or you can consider using @Published in the following way:

     class ViewModel: ObservableObject {
    private var model: Model
    @Published var score: Int

    init() {
        model = Model()
        score = model.score
    }

    func incScore() {
        model.score += 1
        score = model.score
        
    }
}
    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search