SwiftUI: @State variable never get updated from @Published-CodePudding

I'm trying to trigger an alert when is an error in the model but it never get updated to show the alert:

Here is my implementation in the view:

struct ContentView: View {
    @ObservedObject var viewModel: ViewModel
    @State var showAlert = false
    init() {
        viewModel = ViewModel()
        showAlert = viewModel.showAlert
    }
    var body: some View {
        NavigationView {
            Text("Hello, world!")
                .padding()
        }
        .alert(isPresented: $showAlert) {
            Alert(title: Text("This works"),
                  message: Text("Hello"),
                  dismissButton: .default(Text("got it"))
        )}
    }
}

Here is my models:

class ViewModel: ObservableObject {
    @Published var showAlert = false
    var cancellables = Set<AnyCancellable>()
    
    init() {
        DoSomething.shared.showAlert.sink { _ in
            print("got new Value")
        } receiveValue: {[weak self] value in
            print("value")
            self?.showAlert = value
        }.store(in: &cancellables)
    }
}
class DoSomething {
    let showAlert = PassthroughSubject<Bool, Never>()
    static let shared = DoSomething()
    private init() {
        checkToShowAlert()
    }
    func checkToShowAlert() {
        DispatchQueue.main.asyncAfter(deadline: .now()   5) { [weak self] in
            print("change value")
            self?.showAlert.send(true)
        }
    }
}

Any of you knows why the showAlert variable it never gets updated?

I'll really appreciate your help

CodePudding user response：

In your current code, you're setting ContentView's showAlert to the ViewModel's showAlert at that point in time:

init() {
  viewModel = ViewModel()
  showAlert = viewModel.showAlert //<-- assignment at the time of init
}

Meaning, it's false at the time of assignment. Because it's just a Bool getting assigned to another Bool, there's no mechanism to keep it updated if ViewModel's showAlert changes.

The simplest solution is to get rid of your @State variable and observe the @Published property directly:

struct ContentView: View {
    @ObservedObject var viewModel: ViewModel = ViewModel()

    var body: some View {
        NavigationView {
            Text("Hello, world!")
                .padding()
        }
        .alert(isPresented: $viewModel.showAlert) {
            Alert(title: Text("This works"),
                  message: Text("Hello"),
                  dismissButton: .default(Text("got it"))
        )}
    }
}