SwiftUI CurrentValueSubject Behavior

Question

I'm new in SwiftUI. I'm worked with UIKit and Combine framework building an architecture with ViewModel, UseCases and Repositories. All my architecture is based on the loading states. My loading states are build in this way:

/// Equivalent to @Published with `LoadingState<T, E>` property wrapper
@propertyWrapper public class Loading<T, E: Swift.Error> {
public typealias State = LoadingState<T, E>

public var wrappedValue: State {
    willSet {
        subject.send(newValue)
    }
}

public init(wrappedValue: State) {
    self.wrappedValue = wrappedValue
}

private lazy var subject = CurrentValueSubject<State, Never>(wrappedValue)

public var projectedValue: AnyPublisher<State, Never> {
    return subject.eraseToAnyPublisher()
}
}

and my ViewModel works in this way:

@Loading<MyData, MyError> var myDataLoadingState = .idle
public func getMyData(ID: String) {
    myDataLoadingState = .loading
    
    myDataUseCase.execute(ID: ID)
        .receive(on: DispatchQueue.main)
        .sink { completion in
            guard case .failure(let error) = completion else { return }
            myDataLoadingState = .failure(error)
        } receiveValue: { myData in
            self. myDataLoadingState = .success(myData)
        }
        .store(in: &self.cancellables)
}

The controller works in this way:

viewModel.$myDataLoadingState
       .sink { state in
           switch state {
           case .idle:
            break
           case .loading:
             self.showLoader()
           case .success(let myData):
            print(myData)
           case .failure(let error):
            self.print(error)
            self.hideLoader()
           }
       }
       .store(in: &cancellables)

Can I use the loading state and my ViewModel in SwiftUI? I tried in this way but seems not works:

struct ContentView: View {
@StateObject var viewModel: MyViewModel

var body: some View {
    switch viewModel.loadingState {
    case .idle:
        Text("Idle")
    case .loading:
        Text("Loading")
    case .success(let myData):
        Text(myData.name)
    case .failure(let error):
        Text(error.localizedDescription)
    }
}
}

The ViewModel now is an ObservableObject

public class MyViewModel: ObservableObject {

}

Thanks in advance

Does your `myDataLoadingState` have `@Published` wrapper? – Asperi Nov 19 '21 at 11:28 — Asperi, Nov 19 '21 at 11:28
mmm no. can you explain me better? – nunzio giulio caggegi Nov 19 '21 at 11:37 — nunzio giulio caggegi, Nov 19 '21 at 11:37

score 0 · Answer 1 · answered Nov 19 '21 at 11:52

Just recognised that you already has property wrapper for myDataLoadingState, so not sure if you are allowed to make it @Published instead. Anyway to inform @StateObject wrapper (who is a listener) you should fire event about changes in view model. A possible way is to use objectWillChange directly, like

myDataUseCase.execute(ID: ID)
    .receive(on: DispatchQueue.main)
    .sink { completion in
        guard case .failure(let error) = completion else { return }

        self.objectWillChange.send()             // << here !!
        myDataLoadingState = .failure(error)
    } receiveValue: { myData in
        self.objectWillChange.send()             // << here !!
        self.myDataLoadingState = .success(myData)
    }
    .store(in: &self.cancellables)

mmm ok it's a solution! thanks! in your opinion, there is a way to not edit every viewmodel and use directly my loading state? — nunzio giulio caggegi, Nov 19 '21 at 12:55
No. I don't know why do you need it, actually all of needed is `@Published var myDataLoadingState`. — Asperi, Nov 19 '21 at 14:02

SwiftUI CurrentValueSubject Behavior

1 Answers1