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Mark
Mark

Reputation: 7409

SwiftUI MVVM AnyViewModel not propagating state changes

I'm trying to implement MVVM in my SwiftUI app in a way that decouples the view from the view model itself. In my research I came across this article outlining one strategy: https://quickbirdstudios.com/blog/swiftui-architecture-redux-mvvm/

Here's a summary of how it works:

// ViewModel.swift

protocol ViewModel: ObservableObject where ObjectWillChangePublisher.Output == Void {
    associatedtype State
    associatedtype Event

    var state: State { get }
    func trigger(_ event: Event)
}

// AnyViewModel.swift

final class AnyViewModel<State, Event>: ObservableObject {
    private let wrappedObjectWillChange: () -> AnyPublisher<Void, Never>
    private let wrappedState: () -> State
    private let wrappedTrigger: (Event) -> Void

    var objectWillChange: some Publisher {
        wrappedObjectWillChange()
    }

    var state: State {
        wrappedState()
    }

    func trigger(_ input: Event) {
        wrappedTrigger(input)
    }

    init<V: ViewModel>(_ viewModel: V) where V.State == State, V.Event == Event {
        self.wrappedObjectWillChange = { viewModel.objectWillChange.eraseToAnyPublisher() }
        self.wrappedState = { viewModel.state }
        self.wrappedTrigger = viewModel.trigger
    }
}

// MyView.swift

extension MyView {
    enum Event {
        case onAppear
    }

    enum ViewState {
        case loading
        case details(Details)
    }

    struct Details {
        let title: String
        let detail: String
    }
}

struct MyView: View {
    @ObservedObject var viewModel: AnyViewModel<ViewState, Event>

    var body: some View { ... }
}

// ConcreteViewModel.swift

class ConcreteViewModel: ViewModel {
    @Published var state: MyView.ViewState = .loading

    func trigger(_ event: MyView.Event) {
        ...
        state = .details(...) // This gets called by my app and the state is updated.
        ...
    }
}

// Constructing MyView

let view = MyView(viewModel: AnyViewModel(ConcreteViewModel))

This succeeds in separating the view from the view model (using AnyViewModel as a wrapper), but the issue is updates to the state property in ConcreteViewModel are not reflected in MyView.

My suspicion is that the problem lies in AnyViewModel and the wrappedObjectWillChange closure, but I am having difficulty debugging it. Do I need to do something with the objectWillChange publisher explicitly, or should @Published handle it automatically?

Any help is much appreciated.

Upvotes: 0

Views: 584

Answers (1)

Hayden Malcomson
Hayden Malcomson

Reputation: 83

I believe the var objectWillChange: some Publisher is not being resolved correctly by SwiftUI type checker. Setting it to the matching type var objectWillChange: AnyPublisher<Void, Never> should fix the bug.

See: https://gist.github.com/LizzieStudeneer/c3469eb465e2f88bcb8225df29fbbb77

Upvotes: 1

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