SwiftUI with associated ViewModel Type can't be initialised from its hosting ViewController with Swift 5.7

Question

I am defining a SwiftUI view with an associated ViewModel protocol type. However, I getting this error while building Type 'any MyViewModelType' cannot conform to 'MyViewModelType'.

Here's my full code.

protocol MyViewModelType: ObservableObject {
    var loadData: CGFloat { get }
}


struct MyView<ViewModel>: View where ViewModel: MyViewModelType {
    @ObservedObject private var viewModel: ViewModel

    init(viewModel: any MyViewModelType) {
        self.viewModel = viewModel as! ViewModel
    }

    var body: some View {
        Text("Hi")
    }
}

class SubscriptionViewV2Controller: UIHostingController<MyView<MyViewModelType>> {
    init(viewModel: any MyViewModelType) {
        super.init(rootView: MyView(viewModel: viewModel))
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        navigationController?.isNavigationBarHidden = true
    }

    override var preferredStatusBarStyle: UIStatusBarStyle {
        return .darkContent
    }

    @MainActor @objc required dynamic init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

I am not sure what am I doing wrong?

Additionally, I dont know why Xcode would throw error here

init(viewModel: any MyViewModelType) {
    self.viewModel = viewModel as! ViewModel
}

Because of that error, I need to force cast viewModel assignment.

burnsi · Accepted Answer · 2022-09-27T14:31:25.933

SwiftUI generics need one concrete Type to work with. This could be any MyViewModelType. But you have to specify that it will be only one specific. any is the exact opposite.

protocol MyViewModelType: ObservableObject {
    var loadData: CGFloat { get }
}

struct MyView<T>: View where T: MyViewModelType {
    @ObservedObject private var viewModel: T

    init(viewModel: T) {
        self.viewModel = viewModel
    }

    var body: some View {
        Text("Hi")
    }
}

class SubscriptionViewV2Controller<T>: UIHostingController<MyView<T>> where T: MyViewModelType {
    init(viewModel: T) {
        super.init(rootView: MyView(viewModel: viewModel))
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        navigationController?.isNavigationBarHidden = true
    }

    override var preferredStatusBarStyle: UIStatusBarStyle {
        return .darkContent
    }

    @MainActor @objc required dynamic init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

This could shed some light on it.

Also this.

SwiftUI with associated ViewModel Type can't be initialised from its hosting ViewController with Swift 5.7

1 Answers1