How to define a protocol as a type for a @ObservedObject property?

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Wrappers and stored properties are not allowed in swift protocols and extensions, at least for now. So I would go with the following approach mixing protocols, generics and classes… (all compilable and tested with Xcode 11.2 / iOS 13.2)

// base model protocol
protocol ItemViewModel: ObservableObject {
    var title: String { get set }

    func save()
    func delete()
}

// generic view based on protocol
struct ItemView<Model>: View where Model: ItemViewModel {
    @ObservedObject var viewModel: Model

    var body: some View {
        VStack {
            TextField("Item Title", text: $viewModel.title)
            Button("Save") { self.viewModel.save() }
        }
    }
}

// extension with default implementations
extension ItemViewModel {
    
    var title: String {
        get { "Some default Title" }
        set { }
    }
    
    func save() {
        // some default behaviour
    }

    func delete() {
        // some default behaviour
    }
}

// concrete implementor
class SomeItemModel: ItemViewModel {
    @Published var title: String
    
    init(_ title: String) {
        self.title = title
    }
}

// testing view
struct TestItemView: View {
    var body: some View {
        ItemView(viewModel: SomeItemModel("test"))
    }
}

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