SwiftUI views with protocol interfaces to view models.

swift-ui-protocol-view-models.swift

import SwiftUI

/// View model protocol
protocol ViewModel: ObservableObject {
    var count: Int { get }
    func increase()
}

/// Concrete implementation
class MyViewModel: ViewModel {
    @Published var count: Int = 0
    func increase() {
        count += 1
    }
}

/// Alternative implementation
class MyOtherViewModel: ViewModel {
    @Published var count: Int = 0
    func increase() {
        count += 2
    }
}

/// View with generic parameter that is constrained to
/// the ViewModel protocol
struct MyView<VM>: View where VM: ViewModel {
    
//    @ObservedObject var viewModel: ViewModel // <--- Compiler errors
    @ObservedObject var viewModel: VM // <--- Compiler happy :)
    
    var body: some View {
        VStack(spacing: 20) {
            Text("Count = \(viewModel.count)")
            Button(action: {
                self.viewModel.increase()
            }) {
                Text("Increase")
            }
        }

    }
    
}

struct MyView_Preview: PreviewProvider {
    
    static var previews: some View {
        HStack {
            MyView(viewModel: MyViewModel())
            Spacer()
            MyView(viewModel: MyOtherViewModel())
        }.padding()
    }
    
}