Procotol composition on the same property - swift

Question

Is there a way to achieve this ?
protocol VCProtocol1: UIViewController {
var viewModel: VMProtocol1? { get set }
}
protocol VCProtocol2: UIViewController {
var viewModel: VMProtocol2? { get set }
}
class VC: UIViewController, VCProtocol1, VCProtocol2 {
var viewModel: (VMProtocol1 & VMProtocol2)?
}
What I want to do is composition on ViewController to avoid re-implementing the same code in multiple ViewControllers.
Edit:
To be more precise, the problem here is I want my viewModel property to be shared between both protocols because ultimately I want to implement something like this:
- View Model
protocol VMProtocol1 {
func vmTest1()
}
protocol VMProtocol2 {
func vmTest2()
}
class ViewModel: VMProtocol1, VMProtocol2 {
func vmTest1() { }
func vmTest2() { }
}
- View Controller
protocol VCProtocol1: UIViewController {
var viewModel: VMProtocol1 { get set }
func vcTest1()
}
extension VCProtocol1 {
func vcTest1() {
// This is the key point, I want to be able to refer to my viewModel property internally in each protocol.
self.viewModel.vmTest1()
}
}
protocol VCProtocol2: UIViewController {
var viewModel: VMProtocol2 { get set }
}
class VC: UIViewController, VCProtocol1, VCProtocol2 {
var viewModel: (VMProtocol1 & VMProtocol2)?
}

Answer 1

This is actually not possible. One entity that implements multiple protocols which declare the same property (here viewModel) but with a different type is simply not possible in Swift.
The closest you can do is to use a protocol which define the viewModel type as an associated type and provide conditional extensions (it will force you to use a generic implementation).
Please note that this is a complex implementation that require a good knowledge of generic protocols. I would not recommend using a such complicated design without understanding exactly what you are doing.
View model
protocol VMProtocol1 {
func vmTest1()
}
protocol VMProtocol2 {
func vmTest2()
}
class ViewModel: VMProtocol1, VMProtocol2 {
func vmTest1() { }
func vmTest2() { }
}
Implementation
protocol VCProtocolBase {
associatedtype ViewModel
var viewModel: ViewModel { get }
}
protocol VCProtocol1: VCProtocolBase {
func vmTest1()
}
protocol VCProtocol2: VCProtocolBase {
func vmTest2()
}
extension VCProtocolBase where Self: VCProtocol1, Self.ViewModel: VMProtocol1 {
func vmTest1() {
viewModel.vmTest1()
}
}
extension VCProtocolBase where Self: VCProtocol2, Self.ViewModel: VMProtocol2 {
func vmTest2() {
viewModel.vmTest2()
}
}
final class VC<ViewModel: VMProtocol1 & VMProtocol2>: VCProtocolBase, VCProtocol1, VCProtocol2 {
var viewModel: ViewModel
init(viewModel: ViewModel) {
self.viewModel = viewModel
}
}
VC(viewModel: ViewModel()).vmTest1()
VC(viewModel: ViewModel()).vmTest2()
The idea here is to rely on generics and conditional extensions to provide the default implementations for the VCProtocolX, which is the role of VCProtocolBase.