Overriding Swift property with descendant subclass

Question

I'd like to override descendant property, because I want to use methods from TestViewModel class directly without casting ViewModel to TestViewModel. As I suppose it's not possible. Could you advise some nice solution. Code and error below:

class ViewModel {
}

class View: UIView {
    weak var viewModel: ViewModel!
}

class TestViewModel {
}

class TestView: View {
    weak var viewModel: TestViewModel! // error here
}

Got this error:

/Users/evgeniirtishchev/Documents/Development/rdfuturesales/RDProject/View/Identification/IdentificationView.swift:12:26: Cannot override mutable property 'viewModel' of type 'ViewModel!' with covariant type 'IdentificationViewModel!'

Answer 1

Swift does not allow you to change the class type of any variables or properties.

See related question: Overriding superclass property with different type in Swift