Contract with singleton

ViewModel+Contract.swift

final class ViewModel: ViewModelType {
    
    var input: Input
    var output: Output!
    
    struct Input {}
    
    struct Output {}

    static var shared: ViewModel {
        let viewModel = ViewModel()
        viewModel.transform()
        return viewModel
    }
    
    private init() {
        input = Input()
    }
    
    private func transform() {
        self.parser()
        output = Output()
    }

    private func parser() {

    }
}

ViewModelType.swift

protocol ViewModelType {
  associatedtype Input
  associatedtype Output
  
  func transform(input: Input) -> Output
}