SayHelloViewModel.swift

final class SayHelloViewModel: ViewModelType {
  struct Input {
    let name: Observable<String>
    let validate: Observable<Void>
  }

  struct Output {
    let greeting: Driver<String>
  }

  func transform(input: Input) -> Output {
    let greeting = input.validate
      .withLatestFrom(input.name)
      .map { name in
        return "Hello \(name)!"
      }
      .startWith("")
      .asDriver(onErrorJustReturn: ":-(")

    return Output(greeting: greeting)
  }
}