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)
    }
}