a.swift

class ViewModel {
  class Input {
    let fetch = PublishSubject<()>()
  }
  class Output {
    let friends: Driver<[User]>
  }
  let apiClient: APIClient
  let input: Input
  let output: Output
  init(apiClient: APIClient) {
    self.apiClient = apiClient
    // Connect input and output
  }
}