title: How to perform subscription within subscription using RxSwift?
body: <p>Currently I have the following working code:</p>
  <pre><code>    mainView.saveButton.rx.tap.bind { [weak self] in
          if let self = self {
              // start indicator
              self.viewModel.save() // Completable
                  .subscribe(onCompleted: { [weak self] in
                      // completed
                  }, onError: { error in
                      // error
                  })
                  .disposed(by: self.disposeBag)
          }
      }.disposed(by: disposeBag)
  </code></pre>
  <p>But I know it is not a good approach (due to nested subscriptions), so I am trying to create working equivalent (now with no success):</p>
  <pre><code>    mainView.saveButton.rx.tap
          .do(onNext: { [weak self] in
              // start indicator
          })
          .flatMapFirst { _ in
              self.viewModel.save() // Completable
          }
          .subscribe(onError: { error in
              // error
          }, onCompleted: { [weak self] in
              // completed
          })
          .disposed(by: disposeBag)
  </code></pre>
  <p>Subscribe closure is not calling at all. Why?</p>