hoge.swift

button.rx_tap
    .flatMap { () -> Observable<String> in
     if (self.textField3.text?.characters.count > 0) ?? false {
        return just(String(self.textField3.text!))
      } else {
        return failWith(NSError(domain: "app error", code: -1, userInfo: nil))
      }
    }
    .subscribe(
      onNext: { self.showAlert("値は\($0)") },
      onError: { _ in self.showAlert("空ですな...")},
      onCompleted: nil,
      onDisposed: nil
    )
    .addDisposableTo(disposeBag)