How do I wait for a void function to finish executing before triggering a completable in Android?

Question

I'm very new to RxJava/RxAndroid and I'm very confused with how to achieve this workflow:

• Trigger void function
• Once it completes, provide some sort of callback to start a Completable function.

I tried wrapping my void function in an Observable that emits a , but honestly I've no clue if that's the right thing to do.

fun observableFoo(): Observable<Unit> {
        return Observable.create<Unit> { emitter ->
            emitter.onNext( voidFunction() )
        }
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
    }

I have another completeable function inside which I have to subscribe to this Observable as follows:

fun onClick(): Completable { 

  observablefoo().subscribe {
      Logger.e("demoApp", "inside subscribe - value = $it") 
  }

  val x = someCompletableFunction()
  return x
}

I want to be able to first wait for the subscription to finish and then trigger someCompletableFunction(). Is my approach even correct?

I've been stuck on this problem for a day and would really appreciate some help from anyone. Thank you.