【Kotlin/Android】RxJavaのflatMapメソッドの使い方！mapとの違い

@CheckReturnValue
@SchedulerSupport(SchedulerSupport.NONE)
public final <R> Observable<R> flatMap(Function<? super T, ? extends ObservableSource<? extends R>> mapper) {
    return flatMap(mapper, false);
}

val observable1 = Observable.just(1)
val observable2 = Observable.just(2)

Observable.combineLatest(
      observable1,
      observable2,
      { num1, num2 -> num1 + num2 }
  ).flatMap { Observable.just(it * 3) }  // .map { it * 3 } と同じ挙動
  .subscribeBy(
      onNext = { result -> Log.d("RxJava", "Received: $result") },
      onComplete = { Log.d("RxJava","Completed") }
  ).addTo(compositeDisposable)

val observable1 = Observable.just(1, 2, 3)
val observable2 = Observable.just(4, 5, 6)

observable1
    .flatMap { num ->
        observable2.map { letter ->
            num + letter
        }
    }
    .subscribe { result ->
        Log.d("RxJava", "Received: $result")
    }
    .addTo(compositeDisposable)

Received: 5
Received: 6
Received: 7
Received: 6
Received: 7
Received: 8
Received: 7
Received: 8
Received: 9