src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable concatMapCompletable(Function<? super T, ? extends CompletableSource> mapper) {
        return concatMapCompletable(mapper, 2);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable concatMapCompletable(Function<? super T, ? extends CompletableSource> mapper) {
        return concatMapCompletable(mapper, 2);
    }
@ CheckReturnValue @ SchedulerSupport ( SchedulerSupport . NONE ) public final Completable concatMapCompletable ( Function < ? super T , ? extends CompletableSource > mapper ) { return concatMapCompletable ( mapper , 2 ) ; }
Maps each element of the upstream Observable into CompletableSources, subscribes to them one at a time in
order and waits until the upstream and all CompletableSources complete.
<p>
<img width="640" height="505" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/concatMapCompletable.o.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatMapCompletable} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
<p>History: 2.1.6 - experimental
@param mapper
a function that, when applied to an item emitted by the source ObservableSource, returns a CompletableSource
@return a Completable that signals {@code onComplete} when the upstream and all CompletableSources complete
@since 2.2
Maps each element of the upstream Observable into CompletableSources subscribes to them one at a time in order and waits until the upstream and all CompletableSources complete . <p > <img width = 640 height = 505 src = https : // raw . github . com / wiki / ReactiveX / RxJava / images / rx - operators / concatMapCompletable . o . png alt = > <dl > <dt > <b > Scheduler : < / b > < / dt > <dd > {