src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable concatMapCompletable(Function<? super T, ? extends CompletableSource> mapper, int capacityHint) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.verifyPositive(capacityHint, "capacityHint");
        return RxJavaPlugins.onAssembly(new ObservableConcatMapCompletable<T>(this, mapper, ErrorMode.IMMEDIATE, capacityHint));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable concatMapCompletable(Function<? super T, ? extends CompletableSource> mapper, int capacityHint) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.verifyPositive(capacityHint, "capacityHint");
        return RxJavaPlugins.onAssembly(new ObservableConcatMapCompletable<T>(this, mapper, ErrorMode.IMMEDIATE, capacityHint));
    }
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

@param capacityHint
the number of upstream items expected to be buffered until the current CompletableSource,  mapped from
the current item, completes.
@return a Completable that signals {@code onComplete} when the upstream and all CompletableSources complete
@since 2.2