src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.PASS_THROUGH)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> concatWith(@NonNull CompletableSource other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new FlowableConcatWithCompletable<T>(this, other));
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.PASS_THROUGH)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> concatWith(@NonNull CompletableSource other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new FlowableConcatWithCompletable<T>(this, other));
    }
Returns a {@code Flowable} that emits items from this {@code Flowable} and when it completes normally, the
other {@link CompletableSource} is subscribed to and the returned {@code Flowable} emits its terminal events.
<p>
<img width="640" height="380" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/concat.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator does not interfere with backpressure between the current Flowable and the
downstream consumer (i.e., acts as pass-through). When the operator switches to the
{@code Completable}, backpressure is no longer present because {@code Completable} doesn't
have items to apply backpressure to.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatWith} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
<p>History: 2.1.10 - experimental
@param other the {@code CompletableSource} to subscribe to once the current {@code Flowable} completes normally
@return the new Flowable instance
@since 2.2