src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @NonNull
    @BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Flowable<R> concatMapSingle(Function<? super T, ? extends SingleSource<? extends R>> mapper, int prefetch) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new FlowableConcatMapSingle<T, R>(this, mapper, ErrorMode.IMMEDIATE, prefetch));
    }
@CheckReturnValue
    @NonNull
    @BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Flowable<R> concatMapSingle(Function<? super T, ? extends SingleSource<? extends R>> mapper, int prefetch) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new FlowableConcatMapSingle<T, R>(this, mapper, ErrorMode.IMMEDIATE, prefetch));
    }
Maps the upstream items into {@link SingleSource}s and subscribes to them one after the
other succeeds, emits their success values or terminates immediately if
either this {@code Flowable} or the current inner {@code SingleSource} fail.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/concatMap.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator expects the upstream to support backpressure and honors
the backpressure from downstream. If this {@code Flowable} violates the rule, the operator will
signal a {@code MissingBackpressureException}.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatMapSingle} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
<p>History: 2.1.11 - experimental
@param <R> the result type of the inner {@code SingleSource}s
@param mapper the function called with the upstream item and should return
a {@code SingleSource} to become the next source to
be subscribed to
@param prefetch The number of upstream items to prefetch so that fresh items are
ready to be mapped when a previous {@code SingleSource} terminates.
The operator replenishes after half of the prefetch amount has been consumed
and turned into {@code SingleSource}s.
@return a new Flowable instance
@see #concatMapSingle(Function)
@see #concatMapSingleDelayError(Function, boolean, int)
@since 2.2