src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> flatMapSingle(Function<? super T, ? extends SingleSource<? extends R>> mapper, boolean delayErrors) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new ObservableFlatMapSingle<T, R>(this, mapper, delayErrors));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> flatMapSingle(Function<? super T, ? extends SingleSource<? extends R>> mapper, boolean delayErrors) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new ObservableFlatMapSingle<T, R>(this, mapper, delayErrors));
    }
Maps each element of the upstream Observable into SingleSources, subscribes to them
and merges their onSuccess values, in no particular order, into a single Observable sequence,
optionally delaying all errors.
<p>
<img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/flatMapSingle.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code flatMapSingle} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param <R> the result value type
@param mapper the function that received each source value and transforms them into SingleSources.
@param delayErrors if true errors from the upstream and inner SingleSources are delayed until each of them
terminates.
@return the new Observable instance