src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <V> Observable<T> timeout(Function<? super T, ? extends ObservableSource<V>> itemTimeoutIndicator) {
        return timeout0(null, itemTimeoutIndicator, null);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <V> Observable<T> timeout(Function<? super T, ? extends ObservableSource<V>> itemTimeoutIndicator) {
        return timeout0(null, itemTimeoutIndicator, null);
    }
Returns an Observable that mirrors the source ObservableSource, but notifies observers of a
{@code TimeoutException} if an item emitted by the source ObservableSource doesn't arrive within a window of
time after the emission of the previous item, where that period of time is measured by an ObservableSource that
is a function of the previous item.
<p>
<img width="640" height="400" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/timeout3.png" alt="">
<p>
Note: The arrival of the first source item is never timed out.
<dl>
<dt><b>Scheduler:</b></dt>
<dd>This version of {@code timeout} operates by default on the {@code immediate} {@link Scheduler}.</dd>
</dl>

@param <V>
the timeout value type (ignored)
@param itemTimeoutIndicator
a function that returns an ObservableSource for each item emitted by the source
ObservableSource and that determines the timeout window for the subsequent item
@return an Observable that mirrors the source ObservableSource, but notifies observers of a
{@code TimeoutException} if an item emitted by the source ObservableSource takes longer to arrive than
the time window defined by the selector for the previously emitted item
@see <a href="http://reactivex.io/documentation/operators/timeout.html">ReactiveX operators documentation: Timeout</a>