src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <V> Observable<T> timeout(Function<? super T, ? extends ObservableSource<V>> itemTimeoutIndicator,
            ObservableSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return timeout0(null, itemTimeoutIndicator, other);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <V> Observable<T> timeout(Function<? super T, ? extends ObservableSource<V>> itemTimeoutIndicator,
            ObservableSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return timeout0(null, itemTimeoutIndicator, other);
    }
@ CheckReturnValue @ SchedulerSupport ( SchedulerSupport . NONE ) public final < V > Observable < T > timeout ( Function < ? super T , ? extends ObservableSource < V > > itemTimeoutIndicator , ObservableSource < ? extends T > other ) { ObjectHelper . requireNonNull ( other , "other is null" ) ; return timeout0 ( null , itemTimeoutIndicator , other ) ; }
Returns an Observable that mirrors the source ObservableSource, but that switches to a fallback ObservableSource 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/timeout4.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, that
determines the timeout window for the subsequent item
@param other
the fallback ObservableSource to switch to if the source ObservableSource times out
@return an Observable that mirrors the source ObservableSource, but switches to mirroring a fallback ObservableSource
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>
Returns an Observable that mirrors the source ObservableSource but that switches to a fallback ObservableSource 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 / timeout4 . 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 >