src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <B> Observable<Observable<T>> window(ObservableSource<B> boundary) {
        return window(boundary, bufferSize());
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <B> Observable<Observable<T>> window(ObservableSource<B> boundary) {
        return window(boundary, bufferSize());
    }
Returns an Observable that emits non-overlapping windows of items it collects from the source ObservableSource
where the boundary of each window is determined by the items emitted from a specified boundary-governing
ObservableSource.
<p>
<img width="640" height="475" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/window8.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>This version of {@code window} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <B>
the window element type (ignored)
@param boundary
an ObservableSource whose emitted items close and open windows
@return an Observable that emits non-overlapping windows of items it collects from the source ObservableSource
where the boundary of each window is determined by the items emitted from the {@code boundary}
ObservableSource
@see <a href="http://reactivex.io/documentation/operators/window.html">ReactiveX operators documentation: Window</a>