src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.ERROR)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <B> Flowable<Flowable<T>> window(Callable<? extends Publisher<B>> boundaryIndicatorSupplier) {
        return window(boundaryIndicatorSupplier, bufferSize());
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.ERROR)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <B> Flowable<Flowable<T>> window(Callable<? extends Publisher<B>> boundaryIndicatorSupplier) {
        return window(boundaryIndicatorSupplier, bufferSize());
    }
Returns a Flowable that emits windows of items it collects from the source Publisher. The resulting
Publisher emits connected, non-overlapping windows. It emits the current window and opens a new one
whenever the Publisher produced by the specified {@code closingSelector} emits an item.
<p>
<img width="640" height="455" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/window1.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator consumes the source {@code Publisher} in an unbounded manner.
The returned {@code Publisher} doesn't support backpressure as it uses
the {@code closingSelector} to control the creation of windows. The returned inner {@code Publisher}s honor
backpressure but have an unbounded inner buffer that <em>may</em> lead to {@code OutOfMemoryError}
if left unconsumed.</dd>
<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 element type of the boundary Publisher
@param boundaryIndicatorSupplier
a {@link Callable} that returns a {@code Publisher} that governs the boundary between windows.
When the source {@code Publisher} emits an item, {@code window} emits the current window and begins
a new one.
@return a Flowable that emits connected, non-overlapping windows of items from the source Publisher
whenever {@code closingSelector} emits an item
@see <a href="http://reactivex.io/documentation/operators/window.html">ReactiveX operators documentation: Window</a>