src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.ERROR)
    @SchedulerSupport(SchedulerSupport.COMPUTATION)
    public final Flowable<Flowable<T>> window(long timespan, long timeskip, TimeUnit unit) {
        return window(timespan, timeskip, unit, Schedulers.computation(), bufferSize());
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.ERROR)
    @SchedulerSupport(SchedulerSupport.COMPUTATION)
    public final Flowable<Flowable<T>> window(long timespan, long timeskip, TimeUnit unit) {
        return window(timespan, timeskip, unit, Schedulers.computation(), bufferSize());
    }
Returns a Flowable that emits windows of items it collects from the source Publisher. The resulting
Publisher starts a new window periodically, as determined by the {@code timeskip} argument. It emits
each window after a fixed timespan, specified by the {@code timespan} argument. When the source
Publisher completes or Publisher completes or encounters an error, the resulting Publisher emits the
current window and propagates the notification from the source Publisher.
<p>
<img width="640" height="335" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/window7.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
time 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} operates by default on the {@code computation} {@link Scheduler}.</dd>
</dl>

@param timespan
the period of time each window collects items before it should be emitted
@param timeskip
the period of time after which a new window will be created
@param unit
the unit of time that applies to the {@code timespan} and {@code timeskip} arguments
@return a Flowable that emits new windows periodically as a fixed timespan elapses
@see <a href="http://reactivex.io/documentation/operators/window.html">ReactiveX operators documentation: Window</a>