code: @CheckReturnValue
      @BackpressureSupport(BackpressureKind.PASS_THROUGH)
      @SchedulerSupport(SchedulerSupport.NONE)
      public static <T> Flowable<T> defer(Callable<? extends Publisher<? extends T>> supplier) {
          ObjectHelper.requireNonNull(supplier, "supplier is null");
          return RxJavaPlugins.onAssembly(new FlowableDefer<T>(supplier));
      }
docs: Returns a Flowable that calls a Publisher factory to create a Publisher for each new Subscriber
  that subscribes. That is, for each subscriber, the actual Publisher that subscriber observes is
  determined by the factory function.
  <p>
  <img width="640" height="340" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/defer.png" alt="">
  <p>
  The defer Subscriber allows you to defer or delay emitting items from a Publisher until such time as a
  Subscriber subscribes to the Publisher. This allows a {@link Subscriber} to easily obtain updates or a
  refreshed version of the sequence.
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>The operator itself doesn't interfere with backpressure which is determined by the {@code Publisher}
  returned by the {@code supplier}.</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code defer} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param supplier
  the Publisher factory function to invoke for each {@link Subscriber} that subscribes to the
  resulting Publisher
  @param <T>
  the type of the items emitted by the Publisher
  @return a Flowable whose {@link Subscriber}s' subscriptions trigger an invocation of the given
  Publisher factory function
  @see <a href="http://reactivex.io/documentation/operators/defer.html">ReactiveX operators documentation: Defer</a>