private void subscribeTaskForRequestCancellation(
      final AtomicBoolean isCancelled, ProducerContext producerContext) {
    producerContext.addCallbacks(
        new BaseProducerContextCallbacks() {
          @Override
          public void onCancellationRequested() {
            isCancelled.set(true);
          }
        });
  }
Partial disk cache producer.

<p>This producer looks in the disk cache to see if it holds part of the requested image. If the
image is found, then it is passed to the consumer as a non-final result, but an adjusted request
is still sent further along to request the remainder of the image.

<p>When the final result comes from the input producer, the two parts are stitched back together
and returned as a whole.
imagepipeline/src/main/java/com/facebook/imagepipeline/producers/PartialDiskCacheProducer.java