@Nullable
              @Override
              public Object answer(InvocationOnMock invocation) throws Throwable {
                Consumer consumer = (Consumer) invocation.getArguments()[0];
                consumer.onFailure(mException);
                return null;
              }
Checks basic properties of disk cache producer operation, that is: - it delegates to the {@link
BufferedDiskCache#get(CacheKey key, AtomicBoolean isCancelled)} - it returns a 'copy' of the
cached value - if {@link BufferedDiskCache#get(CacheKey key, AtomicBoolean isCancelled)} is
unsuccessful, then it passes the request to the next producer in the sequence. - if the next
producer returns the value, then it is put into the disk cache.
imagepipeline/src/test/java/com/facebook/imagepipeline/producers/DiskCacheReadProducerTest.java