@Before
  public void setUp() {
    mSettableProducerContext =
        new SettableProducerContext(
            mImageRequest,
            mRequestId,
            mock(ProducerListener2.class),
            mock(Object.class),
            ImageRequest.RequestLevel.FULL_FETCH,
            false,
            true,
            Priority.MEDIUM,
            mConfig);
    mCallbacks1 = mock(ProducerContextCallbacks.class);
    mCallbacks2 = mock(ProducerContextCallbacks.class);
  }
Checks basic properties of NullProducer, that is that it always returns null.
imagepipeline/src/test/java/com/facebook/imagepipeline/producers/SettableProducerContextTest.java