@Test
  public void testNoCallbackCalledWhenIsPrefetchDoesNotChange() {
    assertFalse(mSettableProducerContext.isPrefetch());
    mSettableProducerContext.addCallbacks(mCallbacks1);
    mSettableProducerContext.setIsPrefetch(false);
    verify(mCallbacks1, never()).onIsPrefetchChanged();
  }
Checks basic properties of NullProducer, that is that it always returns null.
imagepipeline/src/test/java/com/facebook/imagepipeline/producers/SettableProducerContextTest.java