@Test
  public void testCallbackCalledWhenIsPrefetchChanges() {
    assertFalse(mSettableProducerContext.isPrefetch());
    mSettableProducerContext.addCallbacks(mCallbacks1);
    mSettableProducerContext.addCallbacks(mCallbacks2);
    mSettableProducerContext.setIsPrefetch(true);
    assertTrue(mSettableProducerContext.isPrefetch());
    verify(mCallbacks1).onIsPrefetchChanged();
    verify(mCallbacks1, never()).onCancellationRequested();
    verify(mCallbacks2).onIsPrefetchChanged();
    verify(mCallbacks2, never()).onCancellationRequested();
  }
Checks basic properties of NullProducer, that is that it always returns null.
imagepipeline/src/test/java/com/facebook/imagepipeline/producers/SettableProducerContextTest.java