MessageBasedVerification

Message-based verification is a SoftwareTesting technique that involves making assertions about the sequence of Messages the SystemUnderTest sends to its Collaborators. Often, Spies or Mocks are used to detect the messages. The technique contrasts with StateBasedVerification.

Message-based verification is most useful when the exact sequence and number of the messages is important. For example, when testing a UnitOfCode that sends emails, the number of emails sent is important.

Message-based verification is usually implemented as a layer atop StateBasedVerification techniques. This is because most programming languages don't let code directly observe or reflect on messages being passed. Instead, the messages alter the internal state of a Spy. The test can then get the messages from the spy and make assertions about them.

Mocks are one way to implement "pure" message-based verification, which doesn't rely on inspecting objects' state: the mocks are pre-programmed with an expected sequence of messages so they can perform the assertions themselves. I consider this approach silly, because it imbues the mocks with knowledge of how to make assertions, duplicating behavior of the test framework (see OrthogonalFeatures). Use Spies instead.

On Other Wikis

MartinFowler calls this "behavior verification": https://martinfowler.com/articles/mocksArentStubs.html#ChoosingBetweenTheDifferences