Presentation: Mock Roles Not Object States
In this presentation filmed during QCon London 2007, Nat Pryce and Steve Freeman talk about TDD using Mock Objects. In their opinion, Mock Objects improves the software design and makes the code more easier to maintain and adapt to changing requirements.
There are times when the tests smell. They are complex, long, or hard to maintain. That is a good sign of a design flaw which needs to be addressed. Nat and Steve have come with 6 test smells from their own experience.
Smell #1: the test exposes the internal implementation details of the object tested. It's clear that object encapsulation is broken.
Smell #2: passing too much information to the object tested with constructors having too many parameters. The object tested has too many dependencies, and it may be time to break it up in several smaller objects.
Smell #3: mock everything. One wants to mock dependencies and peers, but not value objects.
Smell #4: the tests just replicate production code. Code should not be replicated.
Smell #5: one needs to mock an object he cannot replace. External objects can be used, but cannot be mocked.
Smell #6: one needs to mock a concrete class. The tests are meant to verify relationships between objects, not to test a specific instance of an object.
A good rule of thumb is that one should try to mock object roles not object states, according to Nat and Steve. The idea is to test interactions between objects, not the objects themselves.
The entire presentation is about 1 hour long.