June 12th, 2008Using Mocks to Drive Design
A recent post on InfoQ made me think about how I use mocks during development. I used to be a classicist tester and thought mocks were only useful to fake external services (usually slow or hard to setup). After learning more about interaction-based testing and BDD, I now see some good advantages of using mocks during development:
Behaviour-Driven Development is more than a variation of TDD. I like Dan North‘s explanation of BDD as an “outside-in” approach to development. We start at the Story level, defining the acceptance criteria as scenarios which create an agreement between the team and the customer on the meaning of DONE. From the outside view, we start digging into the domain model, designing the objects and services needed to implement the story. I find mocks really useful in such scenario, because I can define the interface of the objects that still doesn’t exist, while focusing on the current layer of functionality. As a side effect, my tests become more isolated and when they fail I usually have a good indication of where the problem is located, requiring less debugging.
In this sense, I think the really important scenario for using mocks from Colin Mackay’s article is the last one: “when the real object does not yet exist”. I still mock someone else’s objects in the boundaries of the system, to assert that my code interacts properly with external libraries. But I see much more value in mocking my own objects, designing interactions and isolating behaviour. Which leads to my next point…
CRC with tests
CRC cards was one of the techniques that thought me what good Object-Oriented design looks like. It focus on designing behaviour-rich objects, defining its Responsibilities and Collaborators. In general, while state-based testing is great for defining Responsibilities, mocks give me the ability to precisely describe the Collaborators in my tests. Since I’m all in for using tests as executable documentation, mocks turned out to be a great technique to express my intent. Leading to my last point…
Mocking methods vs. mocking objects
My last point is not exactly an advantage of using mocks, but an example of how a framework can influence how you approach development. Coming from the Java world, I was used to encapsulate behaviour behind well defined interfaces. Now that I spend most of my time developing in Ruby, I not only have the ability of creating a mock/stub object, but also of mocking just one or two methods in a real object. These are usually called Partial Mocks.
Partial mocks allow me to express intent more precisely: instead of having to create a mock from an interface/class and record expectations in a proxy, I can specify the interactions in the real collaborators. It also makes the separation between state-based and interaction-based tests more loose, because I can selectively choose which methods will be mocked and which will be real. Finally, I can use tools such as Synthesis to verify that the interactions I’m mocking in one test are actually being verified in the collaborator’s test.
I know some people don’t feel comfortable using mocks and some even find it complex, but I think there’s great value on using it in the appropriate context. Instead of complaining about different framework’s idiosyncrasies and complexities, I think people should focus on its benefits as a design technique.