Agile in a Flash 49

Field Guide to Mocks – The Code

Agile in a Flash by Jeff Langr and Tim Ottinger (card #49)

> Test double: Any object that emulates another

> Stub: Returns a fixed value to the SUT

> Fake: Emulates a production object

> Mock: Self-verifies

> Partial mock: Combines production and mock methods

> Spy: Records messages for later verification

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Test doubles are emulation objects you build to simplify testing. Surrounding the System Under Test (SUT) with test doubles allows you to exercise and observe it in isolation. Otherwise, accessing database, network, external hardware, or other subsystems can slow or break the test run and cause false positives. Stubs, fakes, mocks (partial or not), and spies are nuanced kinds of test doubles. This field guide, based on XUnit Test Patterns, will help you sort out standard terminology (used in many mock tools).

Test doubles have changed the art of test-driving by allowing tests to be smaller, simpler, and faster running. They can allow you to have a suite of thousands of tests that runs in a minute or less. Such rapid feedback gives you the confidence to dramatically change your code every few minutes. But take care; you should do the following:

> Learn to use test doubles, but employ them only when you need the isolation.

> Use a mock tool (instead of hand-coding them) if it improves test quality.

> Learn the various types of mocks summarized in this field guide.*12

> Read Card 51, Test DoubleTroubles, to avoid their many pitfalls.

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12. If only to avoid being embarrassed by your peers. Or just keep this card handy.

"The secret of life is honesty and fair dealing. IF you can fake that, you've got it made." Groucho Marx