Stubbing

• sometimes a test needs to trick the code

  • to test all cases
  • for speed
  • for clarity

• to stub a method is to replace that method with a block during testing

Stubbing a Mock Clock

• normally, Time.now returns the current time

• in this test, we want to trick the code into thinking it's a different time

  midnight = Time.parse("12:00 am")
  Time.stub(:now) { midnight }
  

• the stub method temporarily replaces a method ("now") with a block ("{ midnight }")

• Before stubbing, Time.now returns the actual current time

• After stubbing, Time.now returns the fake time (midnight)

Stubbing a Time Machine

• since our test has access to the fake time, we can change it at will

• so we can pretend an hour passed in just an instant

  fake_time = Time.parse("12:00 pm")
  Time.stub(:now) { fake_time }
  do_something
  fake_time += 3600  # number of seconds in an hour
  do_something_else
  

• during do_something the code thinks it's noon, but during do_something_else the code thinks it's one o'clock

Test Doubles

• stubs are a form of test doubles, aka mock objects
• others include mocks, fakes, spies, saboteurs...

Mocking Advice

Never mock the object under test. - Alex

*

Never mock values, sometimes mock entities, but mock services freely. - J.B. Rainsberger

• see also http://www.confreaks.com/videos/659-rubyconf2011-why-you-don-t-get-mock-objects