RSpec Magic Part 2: Verifying Doubles

Doubles in RSpec are great - and easy to use. Check this out:

my_double = double

You can give them name to identify them when they fail in some horrible way:

my_double = double "Bicycle"

And you can add some messages and return values in a hash afterwards (I’ll add the brackets for clarity):

my_double = double("Bicycle", {broken?: false, wheels: 2})

One of the problems with doubles in a dynamic language like Ruby is that you’ve got no guarantee that the class you’re mocking actually implements the method you’ve just stubbed. I was taught that the best way to keep an eye on this was to keep notes on what you were methods you need to implement when you make the class ‘for real’.

There is another way though - verifying doubles let you declare the mocks just as above, but with the added bonus that, if the object’s class has been created in the test suite you’re running, it will throw an error if that class doesn’t respond to any of the messages you’ve mocked. It looks like this:

my_double = instance_double "Bicycle", broken?: false, wheels: 2

Now if there’s no Bicycle class about the double acts as previously seen. But if the Bicycle class has been loaded into the test runner, and doesn’t implement either broken? or wheels, then you’ll get an exciting new error message that looks something like this:

Failure/Error: instance_double "Bicycle", broken?: false, wheels: 2
    Player does not implement: broken?

This behaviour could be incredibly useful (a great kick start to defining a new class) or a bit of an annoyance (do I really want RSpec moaning at me about all the methods I’ve not implemented in a class I’ve just started work on?).