More on Mocks: Passing in Input

In a couple of previous posts, I’ve been documenting my foray into the world of mocks. So far, I’ve been using mocks to verify that outgoing messages get sent without incurring unwanted side effects. To do so, I’ve overridden the methods from my real objects so that their mock versions return a unique identifier. For example, my mockIO looked like this:

class MockIO
  def print(message)
    "print was called"
  end

  def gets
    "gets was called"
  end
end

I find this formatting helpful because it does what I want and not more - I test that print and gets are called, but I don’t get things printing to the screen and/or prompting for input (as I would with the real versions). As a bonus, the unique identifiers assure me that I’m calling the right methods; I don’t have to worry about the possibility that my tests are just confirming that a method was called (i.e. if I set all the methods in my mock to return a boolean, I could get a true returned even if I called the wrong method).

Recently, however, I’ve run into a problem: some of the logic of my program relies on taking and using user input. In order to test that, I want to make sure that those methods continue prompting for user input until the get an appropriate response. For example:

def get_input
  response = ''
  until response.to_i >= 0 && response.to_i <= 4
    response = io.gets
  end
end

Putting aside the numerous refactorings that could make this code better (I’ve put in several responsibilities to clarify the context), the irreducible nature of my problem is that my standard mock is going to break the test. get_input keeps waiting and waiting for an integer between 0 and 4, but my mock gets only ever returns “gets was called”.

So, of course, I could separate out the code following the until statement and test that, but I’m still going to want some way of verifying that the loop keeps going until it receives valid input (and I’m also going to want that test to eventually provide valid input, so that the test doesn’t keep waiting indefinitely).

One way I’ve come up with to do that is to pass in input to my mock. Now the mockIO can look like this:

class MockIO
  def initialize(input)
    @input = input ||= ["gets was called"]
  end

  def print(message)
    "print was called"
  end

  def gets
    @input.shift
  end
end

Now, when I initialize an instance of my main class in the tests, I can inject in an instance of my mockIO with whatever input I want to execute. For example, if I pass in ['-7', '13', '2'] to my mockIO and run tests on get_input, I can expect that method to return 2.