How to use mock objects

I’ve been really enjoying test driven development lately. It’s more fun, more relaxing, and I think more productive than the classic “just go for it” or “try to plan everything out ahead” styles of development.

For those who don’t know, “test driven development” (TDD) is a style of building stuff. Typically when you want to build something like software, you start off by planning what you’re going to do, or you just start building. TDD is different. You start off by creating an automatic test that tells you whether you succeeded or not. So if I wanted to write some code that added apples to the bin, I would start of writing something like this:

test "when you add apples to the bin, there should be more apples in the bin" do
  start_number = count_apples_in(the_bin)
  add_apples_to(the_bin)
  end_number = count_apples_in(the_bin)

  end_number.should_be_bigger_than(start_number)
end

Note that we haven’t actually written code that adds apples to the bin. What we did was write some code that can tell us if we have something that works. If we run the test, of course the test will fail. But now we have a goal, and we have a good feedback loop. So we write a little code, and see if the test passes. Write a little more, see if it passes. As soon as the test passes, a little green light lights up, and we’re like “yay! our code works!”

The benefits of this are many. First, it’s emotionally gratifying. It makes coding more fun because getting another test to run is never that far away. And if it is far away, you just write another test that’s halfway there. Second, it lets you write really big software and have some confidence that it all works OK together. Because as you write code you build up more and more tests, and you keep running all those old tests, you have a continuous sense of what is working.

Anyway, that was a bit of a digression. What I really wanted to write about is “mock objects”. Once you’ve written a few tests you start to realize that often you want to test a little bit of code that relies on other objects, but you don’t want to test those other objects together with this one. So, if A depends on B, which depends on C, which depends on D, you want to have four separate tests that test each of those things independently.

But how do you test them independently if they are dependent?

The answer is: mock objects. If you’re testing B, you create a “mock” of C, which is just a fake object that does what C is suppose to do in your particular test case. That way your test isn’t dependent on C working properly.

Here’s an example. I want my Mailer object to get set up properly. But the Mailer depends on the GitoriousConfig object, and I don’t want to test how that is getting set up. So really I just want GitoriousConfig to act right for the Mailer. So I use a mock:

  it "uses smtp server if config says so" do
    GitoriousConfig.should_receive(:has_key?).with('smtp_settings').and_return(true)
    GitoriousConfig.should_receive(:[]).with('smtp_settings').and_return({
      :address  => "smtp.postoffice.net",
    })
      
    Kernel.load("config/initializers/mailer.rb")   
    Mailer.smtp_settings[:address].should == "smtp.postoffice.net"
  end

What this code says is that GitoriousConfig should act as if it has a smtp_settings hash in it, and that the address entry points at smtp.postoffice.net. A depends on B, but I’m using a fake version of B so I can isolate the test to A’s behavior.

That allows me to keep my tests nice and clean and separate. Cool.

Tags: mocks, rspec, tdd