dogbiscuit/mdub/weblog/Tech/Programming/Ruby/SpyingOnYourCodeWithRR

Spying on your code with RR

27 May, 2009

A while back, Melbourne's own Pete Yandell created Not A Mock, an extension to RSpec that supports test-spies. And a damn fine idea it was, too.

I've recently discovered that my current favourite stub/mock framework, Brian Takita's RR, can do test-spies too!

Huh?

What's this "spy" business about? Well, when mocking, before triggering the behaviour you're testing, you set up expectations that a certain methods of collaborating objects will be invoked, with the specified parameters. Like so:

describe TransferEverything do

  before do
    @account1 = Account.new
    @account2 = Account.new
    @transfer = TransferEverything.new(:from => @account1, :to => @account2)
  end

  describe "#execute" do

    it "moves all funds from one account to the other" do

      all_the_money = 1.42
      stub(@account1).balance { all_the_money }

      mock(@account1).withdraw(all_the_money)   # <= set expectations
      mock(@account2).deposit(all_the_money)

      @transfer.execute                         # <= execute
      
    end                                         # <= verify expectations

  end

end

The expectations are typically verified auto-magically, by the mocking framework, at the end of your test.

The spy alternative

Setting up expectations before a call always feels clumsy. Using a test spy makes tests flow more naturally:

1. Stub out collaborators, setting up canned responses where required.
2. Execute the code you're testing.
3. Verify the results, including both:
• the outputs (return values, or resulting state)
• the interactions (ie. the method-invocations you expected your fake collaborators to receive).

Fur egg-sample:

describe TransferEverything do

  # ...

  describe "#execute" do

    it "moves all funds from one account to the other" do

      all_the_money = 1.42
      stub(@account1).balance { all_the_money }
      stub(@account1).withdraw
      stub(@account2).deposit

      @transfer.execute

      @account1.should have_received.withdraw(all_the_money)
      @account2.should have_received.deposit(all_the_money)

    end

  end

end

One thing I find particularly useful about this technique is the ability to execute code in a setup block, then verify the various aspects of it's behaviour in separate test-cases.

describe TransferEverything do

  # ...

  describe "#execute" do

    before do
      @all_the_money = 1.42
      stub(@account1).balance { all_the_money }
      stub(@account1).withdraw
      stub(@account2).deposit
      @transfer.execute
    end

    it "withdraws all funds from source account" do
      @account1.should have_received.withdraw(all_the_money)
    end

    it "deposits funds in receiving account" do
      @account2.should have_received.deposit(all_the_money)
    end

  end

end

This results in smaller, more coherent test-cases.

Using RR test-spies in RSpec

If you're using RSpec, you'll need to use the adapter class that comes with RR, rather than the one that comes with RSpec. That is, in your spec_helper.rb, do this, which provides access to the have_received matcher.

require 'rr'
Spec::Runners.configure do |config|
  config.mock_with RR::Adapters::Rspec
end

Spying on Java

Honourable mention: if you're lucky (*cough*) enough to be coding Java, I HIGHLY recommended Mockito, which also implements test-spies, and is easily the best Java mocking/stubbing library around.

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