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https://github.com/gsmendoza/verified_double

Contract tests for rspec. Featured in http://rubyweekly.com/issues/151.
https://github.com/gsmendoza/verified_double

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Contract tests for rspec. Featured in http://rubyweekly.com/issues/151.

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# VerifiedDouble



[![Build Status](https://travis-ci.org/gsmendoza/verified_double.png)](https://travis-ci.org/gsmendoza/verified_double)



VerifiedDouble is a gem for verifying rspec mocks. The gem works similar to [rspec-fire](https://github.com/xaviershay/rspec-fire). However, instead of checking if a doubled class responds to the methods, the gem looks for tests confirming the signatures of the mocks.



For example, if I mock the created_at method of a model like this:



```ruby

# spec/foo_spec.rb

item = VerifiedDouble.of_instance(Item)

expect(item).to receive(:created_at).and_return(Time.now)

```



When I run the tests, the gem will look for a "contract test" tagged with the method's signature. This test should ensure that calling `#created_at` on Item will return a Time object.



```ruby

# spec/item_spec.rb

describe Item do

  describe '#created_at()=>Time', verifies_contract: true do

    it "tests something" do

      #...

    end

  end

end

```



If this test does not exist, the gem will complain that the mock is not verified.



I got the idea from http://www.infoq.com/presentations/integration-tests-scam, an old (2009) talk that still has some fresh insights on dealing with API changes in your mocked tests.



## Setup



Require `verified_double/rspec_configuration` in your spec_helper.rb to integrate VerifiedDouble with rspec. If you want verified_double to be run only as needed, create a separate verified_spec_helper.rb:



```

# spec/verified_spec_helper.rb

require 'spec_helper'

require 'verified_double/rspec_configuration'

```



And require it when running rspec:



    rspec -r ./spec/verified_spec_helper.rb



## Usage



Let's look again at the example above:



```ruby

# spec/foo_spec.rb

item = VerifiedDouble.of_instance(Item)

expect(item).to receive(:created_at).and_return(Time.now)

```



When you run this test, you'll see a warning saying that there's no test verifying the mock `Item#created_at()=>Time`:



```

The following mocks are not verified:



1. Item#created_at()=>Time

...



```



You can then tag the test for `Item#created_at()=>Time` with the method signature:



```ruby

# spec/item_spec.rb

describe Item do

  describe '#created_at()=>Time', verifies_contract: true do

    it "tests something" do

      #...

    end

  end

end

```



Take note that:



1. The described class must be a class, not a string.

2. The described method must start with `#` if its an instance method and `.` if it's a class method.

3. You need to add the `verifies_contract: true` tag to the test.



If your testing style doesn't follow these conventions, you can tag the test with the whole method signature:



```ruby

# spec/item_spec.rb

describe 'Item' do

  it "has a creation timestamp", verifies_contract: `Item#created_at()=>Time` do

    #...

  end

end

```



Since VerifiedDouble relies on tags to link mocks and contracts together, you'll

need to run the tests containing the contracts along with tests with the mocks in

order to clear the VerifiedDouble warnings.



```

rspec spec/foo_spec.rb spec/item_spec.rb

```



## Booleans



Since Ruby doesn't have Boolean class covering both `TrueClass` and `FalseClass`,

VerifiedDouble supplies its own `VerifiedDouble::Boolean` class. Thus if your method

accepts or returns a boolean value, you'll see `VerifiedDouble::Boolean` in its

method signature e.g. `Student.late?()=>VerifiedDouble::Boolean`.



## Helpers



Most of the time, you'll be mocking accessor methods. VerifiedDouble provides some

helpers to make them easy to verify.



For reader methods, there's the `verify_reader_contract` matcher. The matcher checks

if the method will return an object matching the class specified by the contract:



```ruby

class Collaborator

  attr_reader :value



  def initialize(value)

    @value = value

  end

end



class SomeValue

end



describe Collaborator do

  subject { described_class.new(SomeValue.new) }

  it { should verify_reader_contract('Collaborator#value=>SomeValue') }

end

```



For accessor methods, there's the `verify_accessor_contract` matcher. The matcher

instantiates the return class and tries to pass and retrieve the value

to the method:



```ruby

class Collaborator

  attr_accessor :value

end



describe Collaborator do

  it { should verify_accessor_contract('Collaborator#value=>SomeValue') }

end

```



## Partial and Any Instance mocks



Aside for instance and class doubles, VerifiedDouble also supports partial and "any instance" mocks:



```

object_under_test = ObjectUnderTest.new



expect(VerifiedDouble.wrap(object_under_test))

  .to receive(:some_method).with(input).and_return(output)



expect(VerifiedDouble.wrap(ObjectUnderTest))

  .to receive(:some_method).with(input).and_return(output)

```



```

VerifiedDouble.expect_any_instance_of(Collaborator)

  .to receive(:some_method).with(input).and_return(output)



VerifiedDouble.allow_any_instance_of(Collaborator)

  .to receive(:some_method).with(input).and_return(output)

```



## Complete documentation



You can learn more about using the gem at https://www.relishapp.com/gsmendoza/verified-double.



## Actively tested against



* Ruby 2.0

* RSpec 2.14



## Alternatives



[Bogus](https://www.relishapp.com/bogus/bogus/v/0-0-3/docs/) is the first gem I know to implement contract tests. It doesn't rely on rspec tags to verify contracts, so it's probably a lot smarter than VerifiedDouble :) However, I wasn't able to try it out on my own projects because of its own rr-like mock adapter. But do check it out!



## Special thanks



* [Inner Core Designs](http://icdesign.com.au)

* [Love With Food](https://lovewithfood.com)