Fixtures

A fixture is a pre-defined, reusable test abstraction. The objects under test are specified at runtime, rather than hard-coded.

To get a better understanding of fixtures, imagine a regular test file, but the object (or objects) under test can be swapped so that the same standardized test implementation can be used against multiple objects.

A fixture is just a plain old Ruby object that includes the TestBench API. A fixture has access to the same API that any TestBench test would. By including the TestBench::Fixture module into a Ruby object, the object acquires all of the methods available to a test script, including context, test, assert, refute, assert_raises, refute_raises, comment, and detail.

Fixture API

Callable Object

The only requirement placed on a fixture's API is that it implements Ruby's callable object protocol by implementing the call method, and that the signature of the method is parameterless.

Alternatively, if the call method is not implemented on a fixture, then a block must be given. When a block is given, the fixture object doesn't have to implement the callable object protocol. If call is implemented and a block is given, the block has precedence.

Fixture State and the Initializer

State is passed to a fixture object via the positional parameters of its initializer.

class SomeFixture
  include TestBench::Fixture

  def initialize(something, something_else)
    @something = something
    @something_else = something_else
  end

  def call
    # ...
  end
end

If the fixture class implements a method named build, that method will be used to construct an instance rather than the initializer, leaving the developer the freedom to either implement the initialize method or not, or implement it whatever is preferred.

Implementing Fixtures

The TestBench API is just Ruby methods. There's no special syntax to invoke TestBench API methods, or any special rules or setup.

In the simplest case, a test can be implemented directly in the call method.

class SomeFixture
  include TestBench::Fixture

  attr_accessor :something
  attr_accessor :something_else

  def initialize(something, something_else)
    @something = something
    @something_else = something_else
  end

  def call
    context "Some Context" do
      context "Something" do
        included = something_else.include?(something)

        test "Included in Something Else" do
          assert(included)
        end
      end

      context "Something Else" do
        twice_as_long = something_else.length == something.length * 2

        test "Twice as long as something" do
          assert(twice_as_long)
        end
      end
    end
  end
end

Object Extension

Objects can be dispositioned as fixtures at runtime by having a module named Fixture within their class's namespace. TestBench will extend both the Fixture module and TestBench::Fixture onto the object:

class SomeClass
  attr_accessor :some_attr

  module Fixture
    def assert_some_attr
      test "Some test" do
        assert(!!some_attr)
      end
    end
  end
end

Using Fixtures

In addition to TestBench's core API for writing tests, the API also provides the fixture method for activating fixtures in a test script.

context "Some Fixture" do
  something = 'some value'
  something_else = something * 2

  fixture(SomeFixture, something, something_else)
end

The fixture method runs the test code in the fixture as if its part of the currently running test, maintaining the continuity of the output and indenting.

Other Context
  Some Context
    Something
      Included in Something Else
    Something Else
      Twice as long as something

In addition to class fixtures, fixture accepts objects that include a module named Fixture within their class's namespace:

context "Some Object Fixture" do
  object = SomeClass.new

  fixture(object) do
    object.assert_some_attr
  end
end

API

fixture(fixture_class_or_object, *args, &block)

Parameters

When supplied a fixture class, the fixture method instantiates the fixture class and invokes its call method. The parameters sent to the fixture method are passed along to the fixture's initializer.

When supplied a fixture object, the fixture method extends TestBench::Fixture onto the object as well as the object's Fixture module. If no Fixture module can be resolved, an error is raised.

The fixture method also shares the running test script's output object with the fixture in order to preserve output continuity.

Additional Block

When the generalization provided by a fixture is not enough, the fixture can be specialized for a particular circumstance by supplying an additional block of test code.

When a block is given, the fixture object doesn't have to implement the callable object protocol.

context "Other Context" do
  something = 'some value'
  something_else = 'some value' * 2

  fixture(SomeFixture, something, something_else) do |f|
    context "Something Else" do
      context "First Character" do
        first_character = f.something_else[0]
        last_character = f.something_else[-1]

        same = first_character == last_character

        test "Not the same as the last character" do
          refute(same)
        end
      end
    end
  end
end

Other Context
  Some Context
    Something
      Included in Something Else
    Something Else
      Twice as long as something
  Something Else
    First Character
      Not the same as the last character

Exercising Fixtures

Because fixtures are just Ruby objects, they can be instantiated and exercised just like any other Ruby object.

something = 'some value'
something_else = 'some value'

some_fixture = SomeFixture.new(something, something_else)

some_fixture.()

puts TestBench::Fixture.output(some_fixture)

Some Context
  Something
    Included in Something Else
  Something Else
    Twice as long as something
      Assertion failed

Failure: 1

Testing Fixtures

If a fixture is sufficiently elaborate, it can even be tested like a plain old object.

context "SomeFixture" do
  something = 'some value'
  something_else = 'some value'

  some_fixture = SomeFixture.new(something, something_else)

  some_fixture.()

  context "Included in Something Else" do
    passed = some_fixture.test_session.test_passed?('Included in Something Else')

    test "Passed" do
      assert(passed)
    end
  end

  context "Twice as long as something" do
    failed = some_fixture.test_session.test_failed?('Twice as long as something')

    test "Failed" do
      assert(failed)
    end
  end
end

SomeFixture
  Included in Something Else
    Passed
  Twice as long as something
    Failed

Distributing Fixtures with a Library

Libraries can provide a set of fixtures that users of the library can leverage to test code that uses the library.

A library that ships a set of fixtures doesn't have to take a dependency on the entirety of TestBench. The TestBench::Fixture namespace is its own self-contained package. That package is safe to be packaged with production code, whereas TestBench, being a test framework, should not be distributed with production code.