Fixtures.adoc

Fixtures

It is often the case many tests will share very similar implementations. For instance, suppose an object that handles an event is being tested. In the same program, there might also be other objects that handle other events, but the machinery behind how those events are emitted and passed to handlers might be shared by all event handlers. This may lead to excessive duplication between tests. As an example, suppose there are many tests that structurally resemble this example:

context "An event is handled" do
  handler = SomeHandler.new queue
  event = SomeEvent.new 'some-data'

  queue = EventQueue.new
  queue.attach_handler handler
  queue << event

  test "Another event is published" do
    assert queue do
      published? SomeOtherEvent
    end
  end
end

Test Bench offers a module, TestBench::Fixture, which allows custom objects to make use of Test Bench methods: assert, context, refute, and test. A fixture to suit the above example could look like this:

class Fixtures::Queue::HandleEvent
  include TestBench::Fixture

  def self.call *arguments, &block
    instance = new
    instance.call *arguments, &block
  end

  def call handler, event, &block
    queue = EventQueue.new
    queue.attach_handler handler
    queue << event

    test "Handler receives a `#{event.class.name}'" do
      block.()
    end
  end
end

The test can be rewritten to use the fixture:

context "An event is handled" do
  handler = SomeHandler.new queue
  event = SomeEvent.new 'some-data'

  Fixtures::Queue::HandleEvent.call handler, event do |queue|
    assert queue do
      published? SomeOtherEvent
    end
  end
end

NOTE: Excessive use of fixtures may succeed in de-duplicating code at the expense of readability. It is recommended to employ fixtures only in cases where they not only shorten the tests that use them, but also increase their clarity.

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