cucumber-core

Cucumber Core is the inner hexagon for the Ruby flavour of Cucumber.

It contains the core domain logic to execute Cucumber features. It has no user interface, just a Ruby API. If you're interested in how Cucumber works, or in building other tools that work with Gherkin documents, you've come to the right place.

An overview

The entry-point is a single method on the module Cucumber::Core called #execute. Here's what it does:

Parses the plain-text Gherkin documents into an AST
Compiles the AST down to test cases
Passes the activated test cases through any filters
Executes the test cases, calling back to the report

We've introduced a number of concepts here, so let's go through them in detail.

The AST

The Abstract Syntax Tree or AST is an object graph that represents the Gherkin documents you've passed into the core. Things like Feature, Scenario and ExamplesTable.

These are immutable value objects.

Test cases

Your gherkin might contain scenarios, as well as examples from tables beneath a scenario outline.

Test cases represent the general case of both of these. We compile the AST down to instances of Cucumber::Core::Test::Case, each containing a number of instances of Cucumber::Core::Test::Step. It's these that are then filtered and executed.

Test cases and their test steps are also immutable value objects.

Filters

Once we have the test cases, and they've been activated by the mappings, you may want to pass them through a filter or two. Filters can be used to do things like activate, sort, replace or remove some of the test cases or their steps before they're executed.

Report

A report is how you find out what is happening during your test run. As the test cases and steps are executed, messages are sent to the report.

A report needs to respond to the following methods:

before_test_case(test_case)
after_test_case(test_case, result)
before_test_step(test_step)
after_test_step(test_test, result)
done

That's probably best illustrated with an example.

Example

Here's an example of how you might use Cucumber::Core#execute

require 'cucumber/core'
require 'cucumber/core/filter'

class MyRunner
  include Cucumber::Core
end

class ActivateSteps < Cucumber::Core::Filter.new
  def test_case(test_case)
    test_steps = test_case.test_steps.map do |step|
      activate(step)
    end

    test_case.with_steps(test_steps).describe_to(receiver)
  end

  private
  def activate(step)
    case step.name
    when /fail/
      step.with_action { raise Failure }
    when /pass/
      step.with_action {}
    else
      step
    end
  end
end

class Report
  def before_test_step(test_step)
  end

  def after_test_step(test_step, result)
    puts "#{test_step.name} #{result}"
  end

  def before_test_case(test_case)
  end

  def after_test_case(test_case, result)
  end

  def done
  end
end

feature = Cucumber::Core::Gherkin::Document.new(__FILE__, <<-GHERKIN)
Feature:
  Scenario:
    Given passing
    And failing
    And undefined
GHERKIN

MyRunner.new.execute([feature], Report.new, [ActivateSteps.new])

If you run this little Ruby script, you should see the following output:

passing ✓
failing ✗
undefined ?