2. Add a to_squawk method to String

To update a core class you will have to:

  • Write tests for the desired functionality.

  • Create a file for the code you wish to use.

  • Require that file from your init.rb.

Most plugins store their code classes in the plugin's lib directory. When you add a file to the lib directory, you must also require that file from init.rb. The file you are going to add for this tutorial is lib/core_ext.rb.

First, you need to write the tests. Testing plugins is very similar to testing rails apps. The generated test file should look something like this:

# File: vendor/plugins/yaffle/test/core_ext_test.rb

require 'test/unit'

class CoreExtTest < Test::Unit::TestCase
  # Replace this with your real tests.
  def test_this_plugin
    flunk
  end
end

Start off by removing the default test, and adding a require statement for your test helper.

# File: vendor/plugins/yaffle/test/core_ext_test.rb

require 'test/unit'
require File.dirname(__FILE__) + '/test_helper.rb'

class CoreExtTest < Test::Unit::TestCase
end

Navigate to your plugin directory and run rake test:

cd vendor/plugins/yaffle
rake test

Your test should fail with no such file to load — ./test/../lib/core_ext.rb (LoadError) because we haven't created any file yet. Create the file lib/core_ext.rb and re-run the tests. You should see a different error message:

1.) Failure ...
No tests were specified

Great - now you are ready to start development. The first thing we'll do is to add a method to String called to_squawk which will prefix the string with the word “squawk!”. The test will look something like this:

# File: vendor/plugins/yaffle/init.rb

class CoreExtTest < Test::Unit::TestCase
  def test_string_should_respond_to_squawk
    assert_equal true, "".respond_to?(:to_squawk)
  end

  def test_string_prepend_empty_strings_with_the_word_squawk
    assert_equal "squawk!", "".to_squawk
  end

  def test_string_prepend_non_empty_strings_with_the_word_squawk
    assert_equal "squawk! Hello World", "Hello World".to_squawk
  end
end

# File: vendor/plugins/yaffle/init.rb

require "core_ext"

# File: vendor/plugins/yaffle/lib/core_ext.rb

String.class_eval do
  def to_squawk
    "squawk! #{self}".strip
  end
end

When monkey-patching existing classes it's often better to use class_eval instead of opening the class directly.

To test that your method does what it says it does, run the unit tests. To test this manually, fire up a console and start squawking:

$ ./script/console
>> "Hello World".to_squawk
=> "squawk! Hello World"

If that worked, congratulations! You just created your first test-driven plugin that extends a core ruby class.