== 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:

[source, ruby]
--------------------------------------------------------
# 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.

[source, ruby]
--------------------------------------------------------
# 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:

[source, ruby]
--------------------------------------------------------
# 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
--------------------------------------------------------

[source, ruby]
--------------------------------------------------------
# File: vendor/plugins/yaffle/init.rb

require "core_ext"
--------------------------------------------------------

[source, ruby]
--------------------------------------------------------
# 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.