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== Extending core classes ==
This section will explain how to add a method to String that will be available anywhere in your rails app by:
* Writing tests for the desired behavior
* Creating and requiring the correct files
=== Creating the test ===
In this example you will add a method to String named `to_squawk`. To begin, create a new test file with a few assertions:
*vendor/plugins/yaffle/test/core_ext_test.rb*
[source, ruby]
--------------------------------------------------------
require File.dirname(__FILE__) + '/test_helper.rb'
class CoreExtTest < Test::Unit::TestCase
def test_to_squawk_prepends_the_word_squawk
assert_equal "squawk! Hello World", "Hello World".to_squawk
end
end
--------------------------------------------------------
Navigate to your plugin directory and run `rake test`:
--------------------------------------------------------
cd vendor/plugins/yaffle
rake test
--------------------------------------------------------
The test above should fail with the message:
--------------------------------------------------------
1) Error:
test_to_squawk_prepends_the_word_squawk(CoreExtTest):
NoMethodError: undefined method `to_squawk' for "Hello World":String
./test/core_ext_test.rb:5:in `test_to_squawk_prepends_the_word_squawk'
--------------------------------------------------------
Great - now you are ready to start development.
=== Organize your files ===
A common pattern in rails plugins is to set up the file structure like this:
--------------------------------------------------------
|-- lib
| |-- yaffle
| | `-- core_ext.rb
| `-- yaffle.rb
--------------------------------------------------------
The first thing we need to to is to require our 'lib/yaffle.rb' file from 'rails/init.rb':
*vendor/plugins/yaffle/rails/init.rb*
[source, ruby]
--------------------------------------------------------
require 'yaffle'
--------------------------------------------------------
Then in 'lib/yaffle.rb' require 'lib/core_ext.rb':
*vendor/plugins/yaffle/lib/yaffle.rb*
[source, ruby]
--------------------------------------------------------
require "yaffle/core_ext"
--------------------------------------------------------
Finally, create the 'core_ext.rb' file and add the 'to_squawk' method:
*vendor/plugins/yaffle/lib/yaffle/core_ext.rb*
[source, ruby]
--------------------------------------------------------
String.class_eval do
def to_squawk
"squawk! #{self}".strip
end
end
--------------------------------------------------------
To test that your method does what it says it does, run the unit tests with `rake` from your plugin directory. To see this in action, fire up a console and start squawking:
--------------------------------------------------------
$ ./script/console
>> "Hello World".to_squawk
=> "squawk! Hello World"
--------------------------------------------------------
=== Working with init.rb ===
When rails loads plugins it looks for the file named init.rb. However, when the plugin is initialized, 'init.rb' is invoked via `eval` (not `require`) so it has slightly different behavior.
Under certain circumstances if you reopen classes or modules in 'init.rb' you may inadvertently create a new class, rather than reopening an existing class. A better alternative is to reopen the class in a different file, and require that file from `init.rb`, as shown above.
If you must reopen a class in `init.rb` you can use `module_eval` or `class_eval` to avoid any issues:
*vendor/plugins/yaffle/init.rb*
[source, ruby]
---------------------------------------------------
Hash.class_eval do
def is_a_special_hash?
true
end
end
---------------------------------------------------
Another way is to explicitly define the top-level module space for all modules and classes, like `::Hash`:
*vendor/plugins/yaffle/init.rb*
[source, ruby]
---------------------------------------------------
class ::Hash
def is_a_special_hash?
true
end
end
---------------------------------------------------
|
