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470 lines
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470 lines
14 KiB
Text
h2. The Basics of Creating Rails Plugins
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A Rails plugin is either an extension or a modification of the core framework. Plugins provide:
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* a way for developers to share bleeding-edge ideas without hurting the stable code base
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* a segmented architecture so that units of code can be fixed or updated on their own release schedule
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* an outlet for the core developers so that they don’t have to include every cool new feature under the sun
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After reading this guide you should be familiar with:
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* Creating a plugin from scratch
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* Writing and running tests for the plugin
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This guide describes how to build a test-driven plugin that will:
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* Extend core ruby classes like Hash and String
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* Add methods to ActiveRecord::Base in the tradition of the 'acts_as' plugins
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* Give you information about where to put generators in your plugin.
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For the purpose of this guide pretend for a moment that you are an avid bird watcher.
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Your favorite bird is the Yaffle, and you want to create a plugin that allows other developers to share in the Yaffle
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goodness.
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endprologue.
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h3. Setup
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Before you continue, take a moment to decide if your new plugin will be potentially shared across different Rails applications.
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* If your plugin is specific to your application, your new plugin will be a _vendored plugin_.
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* If you think your plugin may be used across applications, build it as a _gemified plugin_.
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h4. Either generate a vendored plugin...
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Use the +rails generate plugin+ command in your Rails root directory
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to create a new plugin that will live in the +vendor/plugins+
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directory. See usage and options by asking for help:
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<shell>
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$ rails generate plugin new --help
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</shell>
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h4. Or generate a gemified plugin.
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Writing your Rails plugin as a gem, rather than as a vendored plugin,
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lets you share your plugin across different rails applications using
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RubyGems and Bundler.
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Rails 3.1 ships with a +rails plugin new+ command which creates a
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skeleton for developing any kind of Rails extension with the ability
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to run integration tests using a dummy Rails application. See usage
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and options by asking for help:
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<shell>
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$ rails plugin --help
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</shell>
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h3. Testing your newly generated plugin
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You can navigate to the directory that contains the plugin, run the +bundle install+ command
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and run the one generated test using the +rake+ command.
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You should see:
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<shell>
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2 tests, 2 assertions, 0 failures, 0 errors, 0 skips
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</shell>
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This will tell you that everything got generated properly and you are ready to start adding functionality.
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h3. Extending Core Classes
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This section will explain how to add a method to String that will be available anywhere in your rails application.
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In this example you will add a method to String named +to_squawk+. To begin, create a new test file with a few assertions:
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<ruby>
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# yaffle/test/core_ext_test.rb
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require 'test_helper'
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class CoreExtTest < Test::Unit::TestCase
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def test_to_squawk_prepends_the_word_squawk
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assert_equal "squawk! Hello World", "Hello World".to_squawk
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end
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end
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</ruby>
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Run +rake+ to run the test. This test should fail because we haven't implemented the +to_squak+ method:
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<shell>
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1) Error:
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test_to_squawk_prepends_the_word_squawk(CoreExtTest):
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NoMethodError: undefined method `to_squawk' for "Hello World":String
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test/core_ext_test.rb:5:in `test_to_squawk_prepends_the_word_squawk'
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</shell>
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Great - now you are ready to start development.
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Then in +lib/yaffle.rb+ require +lib/core_ext+:
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<ruby>
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# yaffle/lib/yaffle.rb
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require "yaffle/core_ext"
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module Yaffle
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end
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</ruby>
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Finally, create the +core_ext.rb+ file and add the +to_squawk+ method:
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<ruby>
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# yaffle/lib/yaffle/core_ext.rb
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String.class_eval do
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def to_squawk
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"squawk! #{self}".strip
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end
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end
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</ruby>
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To test that your method does what it says it does, run the unit tests with +rake+ from your plugin directory.
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<shell>
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3 tests, 3 assertions, 0 failures, 0 errors, 0 skips
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</shell>
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To see this in action, change to the test/dummy directory, fire up a console and start squawking:
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<shell>
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$ rails console
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>> "Hello World".to_squawk
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=> "squawk! Hello World"
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</shell>
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h3. Add an "acts_as" Method to Active Record
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A common pattern in plugins is to add a method called 'acts_as_something' to models. In this case, you
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want to write a method called 'acts_as_yaffle' that adds a 'squawk' method to your Active Record models.
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To begin, set up your files so that you have:
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<ruby>
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# yaffle/test/acts_as_yaffle_test.rb
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require 'test_helper'
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class ActsAsYaffleTest < Test::Unit::TestCase
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end
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</ruby>
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<ruby>
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# yaffle/lib/yaffle.rb
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require "yaffle/core_ext"
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require 'yaffle/acts_as_yaffle'
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module Yaffle
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end
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</ruby>
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<ruby>
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# yaffle/lib/yaffle/acts_as_yaffle.rb
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module Yaffle
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module ActsAsYaffle
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# your code will go here
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end
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end
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</ruby>
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h4. Add a Class Method
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This plugin will expect that you've added a method to your model named 'last_squawk'. However, the
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plugin users might have already defined a method on their model named 'last_squawk' that they use
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for something else. This plugin will allow the name to be changed by adding a class method called 'yaffle_text_field'.
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To start out, write a failing test that shows the behavior you'd like:
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<ruby>
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# yaffle/test/acts_as_yaffle_test.rb
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require 'test_helper'
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class ActsAsYaffleTest < Test::Unit::TestCase
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def test_a_hickwalls_yaffle_text_field_should_be_last_squawk
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assert_equal "last_squawk", Hickwall.yaffle_text_field
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end
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def test_a_wickwalls_yaffle_text_field_should_be_last_tweet
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assert_equal "last_tweet", Wickwall.yaffle_text_field
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end
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end
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</ruby>
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When you run +rake+, you should see the following:
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<shell>
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1) Error:
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test_a_hickwalls_yaffle_text_field_should_be_last_squawk(ActsAsYaffleTest):
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NameError: uninitialized constant ActsAsYaffleTest::Hickwall
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test/acts_as_yaffle_test.rb:6:in `test_a_hickwalls_yaffle_text_field_should_be_last_squawk'
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2) Error:
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test_a_wickwalls_yaffle_text_field_should_be_last_tweet(ActsAsYaffleTest):
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NameError: uninitialized constant ActsAsYaffleTest::Wickwall
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test/acts_as_yaffle_test.rb:10:in `test_a_wickwalls_yaffle_text_field_should_be_last_tweet'
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5 tests, 3 assertions, 0 failures, 2 errors, 0 skips
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</shell>
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This tells us that we don't have the necessary models (Hickwall and Wickwall) that we are trying to test.
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We can easily generate these models in our "dummy" Rails application by running the following commands from the
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test/dummy directory:
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<shell>
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$ cd test/dummy
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$ rails generate model Hickwall last_squak:string
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$ rails generate model Wickwall last_squak:string last_tweet:string
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</shell>
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Now you can create the necessary database tables in your testing database by navigating to your dummy app
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and migrating the database. First
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<shell>
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$ cd test/dummy
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$ rake db:migrate
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$ rake db:test:prepare
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</shell>
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While you are here, change the Hickwall and Wickwall models so that they know that they are supposed to act
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like yaffles.
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<ruby>
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# test/dummy/app/models/hickwall.rb
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class Hickwall < ActiveRecord::Base
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acts_as_yaffle
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end
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# test/dummy/app/models/wickwall.rb
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class Wickwall < ActiveRecord::Base
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acts_as_yaffle :yaffle_text_field => :last_tweet
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end
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</ruby>
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We will also add code to define the acts_as_yaffle method.
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<ruby>
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# yaffle/lib/yaffle/acts_as_yaffle.rb
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module Yaffle
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module ActsAsYaffle
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extend ActiveSupport::Concern
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included do
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end
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module ClassMethods
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def acts_as_yaffle(options = {})
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# your code will go here
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end
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end
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end
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end
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ActiveRecord::Base.send :include, Yaffle::ActsAsYaffle
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</ruby>
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You can then return to the root directory (+cd ../..+) of your plugin and rerun the tests using +rake+.
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<shell>
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1) Error:
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test_a_hickwalls_yaffle_text_field_should_be_last_squawk(ActsAsYaffleTest):
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NoMethodError: undefined method `yaffle_text_field' for #<Class:0x000001016661b8>
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/Users/xxx/.rvm/gems/ruby-1.9.2-p136@xxx/gems/activerecord-3.0.3/lib/active_record/base.rb:1008:in `method_missing'
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test/acts_as_yaffle_test.rb:5:in `test_a_hickwalls_yaffle_text_field_should_be_last_squawk'
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2) Error:
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test_a_wickwalls_yaffle_text_field_should_be_last_tweet(ActsAsYaffleTest):
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NoMethodError: undefined method `yaffle_text_field' for #<Class:0x00000101653748>
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Users/xxx/.rvm/gems/ruby-1.9.2-p136@xxx/gems/activerecord-3.0.3/lib/active_record/base.rb:1008:in `method_missing'
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test/acts_as_yaffle_test.rb:9:in `test_a_wickwalls_yaffle_text_field_should_be_last_tweet'
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5 tests, 3 assertions, 0 failures, 2 errors, 0 skips
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</shell>
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Getting closer...now we will implement the code of the acts_as_yaffle method to make the tests pass.
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<ruby>
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# yaffle/lib/yaffle/acts_as_yaffle.rb
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module Yaffle
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module ActsAsYaffle
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extend ActiveSupport::Concern
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included do
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end
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module ClassMethods
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def acts_as_yaffle(options = {})
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cattr_accessor :yaffle_text_field
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self.yaffle_text_field = (options[:yaffle_text_field] || :last_squawk).to_s
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end
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end
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end
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end
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ActiveRecord::Base.send :include, Yaffle::ActsAsYaffle
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</ruby>
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When you run +rake+ you should see the tests all pass:
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<shell>
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5 tests, 5 assertions, 0 failures, 0 errors, 0 skips
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</shell>
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h4. Add an Instance Method
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This plugin will add a method named 'squawk' to any Active Record objects that call 'acts_as_yaffle'. The 'squawk'
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method will simply set the value of one of the fields in the database.
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To start out, write a failing test that shows the behavior you'd like:
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<ruby>
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# yaffle/test/acts_as_yaffle_test.rb
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require 'test_helper'
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class ActsAsYaffleTest < Test::Unit::TestCase
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def test_a_hickwalls_yaffle_text_field_should_be_last_squawk
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assert_equal "last_squawk", Hickwall.yaffle_text_field
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end
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def test_a_wickwalls_yaffle_text_field_should_be_last_tweet
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assert_equal "last_tweet", Wickwall.yaffle_text_field
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end
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def test_hickwalls_squawk_should_populate_last_squawk
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hickwall = Hickwall.new
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hickwall.squawk("Hello World")
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assert_equal "squawk! Hello World", hickwall.last_squawk
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end
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def test_wickwalls_squawk_should_populate_last_tweeted_at
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wickwall = Wickwall.new
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wickwall.squawk("Hello World")
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assert_equal "squawk! Hello World", wickwall.last_tweet
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end
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end
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</ruby>
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Run the test to make sure the last two tests fail the an error that contains "NoMethodError: undefined method `squawk'",
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then update 'acts_as_yaffle.rb' to look like this:
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<ruby>
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# yaffle/lib/yaffle/acts_as_yaffle.rb
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module Yaffle
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module ActsAsYaffle
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extend ActiveSupport::Concern
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included do
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end
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module ClassMethods
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def acts_as_yaffle(options = {})
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cattr_accessor :yaffle_text_field
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self.yaffle_text_field = (options[:yaffle_text_field] || :last_squawk).to_s
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end
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end
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def squawk(string)
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write_attribute(self.class.yaffle_text_field, string.to_squawk)
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end
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end
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end
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ActiveRecord::Base.send :include, Yaffle::ActsAsYaffle
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</ruby>
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Run +rake+ one final time and you should see:
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<shell>
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7 tests, 7 assertions, 0 failures, 0 errors, 0 skips
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</shell>
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NOTE: The use of +write_attribute+ to write to the field in model is just one example of how a plugin can interact with the model, and will not always be the right method to use. For example, you could also use <tt>send("#{self.class.yaffle_text_field}=", string.to_squawk)</tt>.
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h3. Generators
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Generators can be included in your gem simply by creating them in a lib/generators directory of your plugin. More information about
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the creation of generators can be found in the "Generators Guide":generators.html
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h3. Publishing your Gem
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Gem plugins in progress can be easily be shared from any Git repository. To share the Yaffle gem with others, simply
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commit the code to a Git repository (like Github) and add a line to the Gemfile of the any application:
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<ruby>
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gem 'yaffle', :git => 'git://github.com/yaffle_watcher/yaffle.git'
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</ruby>
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After running +bundle install+, your gem functionality will be available to the application.
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When the gem is ready to be shared as a formal release, it can be published to "RubyGems":http://www.rubygems.org.
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For more information about publishing gems to RubyGems, see: "http://blog.thepete.net/2010/11/creating-and-publishing-your-first-ruby.html":http://blog.thepete.net/2010/11/creating-and-publishing-your-first-ruby.html
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h3. Non-Gem Plugins
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Non-gem plugins are useful for functionality that won't be shared with another project. Keeping your custom functionality in the
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vendor/plugins directory un-clutters the rest of the application.
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Move the directory that you created for the gem based plugin into the vendor/plugins directory of a generated Rails application, create a vendor/plugins/yaffle/init.rb file that contains "require 'yaffle'" and everything will still work.
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<ruby>
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# yaffle/init.rb
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require 'yaffle'
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</ruby>
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You can test this by changing to the Rails application that you added the plugin to and starting a rails console. Once in the
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console we can check to see if the String has an instance method of to_squawk.
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<shell>
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$ cd my_app
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$ rails console
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$ String.instance_methods.sort
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</shell>
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You can also remove the .gemspec, Gemfile and Gemfile.lock files as they will no longer be needed.
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h3. RDoc Documentation
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Once your plugin is stable and you are ready to deploy do everyone else a favor and document it! Luckily, writing documentation for your plugin is easy.
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The first step is to update the README file with detailed information about how to use your plugin. A few key things to include are:
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* Your name
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* How to install
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* How to add the functionality to the app (several examples of common use cases)
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* Warning, gotchas or tips that might help save users time
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Once your README is solid, go through and add rdoc comments to all of the methods that developers will use. It's also customary to add '#:nodoc:' comments to those parts of the code that are not part of the public api.
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Once your comments are good to go, navigate to your plugin directory and run:
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<shell>
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$ rake rdoc
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</shell>
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h4. References
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* "Developing a RubyGem using Bundler":https://github.com/radar/guides/blob/master/gem-development.md
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* "Using Gemspecs As Intended":http://yehudakatz.com/2010/04/02/using-gemspecs-as-intended/
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* "Gemspec Reference":http://docs.rubygems.org/read/chapter/20
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* "GemPlugins":http://www.mbleigh.com/2008/06/11/gemplugins-a-brief-introduction-to-the-future-of-rails-plugins
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* "Keeping init.rb thin":http://daddy.platte.name/2007/05/rails-plugins-keep-initrb-thin.html
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h3. Changelog
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* March 10, 2011: Minor formatting tweaks.
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* February 13, 2011: Get guide in synch with Rails 3.0.3. Remove information not compatible with Rails 3. Send reader elsewhere
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for information that is covered elsewhere.
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* April 4, 2010: Fixed document to validate XHTML 1.0 Strict. "Jaime Iniesta":http://jaimeiniesta.com
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* November 17, 2008: Major revision by Jeff Dean
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