Rails 3 Engine - Provide Config For Users

Question

I can't seem to find any documentation on how to do the following: I need to provide a config variable for any applications using my engine so that they can easily pass settings to my engine.

Does anyone have any links to a proper or accepted way to do this?

EDIT: As an update, I figured out a decent way to do this. Code is below.

# file: lib/my_engine.rb
module MyEngine

  class Engine < Rails::Engine

    initializer "my_engine.configure_rails_initialization" do |app|
      # Engine configures Rails app here, this is not what my question was about
    end

  end

  # This is what I was trying to figure out
  def self.config(&block)
    @@config ||= MyEngine::Configuration.new

    yield @@config if block

    return @@config
  end

end

This allows any application using my engine to configure it like below in any of their initializers or their environment.rb file, calling any methods defined in the MyEngine::Configuration class:

MyEngine.config do |config|
  config.some_configuration_option = "Whatever"
end

Answer 1

Short answer:

Create a file called your_engine_class.rb and put in config/initializers of your hosting app. Here is an example of this file.

module YourEngineClass
  class Engine < Rails::Engine
    config.param_name = 'value'    
  end
end

Then inside your main engine.rb you can access:

config.param_name

Longer answer: I created a stub engine that includes this and lots of other standard configuration you'll need, you can use it as a starting point:

http://keithschacht.com/creating-a-rails-3-engine-plugin-gem/

Answer 2

I personally like the pattern Devise uses, which defines a class method in your engine's module and then an initializer in the parent application.

Create a class method to call from the parent application

module YourEngine
  class << self
    attr_accessor :your_config_var
  end

  def self.setup(&block)
    # You can yield your own object encapsulating your configuration logic/state
    yield self
  end
end

Call the class method in an initializer in the parent application

YourEngine.setup do |config|
  config.your_config_var = "nyan cat"
end

Now you are free to access your_config_var in your engine's initialization process. You can create a file in your engine's config/initializers directory or use the Rails Initializer hooks.

I like this approach since you have more control on what configuration object you expose to clients and aren't overriding the engine's config method.

Answer 3

Seems better to me as follow :

#lib/my_engine.rb
require 'rails'

module MyEngine
  class Engine < Rails::Engine

  end

  def self.config(&block)
    yield Engine.config if block
    Engine.config
  end
end