A macro to allow object dependencies to be specified at the class level with a block defining a default value. Based on attr_defaultable.

dependencies.rb

module Dependencies
  # Ensure dependencies from the superclass are inherited by the subclass.
  def inherited(subclass)
    if superclass.respond_to?(:dependencies)
      subclass.dependencies.concat dependencies
    end

    super
  end

  # Stores a list of dependency attributes for use by the initializer.
  def dependencies
    @dependencies ||= []
  end

  # Defines a new dependency attribute with a default value.
  def dependency(attribute, default)
    # Define a getter that lazily sets a default value.
    define_method attribute do
      if instance_variable_defined?(:"@#{attribute}")
        instance_variable_get(:"@#{attribute}")
      else
        instance_variable_set(:"@#{attribute}", default.call)
      end
    end

    # Define a setter that lazily sets a default value.
    attr_writer attribute

    # Mark the getter and setter as protected.
    protected attribute, "#{attribute}="

    # Add the attribute to the list of dependencies.
    dependencies << attribute
  end
end

example.rb

require 'dependencies'

class Example
  extend Dependencies
  
  dependency :value, -> { :default }
end

class ExtendedExample < Example
  dependency :another_value, -> { :another_default }
end

example = ExtendedExample.new
example.send(:value) # => :default
example.send(:another_value) # => :another_default