Method: Module#mattr_reader

Defined in:
activesupport/lib/active_support/core_ext/module/attribute_accessors.rb

#mattr_reader(*syms, instance_reader: true, instance_accessor: true, default: nil, location: nil) ⇒ Object Also known as: cattr_reader

Defines a class attribute and creates a class and instance reader methods. The underlying class variable is set to nil, if it is not previously defined. All class and instance methods created will be public, even if this method is called with a private or protected access modifier.

module HairColors
  mattr_reader :hair_colors
end

HairColors.hair_colors # => nil
HairColors.class_variable_set("@@hair_colors", [:brown, :black])
HairColors.hair_colors # => [:brown, :black]

The attribute name must be a valid method name in Ruby.

module Foo
  mattr_reader :"1_Badname"
end
# => NameError: invalid attribute name: 1_Badname

To omit the instance reader method, pass instance_reader: false or instance_accessor: false.

module HairColors
  mattr_reader :hair_colors, instance_reader: false
end

class Person
  include HairColors
end

Person.new.hair_colors # => NoMethodError

You can set a default value for the attribute.

module HairColors
  mattr_reader :hair_colors, default: [:brown, :black, :blonde, :red]
  mattr_reader(:hair_styles) { [:long, :short] }
end

class Person
  include HairColors
end

Person.new.hair_colors # => [:brown, :black, :blonde, :red]
Person.new.hair_styles # => [:long, :short]

Raises:

  • (TypeError) 


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# File 'activesupport/lib/active_support/core_ext/module/attribute_accessors.rb', line 55

def mattr_reader(*syms, instance_reader: true, instance_accessor: true, default: nil, location: nil)
  raise TypeError, "module attributes should be defined directly on class, not singleton" if singleton_class?
  location ||= caller_locations(1, 1).first

  definition = []
  syms.each do |sym|
    raise NameError.new("invalid attribute name: #{sym}") unless /\A[_A-Za-z]\w*\z/.match?(sym)

    definition << "def self.#{sym}; @@#{sym}; end"

    if instance_reader && instance_accessor
      definition << "def #{sym}; @@#{sym}; end"
    end

    sym_default_value = (block_given? && default.nil?) ? yield : default
    class_variable_set("@@#{sym}", sym_default_value) unless sym_default_value.nil? && class_variable_defined?("@@#{sym}")
  end

  module_eval(definition.join(";"), location.path, location.lineno)
end