sonic-pi

Module#mattr_accessor

Defines both class and instance accessors for class attributes.

  module HairColors
    mattr_accessor :hair_colors
  end

  class Person
    include HairColors
  end

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

If a subclass changes the value then that would also change the value for
parent class. Similarly if parent class changes the value then that would
change the value of subclasses too.

  class Male < Person
  end

  Male.hair_colors << :blue
  Person.hair_colors # => [:brown, :black, :blonde, :red, :blue]

To opt out of the instance writer method, pass <tt>instance_writer: false</tt>.
To opt out of the instance reader method, pass <tt>instance_reader: false</tt>.

  module HairColors
    mattr_accessor :hair_colors, instance_writer: false, instance_reader: false
  end

  class Person
    include HairColors
  end

  Person.new.hair_colors = [:brown]  # => NoMethodError
  Person.new.hair_colors             # => NoMethodError

Or pass <tt>instance_accessor: false</tt>, to opt out both instance methods.

  module HairColors
    mattr_accessor :hair_colors, instance_accessor: false
  end

  class Person
    include HairColors
  end

  Person.new.hair_colors = [:brown]  # => NoMethodError
  Person.new.hair_colors             # => NoMethodError

Also you can pass a block to set up the attribute with a default value.

  module HairColors
    mattr_accessor :hair_colors do
      [:brown, :black, :blonde, :red]
    end
  end

  class Person
    include HairColors
  end

  Person.class_variable_get("@@hair_colors") #=> [:brown, :black, :blonde, :red]

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