method
mattr_writer
mattr_writer(*syms, instance_writer: true, instance_accessor: true, default: nil, location: nil)
public
Defines a class attribute and creates a class and instance writer methods to allow assignment to the attribute. 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_writer :hair_colors end class Person include HairColors end HairColors.hair_colors = [:brown, :black] Person.class_variable_get("@@hair_colors") # => [:brown, :black] Person.new.hair_colors = [:blonde, :red] HairColors.class_variable_get("@@hair_colors") # => [:blonde, :red]
To omit the instance writer method, pass instance_writer: false or instance_accessor: false.
module HairColors mattr_writer :hair_colors, instance_writer: false end class Person include HairColors end Person.new.hair_colors = [:blonde, :red] # => NoMethodError
You can set a default value for the attribute.
module HairColors mattr_writer :hair_colors, default: [:brown, :black, :blonde, :red] end class Person include HairColors end Person.class_variable_get("@@hair_colors") # => [:brown, :black, :blonde, :red]