instance method `class_attribute`

Ruby on Rails 2.3.18

Since v2.3.18

Available in: v2.3.18 v3.0.20 v3.1.12 v3.2.22.5 v4.0.13 v4.1.16 v4.2.9 v5.2.8.1 v6.0.6 v6.1.7.10 v7.0.10 v7.1.6 v7.2.3 v8.0.4 v8.1.2

Compare with:

class_attribute(*attrs)

Declare a class-level attribute whose value is inheritable and overwritable by subclasses:

class Base
  class_attribute :setting
end

class Subclass < Base
end

Base.setting = true
Subclass.setting            # => true
Subclass.setting = false
Subclass.setting            # => false
Base.setting                # => true

This matches normal Ruby method inheritance: think of writing an attribute on a subclass as overriding the reader method.

For convenience, a query method is defined as well:

Subclass.setting?           # => false

Instances may overwrite the class value in the same way:

Base.setting = true
object = Base.new
object.setting          # => true
object.setting = false
object.setting          # => false
Base.setting            # => true

To opt out of the instance writer method, pass :instance_writer => false.

object.setting = false  # => NoMethodError

Parameters

attrs rest

Source

# File activesupport/lib/active_support/core_ext/class/attribute.rb, line 40
  def class_attribute(*attrs)
    instance_writer = !attrs.last.is_a?(Hash) || attrs.pop[:instance_writer]

    attrs.each do |name|
      class_eval <<-RUBY, __FILE__, __LINE__ + 1
        def self.#{name}() nil end
        def self.#{name}?() !!#{name} end

        def self.#{name}=(val)
          singleton_class.class_eval do
            remove_possible_method(:#{name})
            define_method(:#{name}) { val }
          end
        end

        def #{name}
          defined?(@#{name}) ? @#{name} : singleton_class.#{name}
        end

        def #{name}?
          !!#{name}
        end
      RUBY

      attr_writer name if instance_writer
    end
  end

Defined in activesupport/lib/active_support/core_ext/class/attribute.rb line 40 · View on GitHub · Improve this page · Find usages on GitHub

Defined in Class

Signature

Parameters