# File activesupport/lib/active_support/core_ext/class/attribute.rb, line 87
def class_attribute(
  *attrs,
  instance_accessor: true,
  instance_reader: instance_accessor,
  instance_writer: instance_accessor,
  instance_predicate: true,
  default: nil
)
  attrs.each do |name|
    singleton_class.silence_redefinition_of_method(name)
    define_singleton_method(name) { default }

    singleton_class.silence_redefinition_of_method("#{name}?")
    define_singleton_method("#{name}?") { !!public_send(name) } if instance_predicate

    ivar = "@#{name}".to_sym

    singleton_class.silence_redefinition_of_method("#{name}=")
    define_singleton_method("#{name}=") do |val|
      redefine_singleton_method(name) { val }

      if singleton_class?
        class_eval do
          redefine_method(name) do
            if instance_variable_defined? ivar
              instance_variable_get ivar
            else
              singleton_class.send name
            end
          end
        end
      end
      val
    end

    if instance_reader
      redefine_method(name) do
        if instance_variable_defined?(ivar)
          instance_variable_get ivar
        else
          self.class.public_send name
        end
      end

      redefine_method("#{name}?") { !!public_send(name) } if instance_predicate
    end

    if instance_writer
      redefine_method("#{name}=") do |val|
        instance_variable_set ivar, val
      end
    end
  end
end

Declare a class-level attribute whose value is inheritable by subclasses. Subclasses can change their own value and it will not impact parent class.

Options

• :instance_reader - Sets the instance reader method (defaults to true).

• :instance_writer - Sets the instance writer method (defaults to true).

• :instance_accessor - Sets both instance methods (defaults to true).

• :instance_predicate - Sets a predicate method (defaults to true).

• :default - Sets a default value for the attribute (defaults to nil).

Examples

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

In the above case as long as Subclass does not assign a value to setting by performing Subclass.setting = something, Subclass.setting would read value assigned to parent class. Once Subclass assigns a value then the value assigned by Subclass would be returned.

This matches normal Ruby method inheritance: think of writing an attribute on a subclass as overriding the reader method. However, you need to be aware when using class_attribute with mutable structures as Array or Hash. In such cases, you don't want to do changes in place. Instead use setters:

Base.setting = []
Base.setting                # => []
Subclass.setting            # => []

# Appending in child changes both parent and child because it is the same object:
Subclass.setting << :foo
Base.setting               # => [:foo]
Subclass.setting           # => [:foo]

# Use setters to not propagate changes:
Base.setting = []
Subclass.setting += [:foo]
Base.setting               # => []
Subclass.setting           # => [:foo]

For convenience, an instance predicate method is defined as well. To skip it, pass instance_predicate: false.

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 reader method, pass instance_reader: false.

object.setting          # => NoMethodError
object.setting?         # => NoMethodError

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

object.setting = false  # => NoMethodError

To opt out of both instance methods, pass instance_accessor: false.

To set a default value for the attribute, pass default:, like so:

class_attribute :settings, default: {}

# File activesupport/lib/active_support/core_ext/class/subclasses.rb, line 21
def descendants
  descendants = []
  ObjectSpace.each_object(singleton_class) do |k|
    next if k.singleton_class?
    descendants.unshift k unless k == self
  end
  descendants
end

Returns an array with all classes that are < than its receiver.

class C; end
C.descendants # => []

class B < C; end
C.descendants # => [B]

class A < B; end
C.descendants # => [B, A]

class D < C; end
C.descendants # => [B, A, D]

# File activesupport/lib/active_support/core_ext/class/subclasses.rb, line 47
def subclasses
  subclasses, chain = [], descendants
  chain.each do |k|
    subclasses << k unless chain.any? { |c| c > k }
  end
  subclasses
end

Returns an array with the direct children of self.

class Foo; end
class Bar < Foo; end
class Baz < Bar; end

Foo.subclasses # => [Bar]