class Class

class_attribute :settings, default: {}

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

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

object.setting = false # => NoMethodError

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

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

To opt out of the instance reader method, pass instance_reader: false.

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

Instances may overwrite the class value in the same way:

Subclass.setting? # => false

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

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

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

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

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

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

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

end
class Subclass < Base

end
class_attribute :setting
class Base

==== Examples

* :default - Sets a default value for the attribute (defaults to nil).
* :instance_predicate - Sets a predicate method (defaults to true).
* :instance_accessor - Sets both instance methods (defaults to true).
* :instance_writer - Sets the instance writer method (defaults to true).
* :instance_reader - Sets the instance reader method (defaults to true).

==== Options

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

def class_attribute(*attrs, instance_accessor: true,
  instance_reader: instance_accessor, instance_writer: instance_accessor, instance_predicate: true, default: nil
)
  class_methods, methods = [], []
  attrs.each do |name|
    unless name.is_a?(Symbol) || name.is_a?(String)
      raise TypeError, "#{name.inspect} is not a symbol nor a string"
    end
    name = name.to_sym
    namespaced_name = :"__class_attr_#{name}"
    ::ActiveSupport::ClassAttribute.redefine(self, name, namespaced_name, default)
    delegators = [
      "def #{name}; #{namespaced_name}; end",
      "def #{name}=(value); self.#{namespaced_name} = value; end",
    ]
    class_methods.concat(delegators)
    if singleton_class?
      methods.concat(delegators)
    else
      methods << <<~RUBY if instance_reader
        silence_redefinition_of_method def #{name}
          if defined?(@#{name})
            @#{name}
          else
            self.class.#{name}
          end
        end
      RUBY
    end
    methods << <<~RUBY if instance_writer
      silence_redefinition_of_method(:#{name}=)
      attr_writer :#{name}
    RUBY
    if instance_predicate
      class_methods << "silence_redefinition_of_method def #{name}?; !!self.#{name}; end"
      if instance_reader
        methods << "silence_redefinition_of_method def #{name}?; !!self.#{name}; end"
      end
    end
  end
  location = caller_locations(1, 1).first
  class_eval(["class << self", *class_methods, "end", *methods].join(";").tr("\n", ";"), location.path, location.lineno)
end

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

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

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

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

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

def descendants
  subclasses.concat(subclasses.flat_map(&:descendants))
end