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Methods
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Instance Public methods
class_attribute(*attrs)

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

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 places but 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.

# File activesupport/lib/active_support/core_ext/class/attribute.rb, line 71
def class_attribute(*attrs)
  options = attrs.extract_options!
  instance_reader = options.fetch(:instance_accessor, true) && options.fetch(:instance_reader, true)
  instance_writer = options.fetch(:instance_accessor, true) && options.fetch(:instance_writer, true)
  instance_predicate = options.fetch(:instance_predicate, true)

  attrs.each do |name|
    define_singleton_method(name) { nil }
    define_singleton_method("#{name}?") { !!public_send(name) } if instance_predicate

    ivar = "@#{name}"

    define_singleton_method("#{name}=") do |val|
      singleton_class.class_eval do
        remove_possible_method(name)
        define_method(name) { val }
      end

      if singleton_class?
        class_eval do
          remove_possible_method(name)
          define_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
      remove_possible_method name
      define_method(name) do
        if instance_variable_defined?(ivar)
          instance_variable_get ivar
        else
          self.class.public_send name
        end
      end
      define_method("#{name}?") { !!public_send(name) } if instance_predicate
    end

    attr_writer name if instance_writer
  end
end
subclasses()

Returns an array with the direct children of self.

Integer.subclasses # => [Fixnum, Bignum]

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

Foo.subclasses # => [Bar]
# File activesupport/lib/active_support/core_ext/class/subclasses.rb, line 35
def subclasses
  subclasses, chain = [], descendants
  chain.each do |k|
    subclasses << k unless chain.any? { |c| c > k }
  end
  subclasses
end
superclass_delegating_accessor(name, options = {})
# File activesupport/lib/active_support/core_ext/class/delegating_attributes.rb, line 7
def superclass_delegating_accessor(name, options = {})
  # Create private _name and _name= methods that can still be used if the public
  # methods are overridden.
  _superclass_delegating_accessor("_#{name}", options)

  # Generate the public methods name, name=, and name?.
  # These methods dispatch to the private _name, and _name= methods, making them
  # overridable.
  singleton_class.send(:define_method, name) { send("_#{name}") }
  singleton_class.send(:define_method, "#{name}?") { !!send("_#{name}") }
  singleton_class.send(:define_method, "#{name}=") { |value| send("_#{name}=", value) }

  # If an instance_reader is needed, generate public instance methods name and name?.
  if options[:instance_reader] != false
    define_method(name) { send("_#{name}") }
    define_method("#{name}?") { !!send("#{name}") }
  end
end