class Class

Public Instance Methods

class_attribute(*attrs) click to toggle source

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, 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
# File lib/dbus/core_ext/class/attribute.rb, line 63
  def class_attribute(*attrs)
    instance_writer = !attrs.last.is_a?(Hash) || attrs.pop[:instance_writer]

    attrs.each do |name|
      class_eval "        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
          val
        end

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

        def #{name}?
          !!#{name}
        end
", __FILE__, __LINE__ + 1

      attr_writer name if instance_writer
    end
  end