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require 'active_support/core_ext/kernel/singleton_class'
require 'active_support/core_ext/module/remove_method'
require 'active_support/core_ext/array/extract_options'
class Class
# 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 <tt>Subclass.setting = _something_ </tt>, <tt>Subclass.setting</tt>
# 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 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
def class_attribute(*attrs)
options = attrs.extract_options!
instance_reader = instance_reader = options.fetch(:instance_reader, true)
instance_writer = options.fetch(:instance_writer, true)
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
if singleton_class?
class_eval do
remove_possible_method(:#{name})
def #{name}
defined?(@#{name}) ? @#{name} : singleton_class.#{name}
end
end
end
val
end
if instance_reader
remove_possible_method :#{name}
def #{name}
defined?(@#{name}) ? @#{name} : self.class.#{name}
end
def #{name}?
!!#{name}
end
end
RUBY
attr_writer name if instance_writer
end
end
private
unless respond_to?(:singleton_class?)
def singleton_class?
ancestors.first != self
end
end
end
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