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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, an instance predicate method is defined as well.
# To skip it, pass <tt>instance_predicate: false</tt>.
#
# 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 <tt>instance_reader: false</tt>.
#
# object.setting # => NoMethodError
# object.setting? # => NoMethodError
#
# To opt out of the instance writer method, pass <tt>instance_writer: false</tt>.
#
# object.setting = false # => NoMethodError
#
# To opt out of both instance methods, pass <tt>instance_accessor: false</tt>.
def class_attribute(*attrs)
options = attrs.extract_options!
# double assignment is used to avoid "assigned but unused variable" warning
instance_reader = 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
private
def singleton_class?
ancestors.first != self
end
end
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