Instance Public methods
class_attribute(*attrs, instance_accessor: true, instance_reader: instance_accessor, instance_writer: instance_accessor, instance_predicate: true, default: nil) Link
Declare a class-level attribute whose value is inheritable by subclasses. Subclasses can change their own value and it will not impact parent class.
Options
-
:instance_reader
- Sets the instance reader method (defaults to true). -
:instance_writer
- Sets the instance writer method (defaults to true). -
:instance_accessor
- Sets both instance methods (defaults to true). -
:instance_predicate
- Sets a predicate method (defaults to true). -
:default
- Sets a default value for the attribute (defaults to nil).
Examples
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 place. Instead 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
.
To set a default value for the attribute, pass default:
, like so:
class_attribute :settings, default: {}
# File activesupport/lib/active_support/core_ext/class/attribute.rb, line 86 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 ::ActiveSupport::ClassAttribute.redefine(self, name, default) unless singleton_class? methods << <<~RUBY if instance_reader silence_redefinition_of_method def #{name} defined?(@#{name}) ? @#{name} : self.class.#{name} 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
descendants() Link
Returns an array with all classes that are < than its receiver.
class C; end
C.descendants # => []
class B < C; end
C.descendants # => [B]
class A < B; end
C.descendants # => [B, A]
class D < C; end
C.descendants # => [B, A, D]