Class: Class
- Defined in:
- lib/active_support/core_ext/class/attribute.rb,
lib/active_support/core_ext/class/subclasses.rb
Instance Method Summary collapse
-
#class_attribute(*attrs) ⇒ Object
Declare a class-level attribute whose value is inheritable by subclasses.
-
#subclasses ⇒ Object
Returns an array with the direct children of
self
.
Instance Method Details
#class_attribute(*attrs) ⇒ Object
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
.
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# File 'lib/active_support/core_ext/class/attribute.rb', line 71 def class_attribute(*attrs) = attrs. instance_reader = .fetch(:instance_accessor, true) && .fetch(:instance_reader, true) instance_writer = .fetch(:instance_accessor, true) && .fetch(:instance_writer, true) instance_predicate = .fetch(:instance_predicate, true) attrs.each do |name| remove_possible_singleton_method(name) define_singleton_method(name) { nil } remove_possible_singleton_method("#{name}?") define_singleton_method("#{name}?") { !!public_send(name) } if instance_predicate ivar = "@#{name}" remove_possible_singleton_method("#{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 remove_possible_method "#{name}?" define_method("#{name}?") { !!public_send(name) } if instance_predicate end if instance_writer remove_possible_method "#{name}=" attr_writer name end end end |
#subclasses ⇒ Object
Returns an array with the direct children of self
.
class Foo; end
class Bar < Foo; end
class Baz < Bar; end
Foo.subclasses # => [Bar]
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# File 'lib/active_support/core_ext/class/subclasses.rb', line 34 def subclasses subclasses, chain = [], descendants chain.each do |k| subclasses << k unless chain.any? { |c| c > k } end subclasses end |