Class | Class |
In: |
lib/dbus/core_ext/class/attribute.rb
|
Parent: | 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, 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\ndef self.\#{name}?() !!\#{name} end\n\ndef self.\#{name}=(val)\nsingleton_class.class_eval do\nremove_possible_method(:\#{name})\ndefine_method(:\#{name}) { val }\nend\nval\nend\n\ndef \#{name}\ndefined?(@\#{name}) ? @\#{name} : singleton_class.\#{name}\nend\n\ndef \#{name}?\n!!\#{name}\nend\n", __FILE__, __LINE__ + 1 attr_writer name if instance_writer end end