class Class
Public Instance Methods
Declare a class-level attribute whose value is inheritable by subclasses. Subclasses can change their own value and it will not impact parent class.
Examples¶ ↑
class Base my_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
# File lib/dbus/core_ext/class/attribute.rb, line 64 def my_class_attribute(*attrs) instance_reader = true instance_writer = true attrs.each do |name| singleton_class.silence_redefinition_of_method(name) define_singleton_method(name) { nil } ivar = "@#{name}".to_sym singleton_class.silence_redefinition_of_method("#{name}=") define_singleton_method("#{name}=") do |val| singleton_class.class_eval do redefine_method(name) { val } end if singleton_class? class_eval do redefine_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 redefine_method(name) do if instance_variable_defined?(ivar) instance_variable_get ivar else self.class.public_send name end end end if instance_writer redefine_method("#{name}=") do |val| instance_variable_set ivar, val end end end end