Class: Class
Instance Method Summary collapse
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#class_attribute(*attrs) ⇒ Object
Declare a class-level attribute whose value is inheritable by subclasses.
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, 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
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# File 'lib/gorillib/metaprogramming/class_attribute.rb', line 62 def class_attribute(*attrs) instance_writer = !attrs.last.is_a?(Hash) || attrs.pop[:instance_writer] attrs.each do |name| class_eval <<-RUBY, __FILE__, __LINE__ + 1 def self.#{name}() nil end def self.#{name}?() !!#{name} end def self.#{name}=(val) singleton_class.class_eval do remove_possible_method(:#{name}) define_method(:#{name}) { val } end val end def #{name} defined?(@#{name}) ? @#{name} : singleton_class.#{name} end def #{name}? !!#{name} end RUBY attr_writer name if instance_writer end end |