Class: Class

Inherits:
Object show all
Defined in:
lib/gorillib/metaprogramming/class_attribute.rb

Instance Method Summary collapse

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