Class Class
In: lib/dbus/core_ext/class/attribute.rb
Parent: Object

Methods

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.

  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

[Source]

# 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

[Validate]