# class Class

### Public Instance Methods

class_attribute(*attrs) click to toggle source

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, 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

To opt out of the instance reader method, pass `instance_reader: false`.

    object.setting          # => NoMethodError
    object.setting?         # => NoMethodError

To opt out of the instance writer method, pass `instance_writer: false`.

    object.setting = false  # => NoMethodError

To opt out of both instance methods, pass `instance_accessor: false`.

    # File lib/dbus/core_ext/class/attribute.rb, line 73
    def class_attribute(*attrs)
      options = attrs.extract_options!
      instance_reader = options.fetch(:instance_accessor, true) && options.fetch(:instance_reader, true)
      instance_writer = options.fetch(:instance_accessor, true) && options.fetch(:instance_writer, true)
      instance_predicate = options.fetch(:instance_predicate, true)

      attrs.each do |name|
        define_singleton_method(name) { nil }
        define_singleton_method("#{name}?") { !!public_send(name) } if instance_predicate

        ivar = "@#{name}"

        define_singleton_method("#{name}=") do |val|
          singleton_class.class_eval do
            remove_possible_method(name)
            define_method(name) { val }
          end

          if singleton_class?
            class_eval do
              remove_possible_method(name)
              define_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
          remove_possible_method name
          define_method(name) do
            if instance_variable_defined?(ivar)
              instance_variable_get ivar
            else
              self.class.public_send name
            end
          end
          define_method("#{name}?") { !!public_send(name) } if instance_predicate
        end

        attr_writer name if instance_writer
      end
    end