class Class

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