# class Class

### Public Instance Methods

my_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.

#### 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
     64 def my_class_attribute(*attrs)
     65   instance_reader    = true
     66   instance_writer    = true
     67
     68   attrs.each do |name|
     69     singleton_class.silence_redefinition_of_method(name)
     70     define_singleton_method(name) { nil }
     71
     72     ivar = "@#{name}".to_sym
     73
     74     singleton_class.silence_redefinition_of_method("#{name}=")
     75     define_singleton_method("#{name}=") do |val|
     76       singleton_class.class_eval do
     77         redefine_method(name) { val }
     78       end
     79
     80       if singleton_class?
     81         class_eval do
     82           redefine_method(name) do
     83             if instance_variable_defined? ivar
     84               instance_variable_get ivar
     85             else
     86               singleton_class.send name
     87             end
     88           end
     89         end
     90       end
     91       val
     92     end
     93
     94     if instance_reader
     95       redefine_method(name) do
     96         if instance_variable_defined?(ivar)
     97           instance_variable_get ivar
     98         else
     99           self.class.public_send name
    100         end
    101       end
    102     end
    103
    104     if instance_writer
    105       redefine_method("#{name}=") do |val|
    106         instance_variable_set ivar, val
    107       end
    108     end
    109   end
    110 end