class Class
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.
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