class Class - activesupport-4.2.4 Documentation

# 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/active_support/core_ext/class/attribute.rb, line 71
    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

subclasses() click to toggle source

Returns an array with the direct children of `self`.

    Integer.subclasses # => [Fixnum, Bignum]

    class Foo; end
    class Bar < Foo; end
    class Baz < Bar; end

    Foo.subclasses # => [Bar]

    # File lib/active_support/core_ext/class/subclasses.rb, line 35
    def subclasses
      subclasses, chain = [], descendants
      chain.each do |k|
        subclasses << k unless chain.any? { |c| c > k }
      end
      subclasses
    end

superclass_delegating_accessor(name, options = {}) click to toggle source

    # File lib/active_support/core_ext/class/delegating_attributes.rb, line 7
    def superclass_delegating_accessor(name, options = {})
      # Create private _name and _name= methods that can still be used if the public
      # methods are overridden.
      _superclass_delegating_accessor("_#{name}", options)

      # Generate the public methods name, name=, and name?.
      # These methods dispatch to the private _name, and _name= methods, making them
      # overridable.
      singleton_class.send(:define_method, name) { send("_#{name}") }
      singleton_class.send(:define_method, "#{name}?") { !!send("_#{name}") }
      singleton_class.send(:define_method, "#{name}=") { |value| send("_#{name}=", value) }

      # If an instance_reader is needed, generate public instance methods name and name?.
      if options[:instance_reader] != false
        define_method(name) { send("_#{name}") }
        define_method("#{name}?") { !!send("#{name}") }
      end
    end

### Private Instance Methods

_stash_object_in_method(object, method, instance_reader = true) click to toggle source

Take the object being set and store it in a method. This gives us automatic inheritance behavior, without having to store the object in an instance variable and look up the superclass chain manually.

    # File lib/active_support/core_ext/class/delegating_attributes.rb, line 32
    def _stash_object_in_method(object, method, instance_reader = true)
      singleton_class.remove_possible_method(method)
      singleton_class.send(:define_method, method) { object }
      remove_possible_method(method)
      define_method(method) { object } if instance_reader
    end

_superclass_delegating_accessor(name, options = {}) click to toggle source

    # File lib/active_support/core_ext/class/delegating_attributes.rb, line 39
    def _superclass_delegating_accessor(name, options = {})
      singleton_class.send(:define_method, "#{name}=") do |value|
        _stash_object_in_method(value, name, options[:instance_reader] != false)
      end
      send("#{name}=", nil)
    end

singleton_class?() click to toggle source

    # File lib/active_support/core_ext/class/attribute.rb, line 123
    def singleton_class?
      ancestors.first != self
    end