mattr_accessor是Rails中特有的类宏。来自active_support。
利用了类变量@@var，类变量相当于类中的全局变量，可以被子类共享。
# File activesupport/lib/active_support/core_ext/module/attribute_accessors.rb
require "active_support/core_ext/array/extract_options"
require "active_support/core_ext/regexp"
# Extends the module object with class/module and instance accessors for
# class/module attributes, just like the native attr* accessors for instance
# attributes.
class Module
# Defines a class attribute and creates a class and instance reader methods.
# The underlying class variable is set to +nil+, if it is not previously
# defined.
#
# module HairColors
# mattr_reader :hair_colors
# end
#
# HairColors.hair_colors # => nil
# HairColors.class_variable_set("@@hair_colors", [:brown, :black])
# HairColors.hair_colors # => [:brown, :black]
#
# The attribute name must be a valid method name in Ruby.
#
# module Foo
# mattr_reader :"1_Badname"
# end
# # => NameError: invalid attribute name: 1_Badname
#
# If you want to opt out the creation on the instance reader method, pass
# <tt>instance_reader: false</tt> or <tt>instance_accessor: false</tt>.
#
# module HairColors
# mattr_reader :hair_colors, instance_reader: false
# end
#
# class Person
# include HairColors
# end
#
# Person.new.hair_colors # => NoMethodError
#
#
# Also, you can pass a block to set up the attribute with a default value.
#
# module HairColors
# mattr_reader :hair_colors do
# [:brown, :black, :blonde, :red]
# end
# end
#
# class Person
# include HairColors
# end
#
# Person.new.hair_colors # => [:brown, :black, :blonde, :red]
def mattr_reader(*syms)
options = syms.extract_options!
syms.each do |sym|
raise NameError.new("invalid attribute name: #{sym}") unless /\A[_A-Za-z]\w*\z/.match?(sym)
class_eval(<<-EOS, __FILE__, __LINE__ + 1)
@@#{sym} = nil unless defined? @@#{sym}
def self.#{sym}
@@#{sym}
end
EOS
unless options[:instance_reader] == false || options[:instance_accessor] == false
class_eval(<<-EOS, __FILE__, __LINE__ + 1)
def #{sym}
@@#{sym}
end
EOS
end
class_variable_set("@@#{sym}", yield) if block_given?
end
end
alias :cattr_reader :mattr_reader
# Defines a class attribute and creates a class and instance writer methods to
# allow assignment to the attribute.
#
# module HairColors
# mattr_writer :hair_colors
# end
#
# class Person
# include HairColors
# end
#
# HairColors.hair_colors = [:brown, :black]
# Person.class_variable_get("@@hair_colors") # => [:brown, :black]
# Person.new.hair_colors = [:blonde, :red]
# HairColors.class_variable_get("@@hair_colors") # => [:blonde, :red]
#
# If you want to opt out the instance writer method, pass
# <tt>instance_writer: false</tt> or <tt>instance_accessor: false</tt>.
#
# module HairColors
# mattr_writer :hair_colors, instance_writer: false
# end
#
# class Person
# include HairColors
# end
#
# Person.new.hair_colors = [:blonde, :red] # => NoMethodError
#
# Also, you can pass a block to set up the attribute with a default value.
#
# module HairColors
# mattr_writer :hair_colors do
# [:brown, :black, :blonde, :red]
# end
# end
#
# class Person
# include HairColors
# end
#
# Person.class_variable_get("@@hair_colors") # => [:brown, :black, :blonde, :red]
def mattr_writer(*syms)
options = syms.extract_options!
syms.each do |sym|
raise NameError.new("invalid attribute name: #{sym}") unless /\A[_A-Za-z]\w*\z/.match?(sym)
class_eval(<<-EOS, __FILE__, __LINE__ + 1)
@@#{sym} = nil unless defined? @@#{sym}
def self.#{sym}=(obj)
@@#{sym} = obj
end
EOS
unless options[:instance_writer] == false || options[:instance_accessor] == false
class_eval(<<-EOS, __FILE__, __LINE__ + 1)
def #{sym}=(obj)
@@#{sym} = obj
end
EOS
end
send("#{sym}=", yield) if block_given?
end
end
alias :cattr_writer :mattr_writer
# Defines both class and instance accessors for class attributes.
#
# module HairColors
# mattr_accessor :hair_colors
# end
#
# class Person
# include HairColors
# end
#
# HairColors.hair_colors = [:brown, :black, :blonde, :red]
# HairColors.hair_colors # => [:brown, :black, :blonde, :red]
# Person.new.hair_colors # => [:brown, :black, :blonde, :red]
#
# If a subclass changes the value then that would also change the value for
# parent class. Similarly if parent class changes the value then that would
# change the value of subclasses too.
#
# class Male < Person
# end
#
# Male.new.hair_colors << :blue
# Person.new.hair_colors # => [:brown, :black, :blonde, :red, :blue]
#
# To opt out of the instance writer method, pass <tt>instance_writer: false</tt>.
# To opt out of the instance reader method, pass <tt>instance_reader: false</tt>.
#
# module HairColors
# mattr_accessor :hair_colors, instance_writer: false, instance_reader: false
# end
#
# class Person
# include HairColors
# end
#
# Person.new.hair_colors = [:brown] # => NoMethodError
# Person.new.hair_colors # => NoMethodError
#
# Or pass <tt>instance_accessor: false</tt>, to opt out both instance methods.
#
# module HairColors
# mattr_accessor :hair_colors, instance_accessor: false
# end
#
# class Person
# include HairColors
# end
#
# Person.new.hair_colors = [:brown] # => NoMethodError
# Person.new.hair_colors # => NoMethodError
#
# Also you can pass a block to set up the attribute with a default value.
#
# module HairColors
# mattr_accessor :hair_colors do
# [:brown, :black, :blonde, :red]
# end
# end
#
# class Person
# include HairColors
# end
#
# Person.class_variable_get("@@hair_colors") # => [:brown, :black, :blonde, :red]
def mattr_accessor(*syms, &blk)
mattr_reader(*syms, &blk)
mattr_writer(*syms)
end
alias :cattr_accessor :mattr_accessor
end