# frozen_string_literal: false # The Singleton module implements the Singleton pattern. # # == Usage # # To use Singleton, include the module in your class. # # class Klass # include Singleton # # ... # end # # This ensures that only one instance of Klass can be created. # # a,b = Klass.instance, Klass.instance # # a == b # # => true # # Klass.new # # => NoMethodError - new is private ... # # The instance is created at upon the first call of Klass.instance(). # # class OtherKlass # include Singleton # # ... # end # # ObjectSpace.each_object(OtherKlass){} # # => 0 # # OtherKlass.instance # ObjectSpace.each_object(OtherKlass){} # # => 1 # # # This behavior is preserved under inheritance and cloning. # # == Implementation # # This above is achieved by: # # * Making Klass.new and Klass.allocate private. # # * Overriding Klass.inherited(sub_klass) and Klass.clone() to ensure that the # Singleton properties are kept when inherited and cloned. # # * Providing the Klass.instance() method that returns the same object each # time it is called. # # * Overriding Klass._load(str) to call Klass.instance(). # # * Overriding Klass#clone and Klass#dup to raise TypeErrors to prevent # cloning or duping. # # == Singleton and Marshal # # By default Singleton's #_dump(depth) returns the empty string. Marshalling by # default will strip state information, e.g. instance variables from the instance. # Classes using Singleton can provide custom _load(str) and _dump(depth) methods # to retain some of the previous state of the instance. # # require 'singleton' # # class Example # include Singleton # attr_accessor :keep, :strip # def _dump(depth) # # this strips the @strip information from the instance # Marshal.dump(@keep, depth) # end # # def self._load(str) # instance.keep = Marshal.load(str) # instance # end # end # # a = Example.instance # a.keep = "keep this" # a.strip = "get rid of this" # # stored_state = Marshal.dump(a) # # a.keep = nil # a.strip = nil # b = Marshal.load(stored_state) # p a == b # => true # p a.keep # => "keep this" # p a.strip # => nil # module Singleton VERSION = "0.1.1" # Raises a TypeError to prevent cloning. def clone raise TypeError, "can't clone instance of singleton #{self.class}" end # Raises a TypeError to prevent duping. def dup raise TypeError, "can't dup instance of singleton #{self.class}" end # By default, do not retain any state when marshalling. def _dump(depth = -1) '' end module SingletonClassMethods # :nodoc: def clone # :nodoc: Singleton.__init__(super) end # By default calls instance(). Override to retain singleton state. def _load(str) instance end def instance # :nodoc: return @singleton__instance__ if @singleton__instance__ @singleton__mutex__.synchronize { return @singleton__instance__ if @singleton__instance__ @singleton__instance__ = new() } @singleton__instance__ end private def inherited(sub_klass) super Singleton.__init__(sub_klass) end end class << Singleton # :nodoc: def __init__(klass) # :nodoc: klass.instance_eval { @singleton__instance__ = nil @singleton__mutex__ = Thread::Mutex.new } klass end private # extending an object with Singleton is a bad idea undef_method :extend_object def append_features(mod) # help out people counting on transitive mixins unless mod.instance_of?(Class) raise TypeError, "Inclusion of the OO-Singleton module in module #{mod}" end super end def included(klass) super klass.private_class_method :new, :allocate klass.extend SingletonClassMethods Singleton.__init__(klass) end end ## # :singleton-method: _load # By default calls instance(). Override to retain singleton state. ## # :singleton-method: instance # Returns the singleton instance. end