diff options
Diffstat (limited to 'lib/singleton.rb')
| -rw-r--r-- | lib/singleton.rb | 417 |
1 files changed, 115 insertions, 302 deletions
diff --git a/lib/singleton.rb b/lib/singleton.rb index 31fb950378..368febc74d 100644 --- a/lib/singleton.rb +++ b/lib/singleton.rb @@ -1,359 +1,172 @@ +# frozen_string_literal: false + # The Singleton module implements the Singleton pattern. # -# Usage: +# == Usage +# +# To use Singleton, include the module in your class. +# # class Klass # include Singleton # # ... # end # -# * this ensures that only one instance of Klass lets call it -# ``the instance'' can be created. +# This ensures that only one instance of Klass can be created. # -# a,b = Klass.instance, Klass.instance -# a == b # => true -# a.new # NoMethodError - new is private ... +# a,b = Klass.instance, Klass.instance # -# * ``The instance'' is created at instantiation time, in other -# words the first call of Klass.instance(), thus +# 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. # -# * This behavior is preserved under inheritance and cloning. +# ObjectSpace.each_object(OtherKlass){} +# # => 0 # +# OtherKlass.instance +# ObjectSpace.each_object(OtherKlass){} +# # => 1 # # -# This is achieved by marking -# * Klass.new and Klass.allocate - as private +# This behavior is preserved under inheritance and cloning. # -# Providing (or modifying) the class methods -# * Klass.inherited(sub_klass) and Klass.clone() - -# to ensure that the Singleton pattern is properly -# inherited and cloned. +# == Implementation # -# * Klass.instance() - returning ``the instance''. After a -# successful self modifying (normally the first) call the -# method body is a simple: +# This above is achieved by: # -# def Klass.instance() -# return @__instance__ -# end +# * Making Klass.new and Klass.allocate private. # -# * Klass._load(str) - calling Klass.instance() +# * Overriding Klass.inherited(sub_klass) and Klass.clone() to ensure that the +# Singleton properties are kept when inherited and cloned. # -# * Klass._instantiate?() - returning ``the instance'' or -# nil. This hook method puts a second (or nth) thread calling -# Klass.instance() on a waiting loop. The return value -# signifies the successful completion or premature termination -# of the first, or more generally, current "instantiation thread". +# * Providing the Klass.instance() method that returns the same object each +# time it is called. # +# * Overriding Klass._load(str) to call Klass.instance(). # -# The instance method of Singleton are -# * clone and dup - raising TypeErrors to prevent cloning or duping +# * 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 and taint +# state, 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" +# a.taint +# +# 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 # -# * _dump(depth) - returning the empty string. Marshalling strips -# by default all state information, e.g. instance variables and -# taint state, from ``the instance''. Providing custom _load(str) -# and _dump(depth) hooks allows the (partially) resurrections of -# a previous state of ``the instance''. - - - module Singleton - # disable build-in copying methods + # 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 - # default marshalling strategy - def _dump(depth=-1) + # By default, do not retain any state when marshalling. + def _dump(depth = -1) '' end -end + module SingletonClassMethods # :nodoc: -class << Singleton - # Method body of first instance call. - FirstInstanceCall = proc do - # @__instance__ takes on one of the following values - # * nil - before and after a failed creation - # * false - during creation - # * sub_class instance - after a successful creation - # the form makes up for the lack of returns in progs - Thread.critical = true - if @__instance__.nil? - @__instance__ = false - Thread.critical = false - begin - @__instance__ = new - ensure - if @__instance__ - class <<self - remove_method :instance - def instance; @__instance__ end - end - else - @__instance__ = nil # failed instance creation - end - end - elsif _instantiate?() - Thread.critical = false - else - @__instance__ = false - Thread.critical = false - begin - @__instance__ = new - ensure - if @__instance__ - class <<self - remove_method :instance - def instance; @__instance__ end - end - else - @__instance__ = nil - end - end - end - @__instance__ - end - - module SingletonClassMethods - # properly clone the Singleton pattern - did you know - # that duping doesn't copy class methods? - def clone + def clone # :nodoc: Singleton.__init__(super) end - + + # By default calls instance(). Override to retain singleton state. + def _load(str) + instance + end + private - - # ensure that the Singleton pattern is properly inherited + def inherited(sub_klass) super Singleton.__init__(sub_klass) end - - def _load(str) - instance - end - - # waiting-loop hook - def _instantiate?() - while false.equal?(@__instance__) - Thread.critical = false - sleep(0.08) # timeout - Thread.critical = true - end - @__instance__ - end - end - - def __init__(klass) - klass.instance_eval { @__instance__ = nil } - class << klass - define_method(:instance,FirstInstanceCall) - end - 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 - - - -if __FILE__ == $0 - -def num_of_instances(klass) - "#{ObjectSpace.each_object(klass){}} #{klass} instance(s)" -end - -# The basic and most important example. - -class SomeSingletonClass - include Singleton -end -puts "There are #{num_of_instances(SomeSingletonClass)}" -a = SomeSingletonClass.instance -b = SomeSingletonClass.instance # a and b are same object -puts "basic test is #{a == b}" - -begin - SomeSingletonClass.new -rescue NoMethodError => mes - puts mes -end - - - -puts "\nThreaded example with exception and customized #_instantiate?() hook"; p -Thread.abort_on_exception = false - -class Ups < SomeSingletonClass - def initialize - self.class.__sleep - puts "initialize called by thread ##{Thread.current[:i]}" - end -end - -class << Ups - def _instantiate? - @enter.push Thread.current[:i] - while false.equal?(@__instance__) - Thread.critical = false - sleep 0.08 - Thread.critical = true - end - @leave.push Thread.current[:i] - @__instance__ - end - - def __sleep - sleep(rand(0.08)) - end - - def new - begin - __sleep - raise "boom - thread ##{Thread.current[:i]} failed to create instance" - ensure - # simple flip-flop - class << self - remove_method :new + class << Singleton # :nodoc: + def __init__(klass) # :nodoc: + klass.instance_eval { + @singleton__instance__ = nil + @singleton__mutex__ = Thread::Mutex.new + } + def klass.instance # :nodoc: + return @singleton__instance__ if @singleton__instance__ + @singleton__mutex__.synchronize { + return @singleton__instance__ if @singleton__instance__ + @singleton__instance__ = new() + } + @singleton__instance__ end + klass end - end - - def instantiate_all - @enter = [] - @leave = [] - 1.upto(9) {|i| - Thread.new { - begin - Thread.current[:i] = i - __sleep - instance - rescue RuntimeError => mes - puts mes - end - } - } - puts "Before there were #{num_of_instances(self)}" - sleep 3 - puts "Now there is #{num_of_instances(self)}" - puts "#{@enter.join '; '} was the order of threads entering the waiting loop" - puts "#{@leave.join '; '} was the order of threads leaving the waiting loop" - end -end - -Ups.instantiate_all -# results in message like -# Before there were 0 Ups instance(s) -# boom - thread #6 failed to create instance -# initialize called by thread #3 -# Now there is 1 Ups instance(s) -# 3; 2; 1; 8; 4; 7; 5 was the order of threads entering the waiting loop -# 3; 2; 1; 7; 4; 8; 5 was the order of threads leaving the waiting loop + private + # extending an object with Singleton is a bad idea + undef_method :extend_object -puts "\nLets see if class level cloning really works" -Yup = Ups.clone -def Yup.new - begin - __sleep - raise "boom - thread ##{Thread.current[:i]} failed to create instance" - ensure - # simple flip-flop - class << self - remove_method :new + 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 - end -end -Yup.instantiate_all - -puts "\n\n","Customized marshalling" -class A - include Singleton - attr_accessor :persist, :die - def _dump(depth) - # this strips the @die information from the instance - Marshal.dump(@persist,depth) - end -end - -def A._load(str) - instance.persist = Marshal.load(str) - instance -end - -a = A.instance -a.persist = ["persist"] -a.die = "die" -a.taint - -stored_state = Marshal.dump(a) -# change state -a.persist = nil -a.die = nil -b = Marshal.load(stored_state) -p a == b # => true -p a.persist # => ["persist"] -p a.die # => nil - - -puts "\n\nSingleton with overridden default #inherited() hook" -class Up -end -def Up.inherited(sub_klass) - puts "#{sub_klass} subclasses #{self}" -end - - -class Middle < Up - include Singleton -end - -class Down < Middle; end - -puts "and basic \"Down test\" is #{Down.instance == Down.instance}\n -Various exceptions" - -begin - module AModule - include Singleton + def included(klass) + super + klass.private_class_method :new, :allocate + klass.extend SingletonClassMethods + Singleton.__init__(klass) + end end -rescue TypeError => mes - puts mes #=> Inclusion of the OO-Singleton module in module AModule -end - -begin - 'aString'.extend Singleton -rescue NoMethodError => mes - puts mes #=> undefined method `extend_object' for Singleton:Module -end + ## + # :singleton-method: _load + # By default calls instance(). Override to retain singleton state. end |