# 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
# 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
