# frozen_string_literal: false

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# The Singleton module implements the Singleton pattern.

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#

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# == Usage

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#

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# To use Singleton, include the module in your class.

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#

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# class Klass

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# include Singleton

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# # ...

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# end

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#

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# This ensures that only one instance of Klass can be created.

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#

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# a,b = Klass.instance, Klass.instance

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#

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# a == b

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# # => true

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#

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# Klass.new

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# # => NoMethodError - new is private ...

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#

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# The instance is created at upon the first call of Klass.instance().

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#

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# class OtherKlass

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# include Singleton

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# # ...

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# end

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#

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# ObjectSpace.each_object(OtherKlass){}

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# # => 0

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#

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# OtherKlass.instance

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# ObjectSpace.each_object(OtherKlass){}

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# # => 1

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#

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#

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# This behavior is preserved under inheritance and cloning.

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#

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# == Implementation

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#

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# This above is achieved by:

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#

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# * Making Klass.new and Klass.allocate private.

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#

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# * Overriding Klass.inherited(sub_klass) and Klass.clone() to ensure that the

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# Singleton properties are kept when inherited and cloned.

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#

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# * Providing the Klass.instance() method that returns the same object each

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# time it is called.

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#

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# * Overriding Klass._load(str) to call Klass.instance().

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#

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# * Overriding Klass#clone and Klass#dup to raise TypeErrors to prevent

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# cloning or duping.

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#

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# == Singleton and Marshal

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#

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# By default Singleton's #_dump(depth) returns the empty string. Marshalling by

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# default will strip state information, e.g. instance variables and taint

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# state, from the instance. Classes using Singleton can provide custom

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# _load(str) and _dump(depth) methods to retain some of the previous state of

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# the instance.

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#

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# require 'singleton'

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#

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# class Example

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# include Singleton

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# attr_accessor :keep, :strip

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# def _dump(depth)

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# # this strips the @strip information from the instance

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# Marshal.dump(@keep, depth)

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# end

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#

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# def self._load(str)

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# instance.keep = Marshal.load(str)

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# instance

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# end

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# end

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#

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# a = Example.instance

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# a.keep = "keep this"

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# a.strip = "get rid of this"

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# a.taint

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#

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# stored_state = Marshal.dump(a)

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#

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# a.keep = nil

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# a.strip = nil

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# b = Marshal.load(stored_state)

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# p a == b # => true

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# p a.keep # => "keep this"

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# p a.strip # => nil

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#

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module Singleton

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# Raises a TypeError to prevent cloning.

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def clone

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raise TypeError, "can't clone instance of singleton #{self.class}"

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end

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# Raises a TypeError to prevent duping.

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def dup

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raise TypeError, "can't dup instance of singleton #{self.class}"

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end

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# By default, do not retain any state when marshalling.

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def _dump(depth = -1)

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''

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end

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module SingletonClassMethods # :nodoc:

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def clone # :nodoc:

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Singleton.__init__(super)

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end

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# By default calls instance(). Override to retain singleton state.

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def _load(str)

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instance

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end

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private

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def inherited(sub_klass)

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super

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Singleton.__init__(sub_klass)

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end

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end

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class << Singleton # :nodoc:

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def __init__(klass) # :nodoc:

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klass.instance_eval {

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@singleton__instance__ = nil

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@singleton__mutex__ = Thread::Mutex.new

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}

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def klass.instance # :nodoc:

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return @singleton__instance__ if @singleton__instance__

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@singleton__mutex__.synchronize {

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return @singleton__instance__ if @singleton__instance__

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@singleton__instance__ = new()

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}

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@singleton__instance__

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end

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klass

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end

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private

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# extending an object with Singleton is a bad idea

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undef_method :extend_object

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def append_features(mod)

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# help out people counting on transitive mixins

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unless mod.instance_of?(Class)

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raise TypeError, "Inclusion of the OO-Singleton module in module #{mod}"

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end

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super

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end

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def included(klass)

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super

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klass.private_class_method :new, :allocate

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klass.extend SingletonClassMethods

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Singleton.__init__(klass)

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end

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end

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##

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# :singleton-method: _load

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# By default calls instance(). Override to retain singleton state.

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end

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