冬季促销! 
Ruby 单例模式讲解和代码示例
单例是一种创建型设计模式， 让你能够保证一个类只有一个实例， 并提供一个访问该实例的全局节点。
单例拥有与全局变量相同的优缺点。 尽管它们非常有用， 但却会破坏代码的模块化特性。
在某些其他上下文中， 你不能使用依赖于单例的类。 你也将必须使用单例类。 绝大多数情况下， 该限制会在创建单元测试时出现。
复杂度：
流行度：
使用示例： 许多开发者将单例模式视为一种反模式。 因此它在 Ruby 代码中的使用频率正在逐步减少。
识别方法： 单例可以通过返回相同缓存对象的静态构建方法来识别。
基础单例
实现一个粗糙的单例非常简单。 你仅需隐藏构造函数并实现一个静态的构建方法即可。
相同的类在多线程环境中会出错。 多线程可能会同时调用构建方法并获取多个单例类的实例。
main.rb: 概念示例
# The Singleton class defines the `instance` method that lets clients access the # unique singleton instance. class Singleton @instance = new private_class_method :new # The static method that controls the access to the singleton instance. # # This implementation let you subclass the Singleton class while keeping just # one instance of each subclass around. def self.instance @instance end # Finally, any singleton should define some business logic, which can be # executed on its instance. def some_business_logic # ... end end # The client code. s1 = Singleton.instance s2 = Singleton.instance if s1.equal?(s2) print 'Singleton works, both variables contain the same instance.' else print 'Singleton failed, variables contain different instances.' end output.txt: 执行结果
Singleton works, both variables contain the same instance. 线程安全单例
为了解决这个问题， 你必须在创建首个单例对象时对线程进行同步。
main.rb: 概念示例
# The Singleton class defines the `instance` method that lets clients access the # unique singleton instance. class Singleton attr_reader :value @instance_mutex = Mutex.new private_class_method :new def initialize(value) @value = value end # The static method that controls the access to the singleton instance. # # This implementation let you subclass the Singleton class while keeping just # one instance of each subclass around. def self.instance(value) return @instance if @instance @instance_mutex.synchronize do @instance ||= new(value) end @instance end # Finally, any singleton should define some business logic, which can be # executed on its instance. def some_business_logic # ... end end # @param [String] value def test_singleton(value) singleton = Singleton.instance(value) puts singleton.value end # The client code. puts "If you see the same value, then singleton was reused (yay!)\n"\ "If you see different values, then 2 singletons were created (booo!!)\n\n"\ "RESULT:\n\n" process1 = Thread.new { test_singleton('FOO') } process2 = Thread.new { test_singleton('BAR') } process1.join process2.join output.txt: 执行结果
If you see the same value, then singleton was reused (yay!) If you see different values, then 2 singletons were created (booo!!) RESULT: FOO FOO 
