如何将一个类的对象数限制为红宝石中的给定数？

Question

我如何将单例模式扩展到一个类的多个对象，即，我最多只能有一个类的5个对象，而在ruby中不超过

Answer 1

示例代码：

# Example class which can be instanciated at most five times
# Naive approach with Class variable

class FiveAtMost
  @@instances = 0

  def initialize()
    if @@instances >= 5
      raise "No more than five instances allowed."
    else
      @@instances += 1
    end
    p "Initialized instance #{@@instances}"
  end
end

one = FiveAtMost.new
two = FiveAtMost.new
three = FiveAtMost.new
four = FiveAtMost.new
five = FiveAtMost.new

# will raise RuntimeError: No more than five instances allowed.
six = FiveAtMost.new

由于无法预测对象何时被垃圾回收 ，因此您需要针对该事件采取某种解决方法。 也许您觉得这很有用： http : //pleac.sourceforge.net/pleac_ruby/classesetc.html#AEN710

Answer 2

具有大小限制的Multiton 。

这是可以在任何类中包含的Multiton模块的幼稚实现。 它可以创建的默认对象数是5，您可以通过定义一个名为max_instances的类方法来覆盖此限制，该方法将在您的类中返回一个数字（允许的最大实例数）。

module Multiton

  MAX_INSTANCES = 5
  COUNT_HOOK = :max_instances

  module MultitonClassMethods
    def instance
      size = @instances.size
      max = respond_to?(COUNT_HOOK) ? self.send(COUNT_HOOK) : MAX_INSTANCES
      @instances << new if size < max
      @instances[rand(size)]
    end
  end

  def self.included(klass)
    klass.class_eval {
      @instances = []
    }
    klass.private_class_method :new
    klass.extend(MultitonClassMethods)
  end

end

将模块包含在类中以使其成为多级。

# Falls back to Multiton::MAX_INSTANCES 
class Person
  include Multiton
end

# Overrides the number of allowed instances in max_instances
class Resource
  include Multiton

  def self.max_instances
    58
  end
end

由于对象是从此multiton中的池中随机返回的，因此您可能无法在短期内恢复所有对象。 但是随着请求更多的对象，它应该均匀。 您可以通过循环遍历对象而不是随机选择对象来在Multiton模块中更改此行为。

people = []
1000.times do
  people << Person.instance
end
# should print 5, but may print a smaller number
p people.uniq.size

resources = []
1000.times do
  resources << Resource.instance
end
# should print 58, but may print a smaller number
p resources.uniq.size