Ruby:调用Singleton实例方法的DRY类方法

Question

我有一个Singleton类ExchangeRegistry，它保存了所有的Exchange。

而不需要调用：ExchangeRegistry.instance.exchanges

我希望能够使用：ExchangeRegistry.exchanges

这是可行的，但我对重复一遍并不满意：

require 'singleton'

# Ensure an Exchange is only created once
class ExchangeRegistry
  include Singleton

  # Class Methods  ###### Here be duplication and dragons

  def self.exchanges
    instance.exchanges
  end

  def self.get(exchange)
    instance.get(exchange)
  end

  # Instance Methods

  attr_reader :exchanges

  def initialize
    @exchanges = {} # Stores every Exchange created
  end

  def get(exchange)
    @exchanges[Exchange.to_sym exchange] ||= Exchange.create(exchange)
  end
end

我对课堂方法中的重复不满意。

我试过使用Forwardable和SimpleDelegator，但似乎无法让它干涸。(大多数示例不是针对类方法，而是针对实例方法)

Answer 1

转发模块将完成这一任务。由于您是转发类方法，所以必须打开本征类并在那里定义转发：

require 'forwardable'
require 'singleton'

class Foo

  include Singleton

  class << self
    extend Forwardable
    def_delegators :instance, :foo, :bar
  end

  def foo
    'foo'
  end

  def bar
    'bar'
  end

end

p Foo.foo    # => "foo"
p Foo.bar    # => "bar"

Answer 2

公认的答案是聪明的，但似乎是不必要的复杂(更不用说method_missing的性能损失了。

解决这个问题的通常方法是将实例赋值给常量。

class ExchangeRegistrySingleton
  include Singleton

  # ...
end

ExchangeRegistry = ExchangeRegistrySingleton.instance

Answer 3

您可以利用method_missing钩子并将方法调用委托给instance。

require 'singleton'

class ExchangeRegistry
  include Singleton

  # Missing methods will be delegated to `instance` if an implementation is available.
  # Else `NoMethodError` will be raised via call to `super`
  def self.method_missing method_name, *args
    if instance.respond_to? method_name
        puts "** Defining new method: '#{method_name}'"
        (class << self; self; end).instance_eval do
            define_method(method_name) do |*args|
                instance.send(method_name, *args)
            end
        end
        instance.send(method_name, *args)
    else
        super
    end
  end

  attr_reader :exchanges

  def initialize
    @exchanges = {} # Stores every Exchange created
  end

  def get(exchange)
    @exchanges[Exchange.to_sym exchange] ||= Exchange.create(exchange)
  end
end

# By default, there is no class method - `exchanges`
p ExchangeRegistry.singleton_methods.grep(/exchanges/)
#=> []

p ExchangeRegistry.exchanges
#=> ** Defining new method: 'exchanges'
#=> {}

# After first call to `exchanges`, a new class method is now available
# Future calls will not hit `method_missing` again.
p ExchangeRegistry.singleton_methods.grep(/exchanges/)
#=> [:exchanges]
p ExchangeRegistry.exchanges
#=> {}

此问题的另一个答案表示处理method_missing存在性能损失。因此，当第一次报告method_missing时，我更新了答案以定义类方法。更新基于文章：在Ruby中动态创建类方法