我们如何在不同的Ruby类之间复制单例方法？

Question

我试图用一个方法定义一个类，一个缺少这些方法的类，然后允许后一个类的对象从前一个类的实例中“学习”这些方法。

这是我的尝试（Ruby 1.9.2） - 当我尝试在lambda绑定中更改'self'的值时，它会中断（在行评论“BREAKS！”）。

如果你能弄清楚如何解决这个问题 - 我会很高兴找到答案。

class Skill

  attr_accessor :name
  attr_accessor :technique

  def initialize(name, &technique_proc)
    @name = name
    @technique = lambda(&proc)
  end

end

class Person

  attr_accessor :name

  def initialize(name)
    @name = name
  end

  def method_missing(m, *args)
    "#{@name} the #{self.class}: I don't know how to #{m}"
  end

  def learn_skill(skill)
    puts "#{@name} the #{self.class} is learning skill: #{skill.name}"
    actual_self = self
    eval "self = #{actual_self}", skill.technique.binding ####### BREAKS!
    define_singleton_method skill.name.to_sym, skill.technique
  end

  def teach_skill(skill_name)
    skill = nil
    if self.respond_to?(skill_name) 
      puts "#{@name} the #{self.class} is teaching skill: #{skill_name}"
      skill_method = self.method(skill_name.to_sym)
      skill_proc = skill_method.to_proc
      skill_lambda = lambda(&skill_proc)
      skill = Skill.new(skill_name, &skill_lambda)
    end
    skill
  end

end

class Teacher < Person

  def speak(sentence)
    "#{@name} the #{self.class} is now saying \"#{sentence}\"!"
  end

  def jump(number_of_feet)
    "#{name} the #{self.class} is now jumping #{number_of_feet} high!"
  end

end

miss_mollyflop = Teacher.new("Miss Mollyflop")
little_billey = Person.new("Little Billy")

puts miss_mollyflop.speak("Good morning, children!")
puts little_billey.speak("Good morning, Miss Mollyflop!")

speak_skill = miss_mollyflop.teach_skill("speak")
little_billey.learn_skill(speak_skill)

puts little_billey.speak("Good morning, Miss Mollyflop!")

这个输出是：

Miss Mollyflop the Teacher is now saying "Good morning, children!"!
Little Billy the Person: I don't know how to speak
Miss Mollyflop the Teacher is teaching skill: speak
Little Billy the Person is learning skill: speak
test.rb:27:in `eval': (eval):1: Can't change the value of self (SyntaxError)
self = #<Person:0x1482270>
      ^
(eval):1: syntax error, unexpected $end
self = #<Person:0x1482270>
                          ^
        from test.rb:27:in `learn_skill'
        from test.rb:64:in `<main>'

Answer 1

使用Object2module： gem install object2module

require 'object2module'

o = Object.new
def o.speak
  puts "hello from o"
end

m = Object.new
m.gen_extend o

m.speak #=> "hello from o"

Answer 2

如果要将方法从一个类复制到另一个类，则可能，但是如果方法修改状态，它将修改原始对象上的状态，而不是该方法随后绑定的对象上的状态（这是因为该方法并未真正复制而不是方法的Proc包装器作为方法绑定到新对象）：

a = Object.new
def a.hello
  puts "hello world from a"
end

b = Object.new
b.define_singleton_method(:hello, &a.method(:hello))
b.hello #=> "hello world from a"