self_vs_default_definee_vs_receiver

最近在学习ruby的过程遇到很多有趣的博客，随记录学习，这篇学习笔记摘自http://yugui.jp/articles/846

#self

ruby中self无处不在，或是显示的调用或是隐含调用，方法调用如果不指明接收者，那么默认也是self。打开pry

ruby --version => ruby 2.4.0p0

p self                      => main

class Persion
  p self                     => Persion
  def hello(param = (p self));end
end

Persion.new.hello       => #<Persion:0x007ff03d3b5468>

class Manager
  class Employ < (p self; self) => Manager
  end
end

#default definee

ruby 中默认存在一个指向 class 的引用，不像 self可以随处调用，这个引用比 self 更加隐含，暂且称之为 default definee, 如果方法定义时不提供默认的接受者，那么方法默认就会作为 default definee 的实例方法,代开pry

def hello;end
Object.instance_method(:hello) => #<UnboundMethod: Object#hello>

class Persion
  def hello;end
end

Persion.instance_method(:hello) => #<UnboundMethod: Persion#hello>

在正常的方法体内(def 定义的方法), self 是方法的接受者，但是内部函数的 default deinee 却是外层的class，例如：

class Persion
   def hello
     def speak;end
   end
 end

p = Persion.new
p.hello
p.method(:hello) => #<Method: Persion#hello>
Persion.instance_method(:speak) => #<UnboundMethod: Persion#speak>

如果方法内部想要定义实例方法可以用self,本质上方法会被添加到对象的单例类上

class Persion
   def hello
     def self.speak;end
   end
 end

p = Persion.new
p.hello
p.method(:hello) => #<Method: Persion#hello>
p.method(:speak) => #<Method: #<Persion:0x007fc5fe1cfe00>.speak>
p.singleton_class.instance_method(:speak) => #<UnboundMethod: #<Class:#<Persion:0x007fc5fe1cfe00>>#speak>

正常方法体内的 default definee 都是外层的class

class Manager;end

$m = Manager.new => #<Manager:0x007f890e474958>
class Employee
  def $m.hello(param = (def speak;end))
    def sing;end
  end
end

$m.hello
Employee.instance_method(:speak) => #<UnboundMethod: Employee#speak>
Employee.instance_method(:sing)  => #<UnboundMethod: Employee#sing>
$m.method(:speak) => NameError: undefined method `speak' for class `#<Class:#<Manager:0x007f890e474958>>'

# eval

# instance_eval

instacne_eval 会执行以下操作：

• 修改 self 为 instance_eval 的接受者
• 修改 default definee 为 instance_eval 的单例类

o = Object.new
o.instance_eval do
  p self
  def hello; end
end

o.method(:hello) => #<Method: #<Object:0x007f890ec8e698>.hello>
o.singleton_class.instance_method(:hello) => #<UnboundMethod: #<Class:#<Object:0x007f890ec8e698>>#hello>

下一个例子

class Persion
  $o = Object.new

  $o.instance_eval do
    def hello(param = (def speak;end))
      def sing;end
    end
  end
end

$o.hello
$o.method(:speak) => #<Method: #<Object:0x007fee5f3f4b98>.speak>
$o.method(:sing) => #<Method: #<Object:0x007fee5f3f4b98>.sing>
Persion.instance_method(:hello) => NameError: undefined method `hello' for class `Persion'
Persion.instance_method(:speak) => NameError: undefined method `speak' for class `Persion'
Persion.instance_method(:sing) =>NameError: undefined method `sing' for class `Persion'
$o.singleton_class.instance_method(:hello) => #<UnboundMethod: #<Class:#<Object:0x007fee5f3f4b98>>#hello>
$o.singleton_class.instance_method(:speak) => #<UnboundMethod: #<Class:#<Object:0x007fee5f3f4b98>>#speak>
$o.singleton_class.instance_method(:sing) => #<UnboundMethod: #<Class:#<Object:0x007fee5f3f4b98>>#sing>

# class_eval

class_eval 会把 self 和 default definee 都修改为class_eval 的接受者

class Persion;end

Persion.class_eval do
  p self           => Persion
  def hello; end
end

Persion.new.method(:hello) => #<Method: Persion#hello>
Persion.instance_method(:hello) => #<UnboundMethod: Persion#hello>

明白了以上几点，那么下面这个例子就很好理解了：

Persion.instance_eval { define_method(:hello) { "hello" } }

Persion.class_eval { define_method(:sing) { "sing" } }

Persion.instance_eval { def speak; 'speak'; end }

Persion.class_eval { def dance; 'dance'; end }

p Persion.new.hello => "hello"
p Persion.new.sing  => "sing"
p Persion.speak.    => "speak"
p Persion.new.dance => "dance"