如何检查method_defined？ 来自 class 中使用的模块，其中包含模块后定义的方法

Question

How can I define a module that checks the existence of an instance method in the class the module is used. The module is normally included at the start of the file while methods are defined afterwards. I'm using Rails.

A module with a hook

module MyModule
  extend ActiveSupport::Concern

  included do
    raise "Foo" if method_defined? :bar
  end 
end 

A Foo error is never raised in the following code, how can I get this to raise the error?

class MyClass 
   include MyModule

   def bar
     puts "Hello from Bar"
   end 
end 

A Foo error IS raised in the following code:

class MyOtherClass 
   def bar
     puts "Hello from Bar"
   end 

   include MyModule
end 

Answer 1

This is a pure-Ruby answer. I don't know if Rails supports callbacks that are not supported by Ruby that would be useful here.

As @Amadan notes, the module is not a mind-reader; to see an instance method defined on the class the method needs to be defined before the module is included. The method Module#included takes as an argument the module in which it being included. It needs that because self is MyModule when that method is executed.

module MyModule
  def self.included(mod)
    puts "self = #{self}"
    puts ":bar is defined" if mod.method_defined? :bar
    puts ":foo is defined" if mod.method_defined? :foo
    puts ":goo is defined" if mod.method_defined? :goo
  end
  def goo
  end  
end 

class MyClass 
  def bar
  end 
  include MyModule
end

prints

self = MyModule
:bar is defined
:goo is defined

Note that Module#included is executed after the instance method defined in MyModule ( goo ) is included in MyClass .