Rails module include way

# Rails module include way
# 以下方式的代码在rails中源码中的相当多见，其中的self.included(base)方法是一个回调方法，当此module被其他名为base的module(或者class)include的时候触发此方法。通过class_eval,include,extend加入了实例方法和类方法到base中，代码划分得很干净。

module ActionController
module Components
def self.included(base)
base.class_eval do
include InstanceMethods
extend ClassMethods
helper HelperMethods
end
end

module ClassMethods
end

module HelperMethods
end

module InstanceMethods
end
end
end

obj.instance_eval and mod.module_eval(mod.class_eval) usage

class C
  def initialize
    @x = 1
  end
end

c = C.new
c.instance_eval { puts @x }
puts c.instance_variable_get "@x"

C.class_eval do
  def some_method
    puts "Created in class_eval"
  end
end
c = C.new
c.some_method

Ruby class_eval example

C = Class.new 
C.class_eval do 
  def some_method 
    puts "Created in class_eval" 
  end 
end 
c = C.new 
c.some_method

var = 'test class_eval'
C.class_eval {define_method('talk') {puts var}}
c.talk