# 法一
def MyClass.a_class_method; end


# 法二
class MyClass
  def self.anther_class_method; end
end


# 法三*
class MyClass
  class << self
    def yet_another_class_method; end
  end
end

module MyModule
  def my_method; ‘hello'; end
end

class MyClass
  class < self
    include MyModule
  end
end

MyClass.my_method

# 法一: 打开单件类来扩展
module MyModule
  def my_method; ‘hello'; end
end

obj = Object.new
class << obj
  include MyModule
end

obj.my_method  # => “hello”
obj.singleton_methods  # => [:my_method]
# 法二：Object#extend方法
module MyModule
  def my_method; ‘hello'; end
end

obj = Object.new
#对象扩展
obj.extend MyModule
obj.my_method  # => “hello” 
#类扩展
class MyClass
  extend MyModule
end

MyClass.my_method # => “hello”