在閱讀開源的 Ruby 代碼和編寫可維護性的代碼經常遇到這兩者的使用，那么他們兩者的共同點和區別是什么呢？
module_function
Ruby 的 module 是 method 和 constants 的集合。module 中的method 又可分為 instance method 和 module method, 當一個 module 被 include 進一個 class ,那么 module 中的 method (注：沒有被 module_function 標記的 method)就是 class 中的 instance method, instance method 需要所在的 class 被實例化之后才能被調用；被 module_function 標記的 method（不管該 method 是 public 或者 private）就是 module method 且 instance method 也會變成 private method，對于被 include 所在的 class 來說是 private method，object.module_name 會出錯。module method 都能被 module_name.method_name 調用，沒有被 module_function 標記的 public method 不能被 module_name.method_name 調用。
module 中的 module_function 會把 module 中的 method 變成 module method 且對于被 include 所在的 class 來說，module method 在 module 中是 private method 故 module_name.module_method 能調用，而不能被 object.module_name 調用。
module 中的 public method 對于被 include 所在的 class 來說是 instance method,故 object.public_method_in_module 能調用。如果想要非 module method 能夠被 module 調用(module_name.not_module_method) ，需要引入 extend self （下文會討論 extend self）
# test.rb
module MyModule
def public_meth
p "a public method, if the module is included to a class , can be call as object.public_meth"
end
def module_method
p "a module method,can be called as module_name.module_method. but can not be call as object.module_method"
end
private
def private_method_to_module_function
p "a private_method, but can be call as module_name.module_method, because it was assigned to module_function"
end
def private_method
p "I am a private method"
end
module_function :module_method, :private_method_to_module_function
end
MyModule.module_method
MyModule.private_method_to_module_function
begin
MyModule.public_meth
rescue
p "public method can not be called by module_name.public_meth"
end
begin
MyModule.private_method
rescue NoMethodError
p "private method can not be called by module_name.module_method"
end
class MyClass
include MyModule
end
obj = MyClass.new
obj.public_meth
begin
obj.private_method
rescue NoMethodError
p "private method in module can not be call by object.method_name"
end
begin
obj.module_method
rescue NoMethodError
p "module method can not be called by object.method_name, for object, module method is private instance method"
end
#調用
ruby test.rb
"a module method,can be called as module_name.module_method. but can not be call as object.module_method"
"a private_method, but can be call as module_name.module_method, because it was assigned to module_function"
"public method can not be called by module_name.public_meth"
"private method can not be called by module_name.module_method"
"a public method, if the module is included to a class , can be call as object.public_meth"
"private method in module can not be call by object.method_name"
"module method can not be called by object.method_name, for object, module method is private instance method"
總結就是
•The method will be copied to class' singleton class
•The instance method's visibility will become private
extend self
Include is for adding methods to an instance of a class and extend is for adding class methods
extend 本質是給 class 或者 module 添加 class method
extend self 讓 module 中的 instance method 能夠被 module_name.instance_method 調用，保留 module 中原本 method 的 public 或 private 屬性，但又不像 module_function 一樣把被標記的 method 變成 private 。
#!/usr/bin/env ruby
# encoding: utf-8
# test_extend.rb
module MyModule
extend self
def public_meth
p "a public_method extended by self can be called by module_name.public_meth and object.public_meth, included by a class"
private_method
end
private
def private_method
p "a private method, can be call in module internal"
end
end
class MyClass
include MyModule
end
MyModule.public_meth
begin
MyModule.private_method
rescue NoMethodError
p "private method in extend self module can not be called module_name.private_method"
end
obj = MyClass.new
obj.public_meth
begin
obj.private_method
rescue NoMethodError
p "private method can not be called by object.private_method"
end
# 調用 ruby test_extend.rb
"a public_method extended by self can be called by module_name.public_meth and object.public_meth, included by a class"
"a private method, can be call in module internal"
"private method in extend self module can not be called module_name.private_method"
"a public_method extended by self can be called by module_name.public_meth and object.public_meth, included by a class"
"a private method, can be call in module internal"
"private method can not be called by object.private_method"
總結就是：
•No method copying involved
•No changes to method visibility
總結
module_function 改變 module 內 原來 method 的 public/private 屬性并把改 method 變成 module method ,能夠被 module_name.module_method 調用。
extend self 就是在 module 自繼承，不改變 module 中 method 的 public/private 屬性，能夠被 module_name.public_method
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