Rails模块中的mattr_accessor是什么?

I couldn't really find this in Rails documentation but it seems like 'mattr_accessor' is the Module corollary for 'attr_accessor' (getter & setter) in a normal Ruby class.

我在Rails文档中找不到这个，但似乎“mattr_accessor”是普通Ruby类中“attr_accessor”(getter & setter)的模块推论。

Eg. in a class

如。在一个类

class User
  attr_accessor :name

  def set_fullname
    @name = "#{self.first_name} #{self.last_name}"
  end
end

Eg. in a module

如。在一个模块

module Authentication
  mattr_accessor :current_user

  def login
    @current_user = session[:user_id] || nil
  end
end

This helper method is provided by ActiveSupport.

这个助手方法由ActiveSupport提供。

2 个解决方案

#1

162

Rails extends Ruby with both mattr_accessor (Module accessor) and cattr_accessor (as well as _reader/_writer versions). As Ruby's attr_accessor generates getter/setter methods for instances, cattr/mattr_accessor provide getter/setter methods at the class or module level. Thus:

Rails通过mattr_accessor(模块访问器)和cattr_accessor(以及_reader/_writer版本)扩展了Ruby。当Ruby的attr_accessor为实例生成getter/setter方法时，cattr/mattr_accessor在类或模块级别提供getter/setter方法。因此:

module Config
  mattr_accessor :hostname
  mattr_accessor :admin_email
end

is short for:

的缩写:

module Config
  def self.hostname
    @hostname
  end
  def self.hostname=(hostname)
    @hostname = hostname
  end
  def self.admin_email
    @admin_email
  end
  def self.admin_email=(admin_email)
    @admin_email = admin_email
  end
end

Both versions allow you to access the module-level variables like so:

两个版本都允许您访问模块级的变量:

>> Config.hostname = "example.com"
>> Config.admin_email = "admin@example.com"
>> Config.hostname # => "example.com"
>> Config.admin_email # => "admin@example.com"

#2

Here's the source for cattr_accessor

这里是cattr_accessor的源代码。

And

和

Here's the source for mattr_accessor

这是mattr_accessor的源代码

As you can see, they're pretty much identical.

正如你所看到的，它们几乎是一样的。

As to why there are two different versions? Sometimes you want to write cattr_accessor in a module, so you can use it for configuration info like Avdi mentions.
However, cattr_accessor doesn't work in a module, so they more or less copied the code over to work for modules also.

为什么有两个不同的版本?有时您想在模块中编写cattr_accessor，因此可以将它用于Avdi提到的配置信息。但是，cattr_accessor不能在模块中工作，因此它们或多或少地将代码复制到模块中。

Additionally, sometimes you might want to write a class method in a module, such that whenever any class includes the module, it gets that class method as well as all the instance methods. mattr_accessor also lets you do this.

此外，有时您可能想要在模块中编写类方法，例如，每当任何类包含该模块时，它都会获得该类方法以及所有实例方法。mattr_accessor也允许您这样做。

However, in the second scenario, it's behaviour is pretty strange. Observe the following code, particularly note the @@mattr_in_module bits

然而，在第二个场景中，它的行为非常奇怪。观察下面的代码，特别注意@@mattr_in_module位。

module MyModule
  mattr_accessor :mattr_in_module
end

class MyClass
  include MyModule
  def self.get_mattr; @@mattr_in_module; end # directly access the class variable
end

MyModule.mattr_in_module = 'foo' # set it on the module
=> "foo"

MyClass.get_mattr # get it out of the class
=> "foo"

class SecondClass
  include MyModule
  def self.get_mattr; @@mattr_in_module; end # again directly access the class variable in a different class
end

SecondClass.get_mattr # get it out of the OTHER class
=> "foo"

#1

162

module Config
  mattr_accessor :hostname
  mattr_accessor :admin_email
end