在 Ruby 1.9 中访问类变量的正确方法是什么？

What is the proper way to access class variables in Ruby 1.9?

提问by Geoffrey H

I'm trying to set some class variables to store paths in a Rails application (but I think this more a ruby question)

我正在尝试设置一些类变量来存储 Rails 应用程序中的路径（但我认为这更像是一个红宝石问题）

Basically my class looks like this

基本上我的班级看起来像这样

class Image < ActiveRecord::Base

   @@path_to_folder = "app/assets"
   @@images_folder = "upimages"
   @@path_to_images = File.join(@@path_to_folder, @@images_folder)

end

But when I try to access @@path_to_imagesfrom my controller by doing Image.path_to_images, I get a NoMethodError

但是当我尝试@@path_to_images通过执行从我的控制器访问时Image.path_to_images，我得到一个 NoMethodError

When I try with Image.class_eval( @@path_to_images ), I get uninitialized class variable @@path_to_images in ImagesController

当我尝试使用时Image.class_eval( @@path_to_images )，我得到uninitialized class variable @@path_to_images in ImagesController

I've searched around and all I've seen says those would work, so I'm very confused about this

我四处搜索，我所看到的一切都说这些会起作用，所以我对此感到非常困惑

What's more, I tried defining simple classes with the ruby console like so

更重要的是，我尝试像这样用 ruby​​ 控制台定义简单的类

 class Bidule
     @@foo = "foo"
     Bar = "bar"
 end

And so I tried, I think, all the ways possible (previous 2 included) to access them but no way I always get an exception raised

所以我尝试了，我想，所有可能的方式（包括前 2 种）来访问它们，但我没有办法总是引发异常

回答by Soundar Rathinasamy

Rails provides class level attribute accessor for this functionality

Rails 为此功能提供了类级别的属性访问器

Try

尝试

class Image < ActiveRecord::Base
  cattr_accessor :path_to_folder
  @@path_to_folder = "app/assets"
end

Then to access path_to_folder class variable just use

然后访问 path_to_folder 类变量只需使用

Image.path_to_folder

But people always suggest to avoid class variables due to its behavior in inheritance.So you can use constants like

但是人们总是建议避免类变量，因为它在继承中的行为。所以你可以使用像这样的常量

class Image < ActiveRecord::Base
   PATH_TO_FOLDER = "app/assets"
end

Then you can access the constant like

然后你可以像这样访问常量

Image::PATH_TO_FOLDER

回答by Peter Alfvin

Although I wouldn't in general recommend it, you can access class variables by passing a string to class_evalas in:

虽然我一般不推荐它，但您可以通过将字符串传递给以下内容来访问类变量class_eval：

Image.class_eval('@@path_to_folder')

at least in later versions of Ruby.

至少在 Ruby 的更高版本中。

Note, however, that class variables are associated with the uppermost class in a subclassed hierarchy. In the case of ActiveRecord subclasses like this one, this means that these class variables really exist in the namespace of ActiveRecord::Base.

但是请注意，类变量与子类层次结构中最上层的类相关联。对于像这样的 ActiveRecord 子类，这意味着这些类变量确实存在于ActiveRecord::Base.

回答by rgtk

If you can't or don't want to extend class use:

如果您不能或不想扩展类使用：

Image.class_variable_get(:@@path_to_images)

回答by Sriram R

Best way is to set and get the value using methods. Below is a sample code

最好的方法是使用方法设置和获取值。下面是一个示例代码

class Planet
  @@planets_count = 0

  def initialize(name)
    @name = name
    @@planets_count += 1
  end

  def self.planets_count
    @@planets_count
  end  

  def self.add_planet
    @@planets_count += 1
  end

  def add_planet_from_obj
    @@planets_count += 1
  end
end


Planet.new("uranus")
Plant.add_planet
obj = Planet.new("earth")
obj.add_planet_from_obj

回答by tadman

Class variables are rarely used in Ruby applications because they have a lot of limitations and also tend to run against the grain of proper Object-Oriented design.

类变量在 Ruby 应用程序中很少使用，因为它们有很多限制，而且往往与正确的面向对象设计的粒度背道而驰。

In nearly every case a class variable can be replaced with a proper constant, a class method, or both.

几乎在所有情况下，类变量都可以用适当的常量、类方法或两者替换。

Your example is probably better described as:

您的示例可能更好地描述为：

class Image < ActiveRecord::Base
  PATH_TO_FOLDER = "app/assets"
  IMAGES_FOLDER = "upimages"
  PATH_TO_IMAGES = File.join(PATH_TO_FOLDER, IMAGES_FOLDER)
end

Class variables are private to the class in question and don't trickle down to sub-classes and are difficult to access from an external context. Using constants allows the use of things like:

类变量是相关类的私有变量，不会渗透到子类，并且难以从外部上下文访问。使用常量允许使用以下内容：

image_path = Image::PATH_TO_FOLDER

There are some circumstances under which a class variable is more reasonable than the alternative, but these are usually very rare.

在某些情况下，类变量比替代方案更合理，但这些通常非常罕见。

回答by Marco Prins

You can do that by wrapping it in a class method, like this:

您可以通过将其包装在类方法中来做到这一点，如下所示：

def self.path_to_images
  @@path_to_images
end

but I should mention that you should try to avoid using class variables in rails

但我应该提到你应该尽量避免在 rails 中使用类变量

回答by GMarx

I would modify it like this:

我会像这样修改它：

class Image < ActiveRecord::Base

  @@path_to_folder = "app/assets"
  @@images_folder = "upimages"
  @@path_to_images = File.join(@@path_to_folder, @@images_folder)

  def initialize
  end 
  ...
  def self.path_to_folder
    @@path_to_folder
  end
end

What you've done here is make the class variable into a method, so you can now access is using a .method name call. Since this is a class variable though, you can only call this on the class itself, not on the instance of a class. You are getting a 'NoMethodError' because you're calling the class variable using a method which has not exist. The code above defines this method on the lines where you say:

您在这里所做的是将类变量变成一个方法，因此您现在可以使用 .method 名称调用进行访问。由于这是一个类变量，您只能在类本身上调用它，而不能在类的实例上调用。您收到“NoMethodError”是因为您正在使用不存在的方法调用类变量。上面的代码在你说的行上定义了这个方法：

def self.path_to_folder
  @@path_to_folder
end

This will now work:

这现在可以工作：

Image.path_to_folder