rspec中的实例变量如何工作?

Here is a bit of code from M Hartl's Ruby on Rails Tutorial. Can anyone explain why an instance variable (@user) is necessary and why not use a local variable. Also, since instance variables are supposed to be the variables in the instance of a class, which class is @user instantiated from?

这里有一些来自M Hartl的Ruby on Rails教程的代码。谁能解释为什么需要实例变量(@user)，为什么不使用局部变量?而且，既然实例变量应该是类实例中的变量，那么@user是从哪个类实例化的呢?

require 'spec_helper'

describe User do

  before { @user = User.new(name: "Example User", email: "user@example.com") }

  subject { @user }

  it { should respond_to(:name) }
  it { should respond_to(:email) }
end

3 个解决方案

#1

Use of a local variable in that instance would mean that its scope would be restricted to the before and hence result in an error. The @user is of type User but is an instance variable of the describe block. Rspec has some magic that at run-time makes a class out of each describe block. Each example (it block) ends up being a subclass of said class. Class inheritance lets the examples see @user.

在该实例中使用局部变量将意味着其范围将被限制在之前，从而导致错误。@user类型为User，但是是description块的实例变量。Rspec有一些神奇之处，即在运行时利用每个description块创建类。每个示例(it块)最后都是类的子类。类继承让示例看到@user。

Edited 2017-05-14

编辑2017-05-14

Linked blog post is no longer available. Updating with Wayback Machine link + inlining relevant section here.

链接的博客文章不再可用。使用回线机链接+在这里插入相关部分进行更新。

Note that this is considered an anti-pattern as detailed in this blog post. Use let instead.

注意，这被认为是一个反模式，在这篇博文中有详细的介绍。使用我们。

let has the following advantages:

让我们拥有以下优势:

It is memoized when used multiple times in one example, but not across examples.
当在一个示例中多次使用它时，它是可记忆的，但不能跨示例使用。
It is lazy-loaded, so you wont waste time initializing the variable for examples that don't reference it.
它是延迟加载的，所以您不会浪费时间为不引用变量的示例初始化变量。
Will raise an exception if you have a typo in your variable name.
如果您的变量名中有一个输入错误，将引发异常。

#2

You can't use a local variable because a local variable exists only in the scope of the local method. before, subject and it generates different scopes within the same class.

您不能使用局部变量，因为局部变量仅存在于本地方法的范围内。在此之前，subject和它在同一个类中生成不同的范围。

The following code

下面的代码

before { user = User.new(name: "Example User", email: "user@example.com") }

will raise an undefined variable when you call it in

当你调用一个未定义的变量时

subject { user }

The instance @user is an instance of the class User (after all, you create it with User.new).

实例@user是类User的一个实例(毕竟，您使用User.new创建它)。

However, instead of instance variables you might want to use the let command. Also, if you define

但是，您可能希望使用let命令而不是实例变量。另外,如果您定义

subject { User.new(name: "Example User", email: "user@example.com") }

the use of before is not required. You'll also get the extra benefit to get a subject method available to access the instance, equal to define let(:subject).

使用之前是不需要的。您还将获得额外的好处，使subject方法可以访问实例，等于定义let(:subject)。

#3

subject ad it blocks are under different scopes, so local variables won't work. @user belongs to class generated by RSpec under the hood.

主题广告在不同的范围内，所以局部变量不起作用。@user属于RSpec在hood下生成的类。

#1