如何在 Rust 中模拟特定方法但不是所有方法? [英] How to mock specific methods but not all of them in Rust?

问题描述

我在为目标结构的方法确定单元测试时遇到了麻烦.

我有一个方法 random_number 根据结构的属性返回一个随机值，还有另一个方法 plus_one 采用第一个方法的结果并执行有它的东西:

pub struct RngTest {酒馆属性:u64，}实现 RngTest {pub fn random_number(&self) ->u64 {让随机= 42；//假设它是随机的返回随机 * self.attr;}pub fn plus_one(&self) ->u64 {返回 self.random_number() + 1;}}

对第一种方法进行单元测试，测试另一种方法的策略是什么?我想为 plus_one() 的单元测试模拟 self.random_number() 输出，以便在单元测试中拥有健全的代码.有一篇很好的帖子比较了 不同的模拟库 并得出结论(很遗憾)它们都不是从其他人中脱颖而出真的很好.

我在阅读这些库的说明时学到的唯一一件事是，我可以模拟方法的唯一方法是将它们移动到一个 trait.我在这些库中没有看到任何示例(我查看了其中的 4 或 5 个)，它们测试了与此类似的案例.

在将这些方法移动到特征(即使它们是)之后，我如何模拟 random_number 以对 RngTest::plus_one 的输出进行单元测试?

pub trait SomeRng {fn random_number(&self) ->u64 {让随机= 42；//假设它是随机的返回随机 * self.attr;}fn plus_one(&self) ->u64 {返回 self.random_number() + 1;}}impl SomeRng for RngTest {}

解决方案

如何在 Rust 中模拟特定方法而不是所有方法?

正如您已经了解到的，您不能替换类型上的方法.您唯一能做的就是将方法移动到特征，然后提供该特征的生产和测试特定的实现.您如何构建特征决定了您能够测试的粒度.

具有默认实现的特征

根据您的用例，您也许可以使用默认实现:

trait SomeRng {fn random_number(&self) ->u64;fn plus_one(&self) ->u64 {self.random_number() + 1}}结构 RngTest(u64);为 RngTest 实现 SomeRng {fn random_number(&self) ->u64 {自我.0}}#[测试]fn plus_one_works() {让 rng = RngTest(41);assert_eq!(rng.plus_one(), 42);}

这里，random_number 是必需的方法，但plus_one 有一个默认实现.实现 random_number 默认为您提供 plus_one.如果可以更有效地执行，您也可以选择实现 plus_one.

真正的 rand crate 有什么作用?

真正的rand crate 使用两个特征:

• Rng

  pub trait Rng: RngCore {/* ... */}

  <块引用>

  RngCore 上自动实现的扩展特征，为采样值和其他便利方法提供高级通用方法.

• RngCore

  pub trait RngCore {/* ... */}

  <块引用>

  随机数生成器的核心.

这将实现的核心有趣部分与辅助方法分开.然后您可以控制核心并测试助手:

trait SomeRngCore {fn random_number(&self) ->u64;}特征 SomeRng: SomeRngCore {fn plus_one(&self) ->u64 {self.random_number() + 1}}impl<R: SomeRngCore>用于 R {} 的 SomeRng结构 RngTest(u64);为 RngTest 实现 SomeRngCore {fn random_number(&self) ->u64 {自我.0}}#[测试]fn plus_one_works() {让 rng = RngTest(41);assert_eq!(rng.plus_one(), 42);}

I have troubles figuring out unit tests for the methods of the target struct.

I have a method random_number that returns a random value based on the attribute of the struct and there is another method plus_one that takes the result of the first method and does something with it:

pub struct RngTest {
    pub attr: u64,
}

impl RngTest {
    pub fn random_number(&self) -> u64 {
        let random = 42; // lets pretend it is random
        return random * self.attr;
    }

    pub fn plus_one(&self) -> u64 {
        return self.random_number() + 1;
    }
}

Having a unit test for the first method, what is the strategy to test the other? I want to mock self.random_number() output for the unit test of plus_one() to have sane code in unit tests. There is a nice post that compares different mocking libraries and concludes (sadly enough) that none of them is really good to stand out from the others.

The only thing I learned while reading instructions for these libraries is that the only way I can mock methods is by moving them to a trait. I didn't see any example in these libraries (I looked at 4 or 5 of them) where they test a case similar to this.

After moving these methods to a trait (even as they are), how do I mock random_number to unit test the output of RngTest::plus_one?

pub trait SomeRng {
    fn random_number(&self) -> u64 {
        let random = 42; // lets pretend it is random
        return random * self.attr;
    }

    fn plus_one(&self) -> u64 {
        return self.random_number() + 1;
    }
}

impl SomeRng for RngTest {}

解决方案

How to mock specific methods but not all of them in Rust?

As you have already learned, you cannot replace methods on a type. The only thing you can do is move the methods to a trait and then provide production and test-specific implementations of that trait. How you structure the trait determines the granularity of what you are able to test.

Trait with a default implementation

Depending on your use case, you might be able to use a default implementation:

trait SomeRng {
    fn random_number(&self) -> u64;

    fn plus_one(&self) -> u64 {
        self.random_number() + 1
    }
}

struct RngTest(u64);
impl SomeRng for RngTest {
    fn random_number(&self) -> u64 {
        self.0
    }
}

#[test]
fn plus_one_works() {
    let rng = RngTest(41);
    assert_eq!(rng.plus_one(), 42);
}

Here, random_number is a required method, but plus_one has a default implementation. Implementing random_number gives you plus_one by default. You could also choose to implement plus_one if you could do it more efficiently.

What does the real rand crate do?

The real rand crate uses two traits:

• Rng

  pub trait Rng: RngCore { /* ... */ }
  

  An automatically-implemented extension trait on RngCore providing high-level generic methods for sampling values and other convenience methods.

• RngCore

  pub trait RngCore { /* ... */ }
  

  The core of a random number generator.

This splits the core interesting parts of the implementation from the helper methods. You can then control the core and test the helpers:

trait SomeRngCore {
    fn random_number(&self) -> u64;
}

trait SomeRng: SomeRngCore {
    fn plus_one(&self) -> u64 {
        self.random_number() + 1
    }
}

impl<R: SomeRngCore> SomeRng for R {}

struct RngTest(u64);
impl SomeRngCore for RngTest {
    fn random_number(&self) -> u64 {
        self.0
    }
}

#[test]
fn plus_one_works() {
    let rng = RngTest(41);
    assert_eq!(rng.plus_one(), 42);
}

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