模拟返回 fs2.Stream 的方法

Question

为什么不能用 Mockito 模拟来模拟返回 fs2.Stream 的方法？

Mockito 抱怨我试图返回 FreeC 而不是 Stream。为什么会这样，我怎样才能让它工作？

以下代码：

import cats.effect.IO
import fs2.Stream
import org.mockito.Mockito.when
import org.scalatest.FlatSpec
import org.scalatest.mockito.MockitoSugar

trait MyTrait {
  def method: Stream[IO, Int]
}

class TestSpec extends FlatSpec with MockitoSugar {

  val m: MyTrait = mock[MyTrait]
  val emptyReturn: Stream[IO, Int] = Stream.emits(List.empty[Int])

  when(m.method).thenReturn(emptyReturn)

  ...
}

给出这个错误：

TestSpec:
TestSpec *** ABORTED ***
  org.mockito.exceptions.misusing.WrongTypeOfReturnValue: Stream cannot be returned by method()
method() should return FreeC
***
If you're unsure why you're getting above error read on.
Due to the nature of the syntax above problem might occur because:
1. This exception *might* occur in wrongly written multi-threaded tests.
   Please refer to Mockito FAQ on limitations of concurrency testing.
2. A spy is stubbed using when(spy.foo()).then() syntax. It is safer to stub spies -
   - with doReturn|Throw() family of methods. More in javadocs for Mockito.spy() method.
  at TestSpec.<init>(TestSpec.scala:15)
  at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
  at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
  at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
  at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
  at java.lang.Class.newInstance(Class.java:442)
  at org.scalatest.tools.Framework$ScalaTestTask.execute(Framework.scala:435)
  at sbt.TestRunner.runTest$1(TestFramework.scala:76)
  at sbt.TestRunner.run(TestFramework.scala:85)
  at sbt.TestFramework$$anon$2$$anonfun$$init$$1$$anonfun$apply$8.apply(TestFramework.scala:202)
  ...

Answer 1

我不确定 mockito 方面，但根本没有必要使用 mocking；

val m: MyTrait = new MyTrait {
  def method: Stream[IO, Int] = Stream.emits(List.empty[Int]).covary[IO]
}

Answer 2

mockito 对模拟 (MyTrait) 的运行时检查和 fs2.Stream 作为 FreeC 的值类似乎发生了一些奇怪的事情。

一种解决方案（除了不模拟）是在测试实现的运行时强制 fs2.Stream 装箱。我通过将 MyTrait 转换为 MyTrait[F[_]] 并将 def 方法： Stream[IO, Int] 转换为 def 方法：F[Int] 来管理此问题

然后你模拟 MyTrait[Stream[IO, ?]]。

希望这个对你有帮助

自mockito-scala v1.0.3 起此问题已得到修复