Advanced MockK
- Kotlin course
- Advanced
- Lesson 45/83
- 5 min
Learn advanced MockK with coEvery and coVerify.
MockK: coEvery and coVerify
MockK is a popular mocking library for Kotlin. For suspend functions, coEvery defines behavior and coVerify checks the call was made.
Example: coEvery { repo.fetchUser(1) } returns "Alex". This makes repo.fetchUser(1) return "Alex". At the end, coVerify { repo.fetchUser(1) } verifies the call. MockK lets you safely mock suspend calls in tests.
Capturing arguments with slot
MockK's slot feature captures an argument passed to a mock. It is useful for callbacks and suspend calls.
Example: val idSlot = slot<Int>(); coEvery { repo.fetchUser(capture(idSlot)) } returns "Alex". After the call, idSlot.captured gives the value passed in. This pattern lets you verify the real argument value afterward.
Practice Mock a suspend repository
Define an interface UserRepo with method suspend fun fetchUser(id: Int): String. In main create a mock with mockk<UserRepo>(), write coEvery { repo.fetchUser(1) } returns "Alex", and inside a runTest block call println(repo.fetchUser(1)).
Starter code
import kotlinx.coroutines.test.runTest
import io.mockk.mockk
import io.mockk.coEvery
// UserRepo interface: kullanıcı getirme metodu
// in main create a mock, define behavior with coEvery, print inside runTestSample solution
import kotlinx.coroutines.test.runTest
import io.mockk.mockk
import io.mockk.coEvery
interface UserRepo {
suspend fun fetchUser(id: Int): String
}
fun main() {
val repo = mockk<UserRepo>()
coEvery { repo.fetchUser(1) } returns "Alex"
runTest {
println(repo.fetchUser(1))
}
}Expected output
Alex
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which MockK function defines the return value of a suspend function?
- given
- every
- when
- coEvery Correct answer
Explanation: coEvery is for suspend functions.
2 Complete the coVerify call
coEvery { repo.fetchUser(1) } returns "Alex"
// verify the call
____ { repo.fetchUser(1) }coEvery { repo.fetchUser(1) } returns "Alex"
// verify the call
coVerify { repo.fetchUser(1) }Explanation: coVerify verifies a suspend call.
3 Match the MockK parts
| Term | Meaning |
|---|---|
coEvery { } |
Behavior definition |
returns |
Return value |
coVerify { } |
Call verification |
slot<T>() |
Argument capture |