Hello everyone I am facing issues when writing unit tests where the object under test internally cal...

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[Prateek]

Hello everyone
I am facing issues when writing unit tests where the object under test internally calls an extension function.
Assume I am writing a unit for my method A inside of which a 'mock' object calls a extension function and this class and its extension function are provided by 3rd party library.
So when I write condition like
whenever(mockObj.callExtFunc()) return something
this would throw error something like:
java.lang.AbstractMethodError: Receiver class kotlin.jvm.functions.Function2$Subclass0 does not define or inherit an implementation of the resolved method 'abstract java.lang.Object invoke(java.lang.Object, java.lang.Object)' of interface kotlin.jvm.functions.Function2. at ....
Any thoughts.
Thank you.

[David Kubecka]

I recommend using mockk: https://mockk.io/#extension-functions

[Prateek]

ah, thanks, wasn't aware of this! This should help 👍

[Michael Krussel]

Other options are to see what the extension functions are doing and mock what they call or know that extensions functions are just static functions and mock the static functions.

I found mockk too slow when I tried to use it even though I like the API better than mockito.

[Prateek]

My codes uses HttpClient.post from Ktor which is defined as an extension function in Ktor.
So I have written conditions in my unit test as mentioned below:

mockkStatic("io.ktor.client.request.BuildersKt")

coEvery {
    httpClientMock.post(any<String>(), any<HttpRequestBuilder.() -> Unit>())
} coAnswers { httpResponseMock }

But I still get this error when executing coEvery block.
What could I be missing?

[Prateek]

Meanwhile I used ktor-client-mock and handled differently but would appreciate if I can get an answer for my problem above.

[Michael Krussel]

I'm not sure what's wrong with the above. My guess would be that BuildersKt should be buildersKt since the file is lowercase in GitHub. But using ktor-client-mock would be my choice of solution. In general using a mocking library is my last choice when creating a test, because it typically creates the most fragile tests.

[AdamW]

Was just about to write that you should use a client with MockEngine or the services block in your TestApplicationEngine instead of a mocking library for this, which should be the last resort in all circumstances.

[Prateek]

@Michael Krussel I too thought it could be 'buildersKt' but then it throws 'ClassNotFoundException' !😬

@AdamW Can you please elaborate a bit for my better understanding?

[AdamW]

MockEngine: https://ktor.io/docs/client-testing.html

External services: https://ktor.io/docs/server-testing.html#external-services

[Prateek]

got it, yeah, I am using MockEngine, thanks.