When using kotlin coroutines, how do I unit test a function that calls a suspend function?

Question

I have a class like this

class SomeClass {
    fun someFun() {
        // ... Some synchronous code
        async {
            suspendfun() 
        }
    }

    private suspend fun suspendFun() {
         dependency.otherFun().await()
         // ... other code
    }
}

I want to unit test someFun() so I wrote a unit test that looks like this:

@Test
fun testSomeFun() {
    runBlocking {
        someClass.someFun()
    }

    // ... verifies & asserts
}

But this doesn't seem to work because runBlocking doesn't actually block execution until everything inside runBlocking is done. If I test suspendFun() directly inside runBlocking it works as expected but I want to be able to test someFun() all together.

Any clue how to test a function with both sync and async code?

s1m0nw1 · Accepted Answer · 2019-02-26T23:16:19.267

21

Fixing async

As implemented, your someFun() will just “fire and forget” the async result. As a result, runBlocking does not make a difference in that test.

If possible, make someFun() return async's Deferred and then, in runBlocking, call await on it.

fun someFun(): Deferred<Unit> {
    // ... Some synchronous code
    return async {
        suspendFun()
    }
}

Then the test:

runBlocking {
    SomeClass().someFun().await()
}

This question/answer is a good resource for further information.

alternative: use launch

It's also possible to avoid async in favor of using suspend functions and a launch-created coroutine:

suspend fun someFun() {
    // ... Some synchronous code
    suspendFun()
}

private suspend fun suspendFun() {
    delay(1000)
    println("executed")
    // ... other code
}

The test uses launch and the outer runBlocking implicitly waits for its completion:

val myScope = GlobalScope
runBlocking {
    myScope.launch {
        SomeClass().someFun()
    }
}

edited Feb 26 '19 at 23:16

answered Jan 05 '18 at 18:27

s1m0nw1

76,759
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196

Does this mean that if you don't call `.await()` an async block will not propogaate any exceptions? – user1809913 Jan 05 '18 at 18:51
3

You shouldn't use `async` if you ignore its return value. You should use `launch` instead so you get unhandled exceptions thrown in the resuming thread. – Marko Topolnik Jan 05 '18 at 19:06
@MarkoTopolnik I see, I was misusing async when I should have been using launch. – user1809913 Jan 05 '18 at 19:19
3

Why `runBlocking { launch { ... }.join() }`? Can't you just use `runBlocking { ... }`? – msrd0 Jan 05 '18 at 23:03
@msrd0 you’re right, that has not always been the case though. I updated the code – s1m0nw1 Feb 26 '19 at 23:17
Using the second approach, would we also need to cancel myScope in the teardown to avoid test bleed? – user1228891 Jul 14 '19 at 13:03

When using kotlin coroutines, how do I unit test a function that calls a suspend function?

1 Answers1

Fixing async

alternative: use launch

Linked