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?

Answer 1

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()     } }