kotlinkotlin-coroutineskotlin-flow

.flowOn(Dispatchers.Main) blocks the main thread

Why does this code block indefinitely?

fun main() = runBlocking {
    val myFlow: Flow<String> = flow {
        emit("abc")
    }.flowOn(Dispatchers.Main)

    myFlow.collect {
        println(it)
    }
}

Everything seems to work when choosing a different dispatcher for flowOn or when removing flowOn entirely. Since only a single value is emitted into the Flow, I would expect this function to return quickly in any case, though.

Solution

  • Core issue is you're using runBlocking which causes a deadlock. When you start the flow flowOn tries to re-post the work to main thread but it's already suspended (and blocked, since it's within runBlocking) by collect.