Suspend function blocks main thread

Question

I'm having a hard time understanding coroutines. This is a very simple setup. Both longComputation and delay are suspend functions. The first one blocks the main thread, the latter doesn't. Why?

CoroutineScope(Dispatchers.Main).launch {
    val result = longComputation() // Blocks
    delay(10_000) // Doesn't block
}

Answer 1

That depends. What does longComputation do exactly? When you mark a function as suspend, this does not mean you can't include blocking code in it. For instance, have a look at this one:

suspend fun blockingSuspendFunction(){
    BigInteger(1500, Random()).nextProbablePrime()
}

The code inside the suspend function is obviously something that utilizes the CPU and blocks the caller. By convention, this should not be done because if you call a suspend function, you expect it to not block the thread:

Convention: suspending functions do not block the caller thread. (https://medium.com/@elizarov/blocking-threads-suspending-coroutines-d33e11bf4761)

To make such a function "behave as a suspending function", the blocking has to be dispatched onto another worker thread, which (by recommendation) should happen with withContext:

suspend fun blockingSuspendFunction() = withContext(Dispatchers.Default) {
    BigInteger.probablePrime(2048, Random())
}