Why does the following code crash with a null exce...

# coroutines

[Thomas]

Why does the following code crash with a null exception?

val channel = ConflatedBroadcastChannel(Unit)
channel.asFlow().produceIn(GlobalScope).poll()!!

If I understand correctly this shouldn’t be happening because the channel has a default Unit value. The following code, however, does not crash:

val channel = ConflatedBroadcastChannel(Unit)
channel.openSubscription().poll()!!

I don’t get why this works but the first example doesn’t. Could someone please explain this behaviour?

[gildor]

In which thread do you run it? Because produceIn explicitly switch thread of Flow, so it may be not enough time to dispatched value

[gildor]

In theory there is shouldn't be context switch if you run it from other Default thread, but even in this case it will be asyncronously redispatched

[Thomas]

@gildor this was exactly the code I wrapped in a runBlocking { } call and put in in a unit test

[gildor]

Exactly, runBlocking uses current thread, GlobalScope will use Default dispatcher, so you have thread switch there, so you cannot get update immidately

[Thomas]

I get it now, appreciate your explanation.

I have a ConflatedBroadcastChannel as a flow in my code. In a unit test I want to check the values of the flow, so I convert the flow to a channel using produceIn so I could use poll(). I'm now using receive() which does do what I want.

[gildor]

Yes, you need suspend receive() for this

[gildor]

I convert the flow to a channel using produceIn so I could use poll()

But why do you need channel for this? You can just use suspend method single() to get 1 value from Flow

[Thomas]

@gildor the flow has more than one value so I need to check it multiple times throughout the unit test. As I understand, single() requires the flow to send one value and be closed or else it won't return. So that's why I'm using `poll()`/`receive()` for this, because I need to check the value (multiple times) without closing the flow.

If you know a better approach, please let me know because I might be missing something obvious.

[gildor]

I see what you mean

[gildor]

maybe than you could just convert all emits to list and assert it?

[Thomas]

@gildor I’m not sure how, as far as I know Flow.toList() won’t work either because it requires the flow to be closed or it won’t return.

[gildor]

yes it’s true. It’s possible to cover such cases, but probably we need some sort testing facilities