Invokes the given action when the given flow is completed o kotlinlang #coroutines

"Invokes the given action when the given flow is completed or cancelled, passing the cancellation exception or failure as cause parameter of action" -> Can some1 explain this documentation for flow.onComplete for me. I don't understand the last part "or failure as cause parameter of action", since an uncaught exception abort the stream. What is this failure refering to ?

For example:

@Test
    fun `flow test`() = runBlockingTest{
        Assert.assertEquals(
            flow<Int>{
                throw IllegalStateException()
            }
                .map { 1 }
                .onCompletion { 2 }
                .toList(),
            listOf(2)
        )
    }

    @Test
    fun `flow test2`() = runBlockingTest{
        Assert.assertEquals(
            flow<Int>{
                throw IllegalStateException()
            }
                .catch {  }
                .map { 1 }
                .onCompletion { if(it != null) emit(2) }
                .toList(),
            listOf<Int>()
        )
    }

The first test crashes, the second test passes. So in what scenario does onComplete emits any exception other than cancellation ?

julian

05/23/2020, 3:06 AM

@myanmarking I haven't used Flows. But from reading the

onCompletion

docs,

onCompletion

doesn't emit an exception. The docs are just saying that the upstream exception (failure) gets passed as the

cause

argument of the

action

(lambda) argument that you provide to

onCompletion

onCompletion

behaves like

finally

, so the first test still crashes even with

onCompletion

applied. Furthermore, flow expects

action

to return

Unit

. So it doesn't matter that you have

in the lambda:

{ 2 }

. That has no effect. You'd need to

emit

to have an effect.

The second test doesn't

map

because you

emit

nothing in the

catch

lambda. In other words, the flow to this point is empty. So nothing to map. And

isn't emitted because the value of

it

in the

onCompletion

action

lambda is the upstream exception:

if(it != null) emit(2) }

. The upstream exception isn't null, so

emit(2)

isn't executed. So the list is empty. So the test passes.

That's how it all looks to me.

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