Hello I got a `Flow` like ```flow while true emit something kotlinlang #coroutines

Davide Giuseppe Farella

09/17/2020, 7:34 AM

Hello, I got a

Flow

flow {
    while (true) {
        emit(something)
        delay(interval)
    }
}.flowOn(injectedDispatcher)

In my test, I inject the dispatcher and test like

val result = mutableListOf<Int>()
val interval = 30.seconds
val flow = getSavedCredentials(interval)
val job = launch {
    flow.toList(result)
}

repo.add(1)
advanceTimeBy(interval)

repo.add(2)
advanceTimeBy(interval)

repo.add(3)
advanceTimeBy(interval)

So I would expect

result

[ 1 ], [ 1, 2 ], [ 1, 2, 3 ]

But instead it is

[ 1, 2, 3 ], [ 1, 2, 3 ], [ 1, 2, 3 ]

What’s wrong with it? 😵

09/17/2020, 7:50 AM

try not exact number, 30 for flow, use a bit gap between, like advanceTimeBy(interval + 5.seconds)

Davide Giuseppe Farella

09/17/2020, 7:51 AM

09/17/2020, 7:52 AM

could you provide a complete example on Kotlin Playground?

Davide Giuseppe Farella

09/17/2020, 7:53 AM

Sure, let me try to reproduce without dependants to project

Can I use coroutines-test on Playground?

May that help?

09/17/2020, 8:23 AM

Can I use coroutines-test on Playground? (edited)

Davide Giuseppe Farella

09/17/2020, 8:27 AM

How to? Deps are not resolved 😵

09/17/2020, 8:50 AM

ahh, TestCoroutineDispatcher wouldn’t be available, indeed, worth to request adding it to Junit run configuration

Davide Giuseppe Farella

09/17/2020, 8:54 AM

So is there something I can do? 🙂

09/17/2020, 8:56 AM

it’s because of closure of mutable property

emit(something)

emit(something.toList())

What is happening, is that by time when flow lambda is invoking, repo alredy changed, essentially when you call

repo += 3

flow is not even dispatched, because it’s blocking code (mutating repo) but launch and flow are asyncronous

also, to make your test pass, swap emit and delay, otherwise flow will emit empty list

Davide Giuseppe Farella

09/17/2020, 8:59 AM

Oh… This is tricky!! So that’s given by our mock! The signature of the method is

Map

, but our test implementation is a

MutableMap

under the hood!

also, to make your test pass, swap emit and delay, otherwise flow will emit empty list

Yup, noticed, May be better to delay the collecting