I have a `Flow<Product>` which emits one by one, how I make it to collect those in list so I ...

# coroutines

[Mohamed Ibrahim]

I have a Flow<Product> which emits one by one, how I make it to collect those in list so I could receive a list in collect()

[louiscad]

You can create a custom operator that creates an empty mutable list before collecting the source flow, and emits the list after each collected element added. You can also use scan.

[Mohamed Ibrahim]

what do you think of this

fun <T> Flow<T>.intoList(): Flow<List<T>> {
    return flow {
        val bag = arrayListOf<T>()
        collect {
            bag.add(it)
        }
        emit(bag)
    }
}

[streetsofboston]

Note that works only if the source Flow ends/completes. Otherwise, nothing will be ever emitted

[Mohamed Ibrahim]

why

[streetsofboston]

If the source Flow does not end, your call to collect will suspend forever. It will keep adding items to bag, but emit(bag) will never be called in that case

[streetsofboston]

• Your source Flow starts as soon as your code calls collect on it, and your collect call suspends.
• The lambda provided to your collect call will be called each time your source/producer Flow calls emit.
• In the mean time your call to collect remains suspended.
• Your call to collect resumes when the lambda provided to your source/producer Flow returns/*ends*.

[louiscad]

@Mohamed Ibrahim You need to call emit after each call to add inside the collect lambda.

[streetsofboston]

That would be like a scan operator, emitting a growing list each time. I think he was more thinking about a final terminating (and suspending) toList() method on the source Flow…. but I could be wrong…

[louiscad]

That one already exists @streetsofboston

[streetsofboston]

I wasn't sure it already existed...😀
But it seems the original poster has some issue with how Flows and their collection exactly works...

[Mohamed Ibrahim]

The issue with me that if I used toList() I have to use try catch. and may I still have RxJava concepts in mind. And not breaking the stream was my purpose

[Mohamed Ibrahim]

I don't know if that even correct for Flow

[Mohamed Ibrahim]

The scan operator is very handy
here is an example of it

val toList = 
flowOf(1, 2,3).scan(emptyList<Int>()) { acc, value -> acc + value }.toList()

[streetsofboston]

Yup, if you are looking on how that list grows for each emission, using scan is a great way to do so.

But if you just need get one final list of all emissions (without any intermediate ones), toList() is your friend, but that requires the flow to end (since a list can only contain a finite amount of items).