Is it safe to combine a flow with something derived from itself? Stupid example: ```val data: Flow&l...

# coroutines

[natario1]

Is it safe to combine a flow with something derived from itself? Stupid example:

val data: Flow<String?> = ....
val hasData: Flow<Boolean> = data.map { it != null }
val combined = data.combine(hasData) { data, hasData -> 
    require(data == null || hasData)
}

Note that hasData does not suspend, and let's assume flow will be collected on a single thread dispatcher.

[ephemient]

it's "safe" but collects data twice, which seems unlikely to be what you want - nothing ensures the two collectors are in sync with each other

[natario1]

I see, thanks! data is shared so it should be fine. I know it's ugly, I'll change this at some point - for now I'm trying to debug an issue and wanted to rule this out

[Trevor Stone]

I think you want to use zip instead of combine here

[Trevor Stone]

combine might have timing issues and emit more values than you expect

[Nick Allen]

It can lead to bugs. A Flow is sequential. Multiple flows, however, are not and can be collected concurrently and there are no guarantees that they will be collected at a similar pace. In your example, data and hasData could be inconsistent.

[Trevor Stone]

Also why not just use an onEach instead?