Double subscriptions on inner flow with kotlin-flow - kotlin

Question

I have the following problem with my code snippet below;
When the set of currently connected barcode-scanners changes, a set with the currently connected scanners will be emitted by the scanners: Flow<Set<BarcodeScanner>> flow.
If I have connected a single scanner, called A, and I scan a barcode this works fine and on the result will be shown on the text-view.
If I connect an additional device, and then scan with device A again, I will see the scan result 2 times.
It looks like that on every change of the Set<> an additional flow subscription is created in the collect { scanner.scan() } and the previous flow is still in place.
How can I solve this? I already played the whole day with different operators etc...
I'm using Flow 1.2.2
uiScope.launch { //CoroutineScope(Job() + Dispatchers.Main)
scanners //Flow<Set<BarcodeScanner>> will emit the set of scanner, currently connected scanners, when one connects/disconnects
.onEach {
txtView.text = "Barcode scanners: ${it.size}\n"
}
.flatMapMerge { // Flatten to Flow<BarcodeScanner>
it.asFlow()
}
.onEach { scanner ->
txtView.append("${scanner.name()} [${scanner.hashCode()}]\n")
}
.collect { scanner ->
//if a new Set<BarcodeScanner> is emitted, this Flow<Barcode> returned by scanner.scan() will be subscribed again, without removing the old stream
//resulting in multiple results on a single scan.
scanner.scan() //returns an Flow<Barcode>
.onEach { barcode ->
findViewById<TextView>(R.id.txtViewBarcodes).append("[${scanner.name()}]: ${barcode.value} (${barcode.type.description})\n")
}
.onCompletion {
Timber.i("[${scanner.name()}]: DISCONNECTED") //Never called
}
.launchIn(scanScope)
}
}

Answer 1

I think rather than:
.collect { scanner ->
scanner.scan()
.onEach { barcode ->
...
You should use:
.flatMapLatest { scanner ->
scanner.scan()
}
.onEach { barcode ->
...
Where the flow is launched in the same UI scope. The problem you have is launchIn(scanScope) is creating a completely separate coroutine that the outer coroutine can't cancel when a new set of barcodes is emitted.