I'm using flow `combine` the first time trying to ...

# coroutines

[Lilly]

I'm using flow combine the first time trying to combine multiple flows to an single object. One of the flows return items which are part of a List. How can I get access to the previous combined object to accumulate the items of the list?

[ephemient]

one option is to wrap the flows with something like

fun <T, R> Flow<T>.withHistory(
    maxSize: Int,
    transform: suspend (List<T>) -> R,
): Flow<R> {
    require(maxSize > 0)
    return flow {
        val window = ArrayDeque<T>(maxSize)
        collect {
            if (window.size == maxSize) window.removeFirst()
            window.addLast(it)
            emit(window.toList())
        }
    }
}

[ephemient]

if you only need one previous item,

fun <T, R> Flow<T>.zipWithPrevious(
    transform: suspend (T?, T) -> R,
): Flow<R> = flow {
    var prev: T? = null
    collect { cur ->
        emit(transform(prev, cur))
        prev = cur
    }
}

[Lilly]

Thanks @ephemient Another related question about `combine`: I use it to combine 4 Flows, 2 Flows come from BroadcastReceivers so they do not emit that frequently. In my logs, I can see that one of the Flows emit but the transform block of combine is not triggered. Does combine has some hidden behavior?

[Lilly]

fun startListening() {
        combine(
            bluetoothLocal.discoverDevices(),
            bluetoothLocal.discoveryState(), // emits a value but transform block is not triggered
            bluetoothLocal.bluetoothAdapterState(),
            protocolState,
        ) { discoveredDevice, isScanning, isBluetoothEnabled, protocolState ->
            ....
       }.launchIn(scope)
     }

I would expect that transform block is called everytime one of the flows emit a value, isnt that the case?

[Lilly]

ok I got it. combine expects initial values for every flow before transform is triggered. bluetoothLocal.bluetoothAdapterState() does not has an initial value. When I emit an initial value like bluetoothLocal.bluetoothAdapterState().onStart { emit(bluetoothLocal.isEnabled) } it works like expected.