Can I use a `StateFlow` in a `select` statement? The IDE doesn't seem to think `onAwait` exists.

# coroutines

[reactormonk]

Can I use a StateFlow in a select statement? The IDE doesn't seem to think onAwait exists.

[ross_a]

Yes, via a Channel

coroutineScope {
   val channel1 = stateFlow1.produceIn(this)
   val channel2 = stateFlow2.produceIn(this)

   // often whileSelect when dealing with streams
   select {
      channel1.onReceiveCatching { result -> }
      channel2.onReceiveCatching { result -> }
   }

   // Remember to close
   channel1.close()
   channel2.close()
}

There's not much point in an onAwait for StateFlow as it always has a value immediately accessible with .value - Unless you are combining it with filter in which case you could do

coroutineScope {
   
   val async1 = async { stateFlow1.filterNotNull().first() } 
   val async2 = async { stateFlow2.filterNotNull().first() } 

   select {
      async1.onAwait { result -> }
      async2.onAwait { result -> }
   }
}

[reactormonk]

Ah, nice, makes sense.

[reactormonk]

So in case I wanna wait for one of the three things to happen, this could work?

val transientToStandby = tvState.filter { it is TVState.TransientToStandby }.produceIn(this)
        val standby = tvState.filter { it is TVState.Standby }.produceIn(this)
        val timeout = produce<Unit> { delay(5.seconds); send(Unit) }
        select<Unit> {
            transientToStandby.onReceive {
            }
            standby.onReceive {
            }
            timeout.onReceive {
            }
        }

[ross_a]

yes it would, but a simpler approach would be

when (val result = withTimeoutOrNull(5.seconds) {
                    tvState.filter { it is TVState.TransientToStandby || it is TVState.Standby }.first()
                }) {
                    TVState.TransientToStandby -> {}
                    TVState.Standby -> {}
                    null -> {}
                }

[ross_a]

select also provides an onTimeout method if you don't want to create your own produce block

[reactormonk]

ah neat. I think I prefer the select version, reads better.