I'm creating a flow out of snapshot state. ```val listFlow = snapshotFlow { appState.myList } viewMo...

# compose

[Colton Idle]

I'm creating a flow out of snapshot state.

val listFlow = snapshotFlow { appState.myList }
viewModelScope.launch {
  listFlow.collectLatest {
Log...

but even when I clear the list and addAll to it, my flow doesn't get fired again (I don't see my log). Am I misunderstanding snapshotFlow, or should this work?

[Stylianos Gakis]

What's the type of myList?

[Colton Idle]

mutableStateListOf<String>

[Stylianos Gakis]

Right, when we use stateFlow, we have to use it with T where T has to be backed by a MutableState so that its observers get notified (this case the snapshotFlow has subscribed to see its changes and is observing that value), instead of passing a State<T> in the stateFlow lambda. That’s my understanding of it.
So this does not work for `snapshotFlow`:

val state: State<String> = produceState("") {
  while(isActive) {
    value = "one"
    delay(1.seconds)
    value = "two"
    delay(1.seconds)
  }
}
LaunchedEffect(Unit) {
  snapshotFlow { state }.collectLatest {
    println("state:$state")
  }
}

But this does:

val state: String by produceState("") {
  while(isActive) {
    value = "one"
    delay(1.seconds)
    value = "two"
    delay(1.seconds)
  }
}
LaunchedEffect(Unit) {
  snapshotFlow { state }.collectLatest {
    println("state:$state")
  }
}

In your case you got the type SnapshotStateList<T> but you want to use snapshotFlow on your <T> instead.

This does not work

val state: SnapshotStateList<String> = mutableStateListOf("One", "Two")
LaunchedEffect(Unit) {
  while (isActive) {
    delay(1.seconds)
    state.add("one")
    delay(1.seconds)
    state.add("two")
    delay(1.seconds)
    state.clear()
  }
}
LaunchedEffect(Unit) {
  snapshotFlow { state }.collectLatest {
    println("state:$state")
  }
}

But this does

This does not work
val state: SnapshotStateList<String> = mutableStateListOf("One", "Two")
LaunchedEffect(Unit) {
  while (isActive) {
    delay(1.seconds)
    state.add("one")
    delay(1.seconds)
    state.add("two")
    delay(1.seconds)
    state.clear()
  }
}
LaunchedEffect(Unit) {
  snapshotFlow { state.toList() }.collectLatest {
    println("state:$state")
  }
}

[Albert Chang]

See this thread. Read the values of the list in the lambda of snapshotFlow.

[Stylianos Gakis]

Can’t believe I didn’t think of just .toList() 😅
So for your original post do instead

val listFlow = snapshotFlow { appState.myList.toList() }
viewModelScope.launch {
  listFlow.collectLatest {
Log...

[Colton Idle]

No way. All I had to do was .toList() Oh man. so much time wasted debugging this yesterday. lmaooo

[Stylianos Gakis]

I was looking into the SnapshotStateList class to see if something that can extract the values exists and there wasn’t anything in there aside from get(index).
But my brain completely forgot to make the connection that SnapshotStateList is in fact a MutableList which is a List which is a Collection which is a Iterable which you can call the public fun <T> Iterable<T>.toList(): List<T> function on it.

Felt like I was back in the CS class on this one, learning Java inheritance and interfaces 😅

[Colton Idle]

I don't really know if this is what happens but I would think .toList() is "wasteful" because... It's already a list so why waste the time converting it to a list again?

[Stylianos Gakis]

I don’t think this converts anything in a wasteful manner. It simply gets the underlying list that is saved inside the SnapshotStateList and since you’re doing it inside the snapshowFlow it also subscribes to any changes it may get.
Could be wrong, but I wouldn’t even think about it.

[Chrimaeon]

Memory-wise it is wastfull. toList() will create a new list with the same content.

[Chrimaeon]

https://github.com/JetBrains/kotlin/blob/ab3d3edb2bfa51698b6c4bc945fa012e76627181/libraries/stdlib/common/src/generated/_Collections.kt#L1334

[Stylianos Gakis]

Yeah but what other option does one have in this case?

[Chrimaeon]

I’d probably change my architechture. Not sure about your use case. But sounds kind of hacky what you are doing. Why do you meet the state then in the first place if you use it as a flow. There is channelFlow builder that you can use to emit your “state”.

[Zach Klippenstein (he/him) [MOD]]

You can use snapshotFlow as the full restart scope. Eg you could do something like this:

viewModelScope.launch {
  snapshotFlow {
    appState.myList.forEach // or w/e
    // put all your list processing here
    // return type doesn't matter
  }.collect()
}

[Colton Idle]

Interesting. I just want to know when the list is cleared or items are added. So maybe a for each will get the job done.