I'm trying to implement a timer using a `LaunchedE...

# compose

[Sean Proctor]

I'm trying to implement a timer using a LaunchedEffect, but it seems that LaunchedEffect gets canceled on recompose.

@Composable
fun TimeDisplay(viewModel: TimeDisplayViewModel) {
  val properties: Map<String, Long> by viewModel.properties.collectAsState()
  var timeRemaining by rememeber { mutableStateOf(0) }
  
  Text(timeRemaining.toString)
  
  val timeProperty = properties["time"]
  LaunchedEffect(timeProperty) {
    val currentTime = System.currentTimeMillis()
    while (timeProperty > currentTime) {
      timeRemaining = timeProperty - currentTime
      delay(1000L)
    }
    timeRemaining = 0L
  }
}

The LaunchedEffect is always canceled during the delay. properties is updated with an unrelated value.

[Stylianos Gakis]

How does LaunchEffect check for key equality? If it’s referential equality, timeProperty seems to be re-calculated on every recomposition and that could cancel the previous LaunchedEffect and launch a new one, meaning that the previous coroutine would be canceled. The first time it suspends and finds an opportunity to stop working is inside the delay function.
Could you try replacing

val timeProperty = properties["time"]

with

val timeProperty by remember(properties) {
    properties["time"]
}

And see if that helps?
This is a wild guess, but that’s what I’d try myself first

[Sean Proctor]

I thought it was value equality, but I haven't checked. It would actually be fine if it stopped the old instance and launched a new one, but it just stops the old one, so I think something else is going on. I ended up moving it into a flow in the view model, which I guess might be a cleaner solution anyway. Thanks for your answer. I still wonder why the LaunchedEffect was canceled and not re-launched.

[Stylianos Gakis]

Btw now that I read it again. Your loop seems to never really end though. You are checking for timeProperty and currentTime which both don’t seem to change inside the while loop. Shouldn’t the currentTimeMillis() be taken again after the delay?

[Sean Proctor]

Yeah, you're right about that. I think it was a mistake in translation from the actual code, but maybe not.

[Stylianos Gakis]

Guess we’ll never know 😅