I have a Compose Text that I want to update every ...

# codereview

[Travis Griggs]

I have a Compose Text that I want to update every second, because it shows a remaining time. Is the following the right/idiomatic way to express that?

var remaining by remember { mutableStateOf(keyInfo.remaining) }
LaunchedEffect(remaining) {
   delay(1.seconds)
   remaining = keyInfo.remaining
}
SettingsLabel(text = remaining.clockPrinted())

[ephemient]

there is no guarantee that the delay will resume or that the composition will be recomposed immediately - it is likely to drift

[ephemient]

what you actually want to do is adjust the delay based on the current time, so that it's independent of any additional delays elsewhere, such as

private fun Duration.truncatedTo(unit: Duration): Duration = unit.multipliedBy(this.dividedBy(unit))

@Composable
fun CountDownTimer(
    goal: Instant = Instant.ofEpochSecond(Int.MAX_VALUE.toLong()),
    tick: Duration = Duration.ofSeconds(1L),
    clock: Clock = Clock.systemUTC(),
) {
    val remaining by flow {
        do {
            val remaining = Duration.between(clock.instant(), goal)
            var next = remaining.truncatedTo(tick)
            if (next >= remaining) next -= tick
            next = next.coerceAtLeast(Duration.ZERO)
            delay((remaining - next).toMillis())
            emit(next)
        } while (next > Duration.ZERO)
    }.collectAsState(Duration.between(clock.instant(), goal).coerceAtLeast(Duration.ZERO).truncatedTo(tick))
    Text(text = remaining.toString())
}

[ephemient]

(Android has https://developer.android.com/reference/android/os/CountDownTimer which also has similar functionality)

[LeoColman]

Sharing a bit on how I solved this particular issue

[LeoColman]

https://github.com/LeoColman/Petals/blob/main/app/src/main/kotlin/br/com/colman/petals/hittimer/HitTimer.kt

[LeoColman]

Mainly this bit:

@IgnoredOnParcel
  val millisLeft = flow {
    while (true) {
      emit(calculateMillisLeft())
      delay(3)
    }
  }

[LeoColman]

The critical part is the calculateMillisLeft. As @ephemient pointed out, we want to avoid losing the small delays here and there.

[ephemient]

having a wakeup every 3ms is absolute overkill

[Davide Giuseppe Farella]

Honestly, I would deal with it in the ViewModel and submit the state in a flow

[ephemient]

Sure, but wherever you implement it, you have to deal with delay skew