Jetpack Compose - no recomposition, why?

im very confused now, because i thought that if a state changes or a composable is altered, like the text of a button, or the size, it would initiate a recomposition of that composable.

In my example there is a suspendfunction timerForCompose() which prints after a certain delay the message: "Timer ended". If the coroutine did stop it should print "Timer cancelled". The suspendfunction is called in the composableFunction Timer(), and will be executed in the lambda of the LaunchedEffect-Composable.

My MainComposablefunction is called TimerScreen1 and consits of a Button. On AppStart or Recomposition the size of the button changes and Timer() receives the initial delay time, provided by the state object timerDuration.

Here is the code:

The supend-function:

suspend fun timerForCompose(time: Long, onTimerEnd: () -> Unit){
delay(timeMillis = time)
onTimerEnd()
}

Timer Composable Function

@Composable
fun Timer(timerDuration: Long) {

LaunchedEffect(key1 = Unit){

    try {
        timerForCompose(timerDuration){
            println("Timer ended successfully")
        }
    }
    catch (ex: Exception){
        println("Timer canceled")
    }}  }

TimerScreen1 function:

@Composable

fun TimerScreen1() {

var dpForButton = remember {
    mutableStateOf(200.dp)
}

var timerDuration = remember {
    mutableLongStateOf(4000L)
}

    Button(onClick = { dpForButton.value = 400.dp }, modifier = Modifier.size(dpForButton.value)) {
        Text(timerDuration.value.toString())
        Timer(timerDuration = timerDuration.value + 2000L)

    } }

The problem: I dont understand why the ButtonComposable is not recomposing every time i click on the button. For me it would be logical that if i click, the whole Button gets recomposed and with that Timer()-function would be called again. But this does not happen.
It only recomposes if i rotate the screen. You might be wondering why timerDuration has a state. Its because the initial Code was like here:

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https://betterprogramming.pub/jetpack-compose-side-effects-launchedeffect-with-example-99c2f51ff463

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I just tried to break it down to that button, cause i expected a recomposition.

The documentation of MutableState says:

> When the value property is written to and changed, a recomposition of any subscribed RecomposeScopes will be scheduled. If value is written to with the same value, no recompositions will be scheduled.

So this is the problem - you always set the same value to dpForButton, so recomposition is not scheduled.

Another problem is that you are calling timerForCompose inside of LaunchedEffect with key = Unit, which means that the code inside of that effect will only run the first time it enters composition. If you want to run it every time it recomposes, you should use SideEffect.