Hi guys, I have simple compose function for counte...

# compose-android

[KotlinLeaner]

Hi guys, I have simple compose function for counter logic. On counter logic I want to change float value. I tried some piece of code using derivedStateOf but it not working. Instead if I used simple condition logic it working fine.

[KotlinLeaner]

@Preview(showBackground = true)
@Composable
fun Experiment() {
    Surface {
        var count by remember { mutableStateOf(0) }
        ExperimentView(count) {
            count++
        }
    }
}

@Composable
fun ExperimentView(count: Int, onCountChange: () -> Unit) {
    Column {
        val floatValue by remember {
            derivedStateOf {
                if (count % 2 == 0) {
                    0.4F
                } else {
                    0.2F
                }
            }
        }
        Text(text = "$count with float value $floatValue")
        Spacer(modifier = Modifier.height(20.dp))
        Button(onClick = { onCountChange() }) {
            Text(text = "Button")
        }
    }
}

[KotlinLeaner]

Float value is never changing with the use of derivedStateOf. If I removed derivedStateOf it working fine. I am adding working example in below

@Composable
fun ExperimentView(count: Int, onCountChange: () -> Unit) {
    Column {
        val floatValue =
            if (count % 2 == 0) {
                0.4F
            } else {
                0.2F
            }

        Text(text = "$count with float value $floatValue")
        Spacer(modifier = Modifier.height(20.dp))
        Button(onClick = { onCountChange() }) {
            Text(text = "Button")
        }
    }
}

So why this is happening in this code? Thanks

[Stylianos Gakis]

derivedStateOf can only be notified for changes for MutableState, not just any value. In your case, your “count” variable is count: Int not count: MutableState<Int>.

[KotlinLeaner]

So what is the solution for this ?

[Stylianos Gakis]

Read: https://medium.com/androiddevelopers/jetpack-compose-when-should-i-use-derivedstateof-63ce7954c11b

[Stylianos Gakis]

For your very specific case, you can do something as simple as

@Composable
fun ExperimentView(count: Int, onCountChange: () -> Unit) {
    val countState by rememberUpdatedState(count)
    Column {
        val floatValue by remember {
            derivedStateOf {
                if (countState % 2 == 0) {
                    0.4F
                } else {
                    0.2F
                }
            }
        }

        Text(text = "$count with float value $floatValue")
        Spacer(modifier = Modifier.height(20.dp))
        Button(onClick = { onCountChange() }) {
            Text(text = "Button")
        }
    }
}

And that should work already, as now you’d be reading from a MutableState inside your derivedStateOf

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

Even if count were a snapshot state object, it wouldn’t make sense to use derivedStateOf here because the calculation is very simple, and the overhead of derivedStateOf would not be worth it.

[KotlinLeaner]

Thank you got it...