Jetpack Compose simple number picker

ComposeNumberPicker.kt

@Composable
fun NumberPicker(
    state: MutableState<Int>,
    modifier: Modifier = Modifier,
    range: IntRange? = null,
    textStyle: TextStyle = LocalTextStyle.current,
    onStateChanged: (Int) -> Unit = {},
) {
    val coroutineScope = rememberCoroutineScope()
    val numbersColumnHeight = 36.dp
    val halvedNumbersColumnHeight = numbersColumnHeight / 2
    val halvedNumbersColumnHeightPx = with(LocalDensity.current) { halvedNumbersColumnHeight.toPx() }

    fun animatedStateValue(offset: Float): Int = state.value - (offset / halvedNumbersColumnHeightPx).toInt()

    val animatedOffset = remember { Animatable(0f) }.apply {
        if (range != null) {
            val offsetRange = remember(state.value, range) {
                val value = state.value
                val first = -(range.last - value) * halvedNumbersColumnHeightPx
                val last = -(range.first - value) * halvedNumbersColumnHeightPx
                first..last
            }
            updateBounds(offsetRange.start, offsetRange.endInclusive)
        }
    }
    val coercedAnimatedOffset = animatedOffset.value % halvedNumbersColumnHeightPx
    val animatedStateValue = animatedStateValue(animatedOffset.value)

    Column(
        modifier = modifier
            .wrapContentSize()
            .draggable(
                orientation = Orientation.Vertical,
                state = rememberDraggableState { deltaY ->
                    coroutineScope.launch {
                        animatedOffset.snapTo(animatedOffset.value + deltaY)
                    }
                },
                onDragStopped = { velocity ->
                    coroutineScope.launch {
                        val endValue = animatedOffset.fling(
                            initialVelocity = velocity,
                            animationSpec = exponentialDecay(frictionMultiplier = 20f),
                            adjustTarget = { target ->
                                val coercedTarget = target % halvedNumbersColumnHeightPx
                                val coercedAnchors = listOf(-halvedNumbersColumnHeightPx, 0f, halvedNumbersColumnHeightPx)
                                val coercedPoint = coercedAnchors.minByOrNull { abs(it - coercedTarget) }!!
                                val base = halvedNumbersColumnHeightPx * (target / halvedNumbersColumnHeightPx).toInt()
                                coercedPoint + base
                            }
                        ).endState.value

                        state.value = animatedStateValue(endValue)
                        onStateChanged(state.value)
                        animatedOffset.snapTo(0f)
                    }
                }
            )
    ) {
        val spacing = 4.dp

        val arrowColor = MaterialTheme.colors.onSecondary.copy(alpha = ContentAlpha.disabled)

        Arrow(direction = ArrowDirection.UP, tint = arrowColor)

        Spacer(modifier = Modifier.height(spacing))

        Box(
            modifier = Modifier
                .align(Alignment.CenterHorizontally)
                .offset { IntOffset(x = 0, y = coercedAnimatedOffset.roundToInt()) }
        ) {
            val baseLabelModifier = Modifier.align(Alignment.Center)
            ProvideTextStyle(textStyle) {
                Label(
                    text = (animatedStateValue - 1).toString(),
                    modifier = baseLabelModifier
                        .offset(y = -halvedNumbersColumnHeight)
                        .alpha(coercedAnimatedOffset / halvedNumbersColumnHeightPx)
                )
                Label(
                    text = animatedStateValue.toString(),
                    modifier = baseLabelModifier
                        .alpha(1 - abs(coercedAnimatedOffset) / halvedNumbersColumnHeightPx)
                )
                Label(
                    text = (animatedStateValue + 1).toString(),
                    modifier = baseLabelModifier
                        .offset(y = halvedNumbersColumnHeight)
                        .alpha(-coercedAnimatedOffset / halvedNumbersColumnHeightPx)
                )
            }
        }

        Spacer(modifier = Modifier.height(spacing))

        Arrow(direction = ArrowDirection.DOWN, tint = arrowColor)
    }
}

@Composable
private fun Label(text: String, modifier: Modifier) {
    Text(
        text = text,
        modifier = modifier.pointerInput(Unit) {
            detectTapGestures(onLongPress = {
                // FIXME: Empty to disable text selection
            })
        }
    )
}

private suspend fun Animatable<Float, AnimationVector1D>.fling(
    initialVelocity: Float,
    animationSpec: DecayAnimationSpec<Float>,
    adjustTarget: ((Float) -> Float)?,
    block: (Animatable<Float, AnimationVector1D>.() -> Unit)? = null,
): AnimationResult<Float, AnimationVector1D> {
    val targetValue = animationSpec.calculateTargetValue(value, initialVelocity)
    val adjustedTarget = adjustTarget?.invoke(targetValue)

    return if (adjustedTarget != null) {
        animateTo(
            targetValue = adjustedTarget,
            initialVelocity = initialVelocity,
            block = block
        )
    } else {
        animateDecay(
            initialVelocity = initialVelocity,
            animationSpec = animationSpec,
            block = block,
        )
    }
}

NumberPickerSample.kt

@Preview
@Composable
fun PreviewNumberPicker() {
    Box(modifier = Modifier.fillMaxSize()) {
        NumberPicker(
            state = remember { mutableStateOf(9) },
            range = 0..10,
            modifier = Modifier.align(Alignment.Center)
        )
    }
}

Has anyone made the picker fling after fast-dragging the picker? Now the picker is very crude - fast-dragging over the picker does not make the picker spin over next values. The picker scrolls only as long the user is dragging over it.

The fling should work I think 🤔 Maybe it's not that apparent as you want it to be. Try playing with the parameters inside fling extension. Mainly you should always fall inside the first branch with animateTo, so the following setup gives much more apparent look to the fling

animateTo(
    targetValue = adjustedTarget,
    animationSpec = spring(
        stiffness = Spring.StiffnessVeryLow,
    ),
    initialVelocity = 0f,
    block = block
)

Hey @vganin. Thanks for the great implementation. I want another implementation like this one but it should be automatically scrolled.
Imagine this: we have two cards with this number picker. one has 60 numbers and another one has 10. the animation speed for both of them should be 3000 ms and they should start simultaneously. Can you help me build this?

Hey @vganin. Thanks for the great implementation. I want another implementation like this one but it should be automatically scrolled. Imagine this: we have two cards with this number picker. one has 60 numbers and another one has 10. the animation speed for both of them should be 3000 ms and they should start simultaneously. Can you help me build this?

Do you mean like when you scroll one picker, then the other scrolls automatically as if you would scroll them simultaneously? Maybe sharing single MutableInteractionSource using remember between two pickers is what you want?

Hey @vganin Thanks for this amazing implementation, I want to ask you about the licence, Is it available under Apache 2.0 license?

Hey @vganin Thanks for this amazing implementation, I want to ask you about the licence, Is it available under Apache 2.0 license?

You're very welcome. Yes, it is. This code is actually a part of my open-source app, and the license can be found here.

There is a generic version here with wrapSelectorWheel parameter