前言
滚轮应该是咱们很经常用到一个控件了,比如日期挑选,时刻挑选,地区挑选等都习惯用滚轮来展现。
滚轮控件的识点
上图是由三个滚轮控件组成的日期挑选器,以此咱们分析所需求的知识点:
- 手势(滑动,惯性翻滚)
- 内容循环翻滚
- 完成滚轮款式
手势(滑动,惯性翻滚)
我首先想到的是 Compose 的 翻滚修饰符 作用如下:
接下来便是处理惯性问题了,我很天然想到列表组件 LazyColumn
就预备去看它的源码是怎样完成时。
好吧,就决议是你了 LazyColumn
内容循环翻滚
既然决议使用 LazyColumn
那内容循环也变的简略,这儿直接贴代码:
val size = data.size
val count = Int.MAX_VALUE
val startIndex = count / 2
val listState = rememberPagerState(initialPage = startIndex - startIndex % size)
LazyColumn(
modifier = Modifier,
state = listState,
flingBehavior = rememberSnapFlingBehavior(listState),
) {
items(count) { index ->
......
}
}
完成滚轮款式
如何通过调整 LazyColumn
的 Item 项款式完成滚轮作用呢,这儿放一张我画的草图:
由上面的草图咱们发现关键点在 Item 项的旋转视点和平移间隔。
原理居然比草图还简略。
既然如此咱们先拿到 Item 项的滑动时的偏移间隔,直接贴代码:
val listState = rememberPagerState(initialPage = startIndex - startIndex % size)
val layoutInfo by remember { derivedStateOf { listState.layoutInfo } }
LazyColumn(
modifier = Modifier,
state = listState,
flingBehavior = rememberSnapFlingBehavior(listState),
) {
items(count) { index ->
val item = layoutInfo.visibleItemsInfo.find { it.index == index }
if (item != null) {
val itemCenterY = item.offset + item.size / 2 //获取Item项的偏移间隔
}
}
}
通过偏离间隔计算调整系数。
/**
* pickerCenterLinePx 滚轮控件中线
* itemCenterY < pickerCenterLinePx 说明Item项在上半部,逐步缩小,反之则逐步扩大
**/
currentsAdjust = 0.75f + 0.25f * if (itemCenterY < pickerCenterLinePx) {
itemCenterY / pickerCenterLinePx
} else {
1 - (itemCenterY - pickerCenterLinePx) / pickerCenterLinePx
}
最后按照惯例贴上完好代码。
@OptIn(ExperimentalFoundationApi::class)
@Composable
fun <T> WheelPicker(
data: List<T>,
selectIndex: Int,
visibleCount: Int,
modifier: Modifier = Modifier,
onSelect: (index: Int, item: T) -> Unit,
content: @Composable (item: T) -> Unit,
) {
BoxWithConstraints(modifier = modifier, propagateMinConstraints = true) {
val density = LocalDensity.current
val size = data.size
val count = size * 10000
val pickerHeight = maxHeight
val pickerHeightPx = density.run { pickerHeight.toPx() }
val pickerCenterLinePx = pickerHeightPx / 2
val itemHeight = pickerHeight / visibleCount
val itemHeightPx = pickerHeightPx / visibleCount
val startIndex = count / 2
val listState = rememberLazyListState(
initialFirstVisibleItemIndex = startIndex - startIndex.floorMod(size) + selectIndex,
initialFirstVisibleItemScrollOffset = ((itemHeightPx - pickerHeightPx) / 2).roundToInt(),
)
val layoutInfo by remember { derivedStateOf { listState.layoutInfo } }
LazyColumn(
modifier = Modifier,
state = listState,
flingBehavior = rememberSnapFlingBehavior(listState),
) {
items(count) { index ->
val currIndex = (index - startIndex).floorMod(size)
val item = layoutInfo.visibleItemsInfo.find { it.index == index }
var currentsAdjust = 1f
if (item != null) {
val itemCenterY = item.offset + item.size / 2
currentsAdjust = 0.75f + 0.25f * if (itemCenterY < pickerCenterLinePx) {
itemCenterY / pickerCenterLinePx
} else {
1 - (itemCenterY - pickerCenterLinePx) / pickerCenterLinePx
}
if (!listState.isScrollInProgress
&& item.offset < pickerCenterLinePx
&& item.offset + item.size > pickerCenterLinePx
) {
onSelect(currIndex, data[currIndex])
}
}
Box(
modifier = Modifier
.fillMaxWidth()
.height(itemHeight)
.graphicsLayer {
alpha = currentsAdjust
scaleX = currentsAdjust
scaleY = currentsAdjust
rotationX = (1 + currentsAdjust) * 180
},
contentAlignment = Alignment.Center,
) {
content(data[currIndex])
}
}
}
}
}
private fun Int.floorMod(other: Int): Int = when (other) {
0 -> this
else -> this - floorDiv(other) * other
}
Thanks
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谢谢~~
源代码地址
- WheelPicker.kt miaowmiaow/fragmject