php小編草今天為大家介紹一款非常實用的功能－LazyColumn 引用回到起始位置。在開發過程中，我們經常需要建立一個垂直滾動列表，而LazyColumn正是為此而生。它可以根據需要動態產生列表項，大大提高了應用程式的效能和效率。同時，LazyColumn還可以回到清單的起始位置，方便我們進行指定位置的操作和處理。無論是開發行動應用程式還是網頁應用，LazyColumn都是一個非常實用的工具，讓我們一起來了解一下吧！
我有一個申請。一張大圖片和底部的lazycolumn。當我轉到第四個（或第五個）連結然後返回時，我的 lazycolumn 最終位於初始位置。連結1在最上面。當我再次想要連結 4 時，我必須「扭轉」。
「本機」導覽列上的「後退」按鈕有所不同。當我回來時，lazycolumn 已經以我留下的形式在等著我。
程式中需要做什麼才能使頂部按鈕的行為與導覽列上的「後退」按鈕相同？
這是我的文件：
navgraph.kt
#
@composable
fun navgraph(navcontroller: navhostcontroller){
navhost(navcontroller = navcontroller, startdestination = screens.contents.route
) {
addcontentsscreen(navcontroller, this)
addscreen1(navcontroller, this)
addscreen2(navcontroller, this)
addscreen3(navcontroller, this)
addscreen4(navcontroller, this)
addscreen5(navcontroller, this)
}
}
private fun addcontentsscreen(
navcontroller: navhostcontroller,
navgraphbuilder: navgraphbuilder
) {
navgraphbuilder.composable(route = screens.contents.route) {
contentsscreen(
navigatescreen1 = { navcontroller.navigate(screens.screens1.route) },
navigatescreen2 = { navcontroller.navigate(screens.screens2.route) },
navigatescreen3 = { navcontroller.navigate(screens.screens3.route) },
navigatescreen4 = { navcontroller.navigate(screens.screens4.route) },
navigatescreen5 = { navcontroller.navigate(screens.screens5.route) },
)
}
}
private fun addscreen1(navcontroller: navhostcontroller, navgraphbuilder: navgraphbuilder) {
navgraphbuilder.composable(route = screens.screens1.route) {
screen1 (navigatecontentsscreen = { navcontroller.navigate(screens.contents.route) })
}
}
private fun addscreen2(navcontroller: navhostcontroller, navgraphbuilder: navgraphbuilder) {
navgraphbuilder.composable(route = screens.screens2.route) {
screen2 (navigatecontentsscreen = { navcontroller.navigate(screens.contents.route) })
}
}
private fun addscreen3(navcontroller: navhostcontroller, navgraphbuilder: navgraphbuilder) {
navgraphbuilder.composable(route = screens.screens3.route) {
screen3 (navigatecontentsscreen = { navcontroller.navigate(screens.contents.route) })
}
}
private fun addscreen4(navcontroller: navhostcontroller, navgraphbuilder: navgraphbuilder) {
navgraphbuilder.composable(route = screens.screens4.route) {
screen4 (navigatecontentsscreen = { navcontroller.navigate(screens.contents.route) })
}
}
private fun addscreen5(navcontroller: navhostcontroller, navgraphbuilder: navgraphbuilder) {
navgraphbuilder.composable(route = screens.screens5.route) {
screen5 (navigatecontentsscreen = { navcontroller.navigate(screens.contents.route) })
}
}螢幕
sealed class screens(val route: string) {
data object contents : screens("contents_screen")
data object screens1 : screens("screen_1");
data object screens2 : screens("screen_2");
data object screens3 : screens("screen_3")
data object screens4 : screens("screen_4");
data object screens5 : screens("screen_5");
}screens.kt
#class item(val link: () -> unit, val name: string)
@composable
fun contentslist(explanation: list<item>) {
val liststate = rememberlazyliststate()
lazycolumn(state = liststate) {
items(explanation.size) { index ->
val item = explanation[index]
contentsbutton(
onclick = item.link,
text = item.name
)
}
}
}
@composable
fun contentsscreen(
navigatescreen1: () -> unit,
navigatescreen2: () -> unit,
navigatescreen3: () -> unit,
navigatescreen4: () -> unit,
navigatescreen5: () -> unit,
) {
val listofquestions = listof(
item( navigatescreen1, "screen1"),
item( navigatescreen2, "screen2"),
item( navigatescreen3, "screen3"),
item( navigatescreen4, "screen4"),
item( navigatescreen5, "screen5"),
)
column {
image(
modifier = modifier.height(650.dp),
painter = painterresource(id = r.drawable.cat),
contentdescription = null,
contentscale = contentscale.crop
)
contentslist(listofquestions)
}
}screen4.kt
@OptIn(ExperimentalMaterial3Api::class)
@Composable
fun Screen4(
navigateContentsScreen: () -> Unit,
) {
val text = "Screen4"
Scaffold(
containerColor = lightGray,
contentColor = darkGrey,
topBar = {
TopAppBar(
modifier = Modifier.height(80.dp),
title = {
Box(
Modifier.fillMaxHeight(),
contentAlignment = Alignment.Center
) {
Text(
maxLines = 1,
overflow = TextOverflow.Ellipsis,
text = text,
color = orange,
)
}
},
navigationIcon = {
Box(
Modifier.fillMaxHeight(),
contentAlignment = Alignment.Center
) {
IconButton(
onClick = navigateContentsScreen
) {
Icon(
imageVector = Icons.Filled.ArrowBack,
contentDescription = "Back",
tint=orange,
)
}
}
},
colors = TopAppBarDefaults.mediumTopAppBarColors(
containerColor = darkGrey
)
)
}
) { contentPadding ->
Column(
modifier = Modifier
.fillMaxSize()
.padding(contentPadding)
.verticalScroll(rememberScrollState()),
) {
Text(
text = "Screen4",
color = darkGrey,
textAlign = TextAlign.Justify,
modifier = Modifier.padding(24.dp)
)
}
}
}我嘗試並蒐索，但一無所獲......
嘗試按如下方式更改您的程式碼：
navGraphBuilder.composable(route = Screens.Screens4.route) {
Screen4 (navigateContentsScreen = { navController.navigateUp() })
}當您使用navcontroller.navigate(screens.contents.route) 時，您將始終為該路由創建一個新的backstackentry ，並且這個新目的地將是全新創建的，沒有任何狀態。請小心，您將使用目前的方法建立一個巨大的後台堆疊。
使用 navcontroller.navigateup() 或 navcontroller.popbackstack()，您將獲得實際的先前目的地。請參閱此 stackoverflow 帖子以了解差異的說明。
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