php小编百草今天为大家介绍一款非常实用的功能——lazycolumn 引用返回起始位置。在开发过程中，我们经常需要创建一个垂直滚动列表，而lazycolumn正是为此而生。它可以根据需要动态生成列表项，大大提高了应用的性能和效率。同时，lazycolumn还可以返回列表的起始位置，方便我们进行指定位置的操作和处理。无论是开发移动应用还是网页应用，lazycolumn都是一个非常实用的工具，让我们一起来了解一下吧！
问题内容
我有一个申请。一张大图片和底部的lazycolumn。当我转到第四个（或第五个）链接然后返回时，我的 lazycolumn 最终位于初始位置。链接1在最上面。当我再次想要链接 4 时，我必须“扭转”。
“本机”导航栏上的“后退”按钮有所不同。当我回来时，lazycolumn 已经以我留下的形式在等着我。
程序中需要做什么才能使顶部按钮的行为与导航栏上的“后退”按钮相同？
这是我的文件：
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) })
}
}
屏幕
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)
)
}
}
}
我尝试并搜索，但一无所获......
解决方法
尝试按如下方式更改您的代码：
navGraphBuilder.composable(route = Screens.Screens4.route) {
Screen4 (navigateContentsScreen = { navController.navigateUp() })
}
当您使用 navcontroller.navigate(screens.contents.route) 时，您将始终为该路由创建一个新的 backstackentry ，并且这个新目的地将是全新创建的，没有任何状态。请小心，您将使用当前的方法建立一个巨大的后台堆栈。
使用 navcontroller.navigateup() 或 navcontroller.navigateup()，您将获得实际的先前目的地。请参阅此 stackoverflow 帖子了解差异的说明。
