本文展示了 Scaffold 的一个简单的基本用法，更多用法请看官方文档

一、Scaffold 脚手架

@OptIn(ExperimentalMaterialApi::class)
@Composable
fun Scaffold() {
    // BottomBar 当前选择项
    var selectedItem by remember { mutableStateOf(0) }
    // TopBar 下拉菜单打开状态
    var dropDownMenuOpen by remember { mutableStateOf(false) }
    // Scaffold 状态，可以用来获取侧边栏打开状态
    val scaffoldState = rememberScaffoldState()
    // 获取组合感知作用域，以便在可组合项外启动协程
    val scope = rememberCoroutineScope()

    // 设置 BottomBar 项
    val items = listOf(
        BottomBarItem("首页", Icons.Outlined.Home),
        BottomBarItem("消息", Icons.Outlined.Email),
        BottomBarItem("我", Icons.Outlined.Face)
    )

    // 设置 TopBar 下拉菜单选项
    val menus = listOf("刷新数据", "设置", "退出")

    // 拦截系统返回按键事件
    BackHandler(
        enabled = scaffoldState.drawerState.isOpen
    ) {
        scope.launch {
            scaffoldState.drawerState.close()
        }
    }

    // 脚手架
    Scaffold (
        topBar = {
            TopAppBar (
                title = {
                    Text("首页")
                },
                navigationIcon = {
                    IconButton(onClick = {
                        scaffoldState.drawerState.apply {
                            scope.launch {
                                if (isOpen) close() else open()
                            }
                        }
                    }) {
                        Icon(imageVector = Icons.Filled.Menu, contentDescription = null)
                    }
                },
                // 设置 TopBar 右侧按钮
                actions = {
                    IconButton(onClick = {
                        dropDownMenuOpen = !dropDownMenuOpen
                    }) {
                       Icon(imageVector = Icons.Filled.Face, contentDescription = null)
                    }
                    // 下拉菜单
                    Menu(options = menus, expanded = dropDownMenuOpen) {
                        dropDownMenuOpen = false
                    }
                }
            )

        },
        bottomBar = {
            BottomNavigation {
                items.forEachIndexed { index, bottomBarItem ->
                    BottomNavigationItem(
                        selected = selectedItem == index,
                        onClick = {
                            selectedItem = index
                        },
                        icon = {
                            Icon(imageVector = bottomBarItem.icon, null)
                        },
                        label = {
                            Text(bottomBarItem.name)
                        }
                    )
                }
            }
        },
        // 侧边栏
        drawerContent = {
            Text(text = "Hello")
        },
        scaffoldState = scaffoldState
    ) {
        Box (
            modifier = Modifier
                .fillMaxSize()
                // 此处设置 padding 以便偏移顶部栏和底部栏（如果存在的话），否则报错
                .padding(it),
            contentAlignment = Alignment.Center
        ) {
            when (selectedItem) {
                0 -> {
                    Text("首页")
                }
                1 -> {
                    Text("消息")
                }
                2 -> {
                    Text("我")
                }
            }
        }
    }


}

@ExperimentalMaterialApi
@Composable
fun Menu(
    options: List<String>,
    expanded: Boolean,
    onDismissRequest: () -> Unit
) {
    DropdownMenu(
        expanded = expanded,
        onDismissRequest = onDismissRequest
    ) {
        Column {
            options.forEach { option ->
                ListItem(text = {
                    Text(option)
                })
            }
        }
    }
}

data class BottomBarItem (val name: String, val icon: ImageVector)