Bottom Navigation：基础知识，BottomNavigationView和TabView

// 使用BottomNavigationView和NavHostFragment的Activity
class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val navView = findViewById<NavigationBarView>(R.id.nav_view)
        val navController = findNavController(R.id.nav_host_fragment)

        // 导航自动同步
        navView.setupWithNavController(navController)

        // 徽章配置
        val badge = navView.getOrCreateBadge(R.id.navigation_messages)
        badge.number = 8
        badge.badgeGravity = BadgeDrawable.TOP_END
    }
}

// Bottom Navigation的XML菜单 (res/menu/bottom_nav_menu.xml)
// <?xml version="1.0" encoding="utf-8"?>
// <menu xmlns:android="http://schemas.android.com/apk/res/android">
//   <item android:id="@+id/navigation_home"
//     android:icon="@drawable/ic_home"
//     android:title="首页" />
//   <item android:id="@+id/navigation_messages"
//     android:icon="@drawable/ic_chat"
//     android:title="聊天" />
//   <item android:id="@+id/navigation_profile"
//     android:icon="@drawable/ic_person"
//     android:title="个人资料" />
// </menu>

import SwiftUI

struct ContentView: View {
    var body: some View {
        TabView {
            HomeView()
                .tabItem {
                    Label("首页", systemImage: "house.fill")
                }
                .badge(3)

            SearchView()
                .tabItem {
                    Label("搜索", systemImage: "magnifyingglass")
                }

            NotificationsView()
                .tabItem {
                    Label("通知", systemImage: "bell.fill")
                }
                .badge("12")

            ProfileView()
                .tabItem {
                    Label("个人资料", systemImage: "person.circle.fill")
                }
        }
    }
}

struct AppTabView: View {
    var body: some View {
        TabView {
            NavigationStack {
                HomeView()
            }
            .tabItem {
                Label("首页", systemImage: "house")
            }

            NavigationStack {
                SearchView()
            }
            .tabItem {
                Label("搜索", systemImage: "magnifyingglass")
            }
        }
    }
}

// HomeView内部 — 堆栈导航
struct HomeView: View {
    var body: some View {
        List(items) { item in
            NavigationLink("\(item.title)", value: item)
        }
        .navigationDestination(for: Item.self) { item in
            ItemDetailView(item: item)
        }
    }
}

<!-- NavigationBarView活动指示器样式 -->
<style name="Widget.MyApp.NavigationBarView.ActiveIndicator"
    parent="Widget.Material3.NavigationBarView.ActiveIndicator">
    <item name="android:height">32dp</item>
    <item name="android:width">64dp</item>
    <item name="android:marginHorizontal">8dp</item>
    <item name="shapeAppearance">
        @style/ShapeAppearance.Material3.Corner.Full
    </item>
    <item name="color">@color/primary_container</item>
</style>