如何每次打开应用时调用指定Fragment？含导航抽屉场景

当然有办法实现啦！核心思路就是在MainActivity启动时，自动把你的主页面Fragment加载到Activity的容器里就行，下面给你分步讲解具体实现：

首先，你的MainActivity布局（比如activity_main.xml）里得有一个容器来承载Fragment，通常用FrameLayout或者FragmentContainerView就行，把它放在Navigation Drawer的布局结构内：

<!-- activity_main.xml 示例 -->
<DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <!-- 主内容区域：Fragment容器 -->
    <FrameLayout
        android:id="@+id/fragment_container"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <!-- Navigation Drawer侧边栏 -->
    <com.google.android.material.navigation.NavigationView
        android:id="@+id/nav_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:fitsSystemWindows="true"
        app:menu="@menu/activity_main_drawer" />

</DrawerLayout>

在MainActivity完成Navigation Drawer的初始化后，添加一段代码来自动加载主Fragment。这里要注意判断savedInstanceState == null，避免屏幕旋转等配置变化时重复添加Fragment：

Java 示例 #

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // 初始化Navigation Drawer的常规代码
    DrawerLayout drawer = findViewById(R.id.drawer_layout);
    NavigationView navigationView = findViewById(R.id.nav_view);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.addDrawerListener(toggle);
    toggle.syncState();
    navigationView.setNavigationItemSelectedListener(this);

    // 关键：首次启动时自动加载主Fragment
    if (savedInstanceState == null) {
        // 实例化你的主页面Fragment
        MainHomeFragment mainFragment = new MainHomeFragment();
        getSupportFragmentManager()
                .beginTransaction()
                .replace(R.id.fragment_container, mainFragment)
                .commit();
        // 可选：让Navigation Drawer默认选中主页面的菜单项
        navigationView.setCheckedItem(R.id.nav_home);
    }
}

Kotlin 示例 #

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

    // 初始化Navigation Drawer的常规代码
    val drawer: DrawerLayout = findViewById(R.id.drawer_layout)
    val navView: NavigationView = findViewById(R.id.nav_view)
    val toggle = ActionBarDrawerToggle(
        this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close
    )
    drawer.addDrawerListener(toggle)
    toggle.syncState()
    navView.setNavigationItemSelectedListener(this)

    // 关键：首次启动时自动加载主Fragment
    if (savedInstanceState == null) {
        val mainFragment = MainHomeFragment()
        supportFragmentManager.beginTransaction()
            .replace(R.id.fragment_container, mainFragment)
            .commit()
        navView.setCheckedItem(R.id.nav_home)
    }
}

如果你的Navigation Drawer里有主页面的菜单项，点击时也要切换到主Fragment，可以在NavigationItemSelectedListener里补充逻辑：

@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {
    int id = item.getItemId();

    if (id == R.id.nav_home) {
        // 点击主菜单时加载主Fragment
        MainHomeFragment mainFragment = new MainHomeFragment();
        getSupportFragmentManager()
                .beginTransaction()
                .replace(R.id.fragment_container, mainFragment)
                .commit();
    } else if (id == R.id.nav_other_page) {
        // 处理其他菜单项的Fragment切换
    }

    DrawerLayout drawer = findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

• 用replace而不是add来切换Fragment，避免多个Fragment重叠在容器里
• 确认你的主Fragment已经正确实现，没有初始化逻辑的问题
• savedInstanceState == null的判断很重要，能防止Activity重建时重复添加Fragment

这样设置后，每次打开应用，MainActivity启动就会自动加载你的主页面Fragment啦！

内容的提问来源于stack exchange，提问作者Eduardo Silva