AHBottomNavigation初始不加载Fragment及切换后异常问题求助

看起来你遇到的两个问题都是Fragment初始化和状态管理的典型场景，我来帮你逐个拆解修复：

问题a：启动时FirstFragment未加载的原因及修复 #

你在createNavItems()里调用了bottomNavigation.setCurrentItem(0)，但这行代码执行时，setOnTabSelectedListener还没绑定到导航栏上！AHBottomNavigation的setCurrentItem在监听器未设置时不会触发选中回调，导致启动时虽然底部导航显示选中了First，但对应的Fragment根本没被加载。

修复方案：
把setCurrentItem(0)移到设置监听器之后执行，确保回调能正常触发：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    bottomNavigation = findViewById(R.id.bottom_navigation);
    this.createNavItems();

    // 先绑定监听器
    bottomNavigation.setOnTabSelectedListener(new AHBottomNavigation.OnTabSelectedListener() {
        @Override
        public boolean onTabSelected(int position, boolean wasSelected) {
            // 原有逻辑保持不变
            if (position==0) {
                FirstFragment firstFragment=new FirstFragment();
                getSupportFragmentManager().beginTransaction().replace(R.id.content_id,firstFragment).commit();
            }else if (position==1) {
                SecondFragment secondFragment=new SecondFragment();
                getSupportFragmentManager().beginTransaction().replace(R.id.content_id, secondFragment).commit();
            }else if (position==2) {
                ThirdFragment thirdFragment=new ThirdFragment();
                getSupportFragmentManager().beginTransaction().replace(R.id.content_id,thirdFragment).commit();
            }else{
                FourthFragment fourthFragment=new FourthFragment();
                getSupportFragmentManager().beginTransaction().replace(R.id.content_id,fourthFragment).commit();
            }
            return true;
        }
    });
    
    // 再设置当前选中项，此时会触发监听器加载Fragment
    bottomNavigation.setCurrentItem(0);
}

// 修改createNavItems方法，移除里面的setCurrentItem
private void createNavItems(){
    navigationAdapter = new AHBottomNavigationAdapter(this, R.menu.navigation);
    navigationAdapter.setupWithBottomNavigation(bottomNavigation);
}

问题b：切换回FirstFragment后子Tab空白、滑动异常的原因及修复 #

每次切换到First时你都new FirstFragment()，这会销毁之前的Fragment实例并创建全新对象，导致ViewPager的状态完全丢失。另外，FirstFragment里的ViewPager使用了Activity的getSupportFragmentManager()，而嵌套在Fragment里的子Fragment必须使用子Fragment管理器（getChildFragmentManager()），否则会出现层级混乱，引发滑动卡顿、状态丢失的问题。

修复方案：

1. 缓存Fragment实例，避免每次切换都创建新对象；
2. 将FirstFragment里的FragmentManager替换为getChildFragmentManager()。

第一步：在MainActivity中缓存Fragment #

public class MainActivity extends AppCompatActivity{
    private AHBottomNavigationAdapter navigationAdapter;
    private AHBottomNavigation bottomNavigation;
    // 用数组缓存四个Fragment实例
    private Fragment[] fragments = new Fragment[4];

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        bottomNavigation = findViewById(R.id.bottom_navigation);
        this.createNavItems();

        // 提前初始化所有Fragment，后续复用
        fragments[0] = new FirstFragment();
        fragments[1] = new SecondFragment();
        fragments[2] = new ThirdFragment();
        fragments[3] = new FourthFragment();

        bottomNavigation.setOnTabSelectedListener(new AHBottomNavigation.OnTabSelectedListener() {
            @Override
            public boolean onTabSelected(int position, boolean wasSelected) {
                // 复用缓存的Fragment，不再每次new
                getSupportFragmentManager().beginTransaction()
                        .replace(R.id.content_id, fragments[position])
                        .commit();
                return true;
            }
        });
        
        bottomNavigation.setCurrentItem(0);
    }

    private void createNavItems(){
        navigationAdapter = new AHBottomNavigationAdapter(this, R.menu.navigation);
        navigationAdapter.setupWithBottomNavigation(bottomNavigation);
    }
}

第二步：修改FirstFragment的ViewPager初始化逻辑 #

public class FirstFragment extends Fragment {

    public FirstFragment() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_first, container, false);
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        // 改用getChildFragmentManager()管理子Fragment，并且在onViewCreated中初始化更稳妥
        ViewPager viewPager = view.findViewById(R.id.viewpager);
        FirstFragmentPagerAdapter adapter = new FirstFragmentPagerAdapter(getChildFragmentManager(), getContext());
        viewPager.setAdapter(adapter);

        TabLayout tabLayout = view.findViewById(R.id.sliding_tabs);
        tabLayout.setupWithViewPager(viewPager);
    }

    // 可以移除onAttach的代码，getContext()在onViewCreated阶段已经可用
}

同时调整FirstFragmentPagerAdapter的构造逻辑，直接使用传入的FragmentManager：

public class FirstFragmentPagerAdapter extends FragmentPagerAdapter {
    private Context context;

    public FirstFragmentPagerAdapter(FragmentManager fm, Context mContext){
        super(fm);
        context = mContext;
    }

    // 其余代码保持不变
}

这样修改后，两个问题都能解决：启动时FirstFragment会正常加载，切换回来时会复用之前的Fragment实例，ViewPager的状态和滑动功能也能完全恢复正常。

内容的提问来源于stack exchange，提问作者Paradox