java.lang.IllegalStateException：片段已添加：

Question

所以我有一个BottomNavigation，它在某些菜单中还有另一个TabLayout，当我单击该菜单时，它像标题一样出现了错误。这是我的代码

FavoriteFragment

其中包含BottomNavigation中的片段

public class FavoriteFragment extends Fragment {

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


@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    return inflater.inflate(R.layout.fragment_favorite, container, false);
}

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    ViewPager viewPager = view.findViewById(R.id.view_pager);
    setViewPager(viewPager);

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

private void setViewPager(ViewPager viewPager){
    SectionsPagerAdapter sectionsPagerAdapter = new SectionsPagerAdapter(getChildFragmentManager());

    sectionsPagerAdapter.addFragment(new MoviesFavoriteFragment(),getResources().getString(R.string.title_movies));
    sectionsPagerAdapter.addFragment(new TvShowFavoriteFragment(),getResources().getString(R.string.title_tv_show));
    viewPager.setAdapter(sectionsPagerAdapter);
}
}

而且我的ViewPager获得了pageradapter

SectionsPagerAdapter

public class SectionsPagerAdapter extends FragmentPagerAdapter {
    private List<Fragment> listFragment = new ArrayList<>();
    private List<String> titleFragment = new ArrayList<>();

public SectionsPagerAdapter(FragmentManager fm) {
    super(fm,BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT);
}

@NonNull
@Override
public Fragment getItem(int position) {
    return listFragment.get(position);
}

@Override
public int getCount() {
    return listFragment.size();
}

public void addFragment(Fragment fragment, String title){
    listFragment.add(fragment);
    titleFragment.add(title);
}

@Override
public CharSequence getPageTitle(int position) {
    return titleFragment.get(position);
}
}

这是TabLayout中的某些片段，发生了错误

MoviesFavoriteFragment

public class MoviesFavoriteFragment extends Fragment {
private RecyclerAdapterFavMovies adapterFavMovies;

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



@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    return inflater.inflate(R.layout.fragment_movies_favorite, container, false);


}

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    RecyclerView rv_fav_movie = view.findViewById(R.id.rv_fav_movies);
    rv_fav_movie.setHasFixedSize(true);

    rv_fav_movie.setLayoutManager(new LinearLayoutManager(view.getContext()));

    adapterFavMovies = new RecyclerAdapterFavMovies(view.getContext());
    rv_fav_movie.setAdapter(adapterFavMovies);

    ArrayList<Movies> listMovie = (ArrayList<Movies>) loadDataFavoriteMovies();

    MainViewModel mainViewModel = ViewModelProviders.of(this).get(MainViewModel.class);
    mainViewModel.setFavMovies(listMovie);
    mainViewModel.getMovieList().observe(getViewLifecycleOwner(),getMovies);
}

private List<Movies> loadDataFavoriteMovies(){
    MovieDatabase database = Room.databaseBuilder(Objects.requireNonNull(getActivity()), MovieDatabase.class, "db_movie")
            .allowMainThreadQueries()
            .build();
    MovieDAO movieDAO = database.getMovieDAO();
    return movieDAO.getMoviesByMovieType(1);
}

private Observer<ArrayList<Movies>> getMovies = new Observer<ArrayList<Movies>>() {
    @Override
    public void onChanged(@Nullable ArrayList<Movies> movies) {
        if (movies != null) {
            adapterFavMovies.setListMovies(movies);
        }
    }
};
}

我不知道当我再次打开android studio时突然让我出错，以前没有任何反应，可以正常工作

Answer 1

也许文档不清楚，但是您应该始终从适配器的getItem返回一个新的片段实例。在FragmentPagerAdapter内部，instantiateItem负责返回一个片段，如果需要，它需要一个新的片段实例：

// Do we already have this fragment?
String name = makeFragmentName(container.getId(), itemId);
Fragment fragment = mFragmentManager.findFragmentByTag(name);
if (fragment != null) {
    if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
    mCurTransaction.attach(fragment);
} else {
    fragment = getItem(position);
    if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
    mCurTransaction.add(container.getId(), fragment,
            makeFragmentName(container.getId(), itemId));
}

在适配器中，getItem将始终返回相同的片段实例。没关系，直到适配器第二次调用该片段时该片段被销毁为止，最终将导致IllegalStateException。

这里是如何修复适配器的示例：

public Fragment getItem(int position) {
    switch (position) {
        case 0: return MoviesFavoriteFragment();
        case 1: return TvShowFavoriteFragment();
    }
}

public CharSequence getPageTitle(int position) {
    switch (position) {
        case 0: return getResources().getString(R.string.title_movies);
        case 1: return getResources().getString(R.string.title_tv_show);
    }
}