来自 viewpager getActivity 的 Android 子片段返回 null

Question

我有一个活动，并通过一个按钮在两个片段（主要和设置）之间切换。在 MAIN 片段中，我有一个带有 4 个子片段的 ViewPager。

第一次运行一切正常，但如果我旋转屏幕，ViewPager 中的片段的getActivity() 将返回 null。

ActivityMain：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    
    // Add or show the fragments
    showHideScreenFragment(FRAGMENT_MAIN);
}

private void showHideScreenFragment(String tag) {   

    FragmentManager fm = getSupportFragmentManager();
    FragmentTransaction ft = fm.beginTransaction();
    
    ft.setCustomAnimations(android.R.anim.fade_in, android.R.anim.fade_out);
    
    // Get the fragment from the backstack if it is existing
    BaseFragment oldFragment = getFragmentFromBackstack(tag);

    // Get the current fragment from the layout
    BaseFragment currentFragment = getCurrentFragment();
    
    if (oldFragment == null) {
        if (currentFragment != null) {
             ft.hide(currentFragment);
        }
        
        ft.add(getMainContainerId(), getFragmentInstance(tag), tag);
    }
    else {

        if (currentFragment != null) {
            if (isSameFragment(oldFragment, currentFragment))
                return;
            
            ft.hide(currentFragment);
        }
        
        if (oldFragment.isHidden())
            ft.show(oldFragment);
    }
        
    ft.commit();
    fm.executePendingTransactions();
}

private BaseFragment getFragmentInstance(String tag) {
    if (tag.equals(FRAGMENT_MAIN)) return getFragmentMain();
    if (tag.equals(FRAGMENT_SETTINGS)) return getFragmentSettings();
    
    throw new RuntimeException("Fragment not found !");
}

private FragmentMain getFragmentMain() {
    return new FragmentMain();
}

private FragmentSettings getFragmentSettings() {
    return new FragmentSettings();
}

private BaseFragment getFragmentFromBackstack(String tag) {
    if (tag.equals(FRAGMENT_MAIN)) return getFragmentMainFromBackstack();
    if (tag.equals(FRAGMENT_SETTINGS)) return getFragmentSettingsFromBackstack();
    
    throw new RuntimeException("Fragment not found !");
}

private FragmentMain getFragmentMainFromBackstack() {
    return (FragmentMain) getSupportFragmentManager().findFragmentByTag(FRAGMENT_MAIN);
}

private FragmentSettings getFragmentSettingsFromBackstack() {
    return (FragmentSettings) getSupportFragmentManager().findFragmentByTag(FRAGMENT_SETTINGS);
}

private boolean isSameFragment(Fragment f1, Fragment f2) {
    return f1.getTag().equals(f2.getTag());
}

FragmentMain：

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {         
        View view = inflater.inflate(R.layout.fragment_main, container, false);
        
        viewPager = (ViewPager) view.findViewById(R.id.viewPager);
        
        // Add the 4 child fragments to the viewpager
        populateViewPager();
        
        // Debugging
        new Handler().postDelayed(new Runnable() {
            @Override
            public void run() {
                _printFragmentStates();
            }
        }, 2500);
        
        return view;
    }

    private void populateViewPager() {      
        ArrayList<BaseMainFragment> fragments = new ArrayList<BaseMainFragment>();
        
        fragments.add(new FragmentSearch());
        fragments.add(new FragmentFavorites());
        fragments.add(new FragmentHouse());
        fragments.add(new FragmentRoom());
        
        adapterMain = new AdapterMain(getChildFragmentManager(), fragments);
        
        viewPager.setOffscreenPageLimit(4);
        viewPager.setAdapter(adapterMain);
    }

    // DEBUGGING
    private void _printFragmentStates() {
        Activity actSearch = null;
        Activity actFav = null;
        Activity actHouse = null;
        Activity actRoom = null;
    
        actSearch = getFragmentSearch().getActivity();
        actFav = getFragmentFavorites().getActivity();
        actHouse = getFragmentHouse().getActivity();
        actRoom = getFragmentRoom().getActivity();
    
        Functions.logd("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");
        Functions.logd("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");
        Functions.logd("Main fragment act, is null: " + (getActivity() == null));
        Functions.logd("Search act, is null: " + (actSearch == null));
        Functions.logd("Favorite act, is null: " + (actFav == null));
        Functions.logd("House act, is null: " + (actHouse == null));
        Functions.logd("Room act, is null: " + (actRoom == null));
        Functions.logd("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");
        Functions.logd("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");
}

private FragmentSearch getFragmentSearch() {
    return (FragmentSearch) adapterMain.getItem(0);
}

private FragmentFavorite getFragmentFavorite() {
    return (FragmentFavorite) adapterMain.getItem(1);
}

private FragmentHouse getFragmentHouse() {
    return (FragmentHouse) adapterMain.getItem(2);
}

private FragmentRoom getFragmentHouse() {
    return (FragmentRoom) adapterMain.getItem(3);
}

正如我所说，起初运行一切正常，但在我旋转屏幕后，getActivity(); 在 4 个子片段中为空：搜索、收藏夹、房屋和房间。

Logcat 调试

1 次运行：

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Main fragment act, is null: false
Search act, is null: false
Favorite act, is null: false
House act, is null: false
Room act, is null: false
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

屏幕方向改变后：

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Main fragment act, is null: false
Search act, is null: true
Favorite act, is null: true
House act, is null: true
Room act, is null: true
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

我做错了什么？

Answer 1

经过数小时的调试，我发现如果您的活动只有 1 个片段（没有子片段或嵌套片段），那么您不需要重新添加片段。

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

    // Add or show the fragments if the savedInstance is null, otherwise let the system reattach your fragment.
    if (savedIstance == null)
       showHideScreenFragment(FRAGMENT_MAIN);
}

你不需要重新附加fragment，android系统会为你做这个。

在子片段中获取NPE getActivity(); 的解决方案是：

使用FragmentStatePagerAdapter 作为ViewPager 的适配器。

并覆盖保存状态方法：

@Override
public Parcelable saveState() {
    return null;
}

我不知道为什么，但setRetainInstance(false); 并没有帮助我，我认为这对我来说仍然是一个谜。

Answer 2

在处理片段时遵循这样的模式是一个好习惯：

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

@Override
public void onViewCreated(View view, Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    view = view.findViewById(R.id.view);
}

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
// do your thing here
populateViewPager();
}

1) 使用OnCreateView 设置片段的视图， 2) OnViewCreated 初始化视图和 3）OnActivityCreated 进行设置。

在OnActivityCreated 中使用getActivity() 可确保getActivity() 不返回null。只有在活动完全初始化后才会调用此方法。 OnAttach、OnCreate、OnCreateView 甚至可能在活动创建之前得到。