StartDestination NullpointerException导航时离开SaveSaveInstance

Question

我有一个抽屉布局，可以在两个片段之间进行导航（就像在创建带有抽屉布局的新项目时所设置的那样：因此，有一个导航图和AppBarConfiguration的常规设置。我只是删除了一大堆东西，所以我最终只会得到两个片段）。

问题：当我导航到第二个片段，并两次更改设备的方向（因此将手机横向翻转，然后返回纵向）时，在起始目标的onSavedInstanceState（）方法中得到了nullpointerException。我想知道为什么会这样吗？有没有办法弹出该片段或某些东西，因为它甚至不可见？我有点希望导航UI能够正确处理此问题，因为我认为这是其目的-但现在，我觉得我可能会缺少一些真正重要的东西

导航图：

<navigation xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/mobile_navigation"
app:startDestination="@+id/nav_home">

<fragment
    android:id="@+id/nav_home"
    android:name="SomeNameHome"
    android:label="@string/menu_auction"
    tools:layout="@layout/fragment_auction" />

<fragment
    android:id="@+id/nav_gallery"
    android:name="SomeNameGallery"
    android:label="@string/menu_basket"
    tools:layout="@layout/fragment_basket" />

</navigation>

然后，家庭活动：

/** Activity containing the Auction and Basket fragments. */
public class HomeActivity extends AppCompatActivity {

    private AppBarConfiguration mAppBarConfiguration;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        NavigationView navigationView = findViewById(R.id.nav_view);
        // Passing each menu ID as a set of Ids because each
        // menu should be considered as top level destinations.
        mAppBarConfiguration = new AppBarConfiguration.Builder(
                R.id.nav_home, R.id.nav_gallery)
                .setDrawerLayout(drawer)
                .build();
        navigationView.setItemIconTintList(null);
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
        NavigationUI.setupActionBarWithNavController(this, navController, mAppBarConfiguration);
        NavigationUI.setupWithNavController(navigationView, navController);
    }

    @Override
    public boolean onSupportNavigateUp() {
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
        return NavigationUI.navigateUp(navController, mAppBarConfiguration)
                || super.onSupportNavigateUp();
    }
}

问题代码，在起始目标片段中：

 @Override
    public void onSaveInstanceState(@NonNull Bundle outState) {
        super.onSaveInstanceState(outState);
        LinearLayoutManager m = (LinearLayoutManager) mSearchResultsList.getLayoutManager();
        outState.putInt(SAVED_INSTANCE_POSITION, m.findFirstCompletelyVisibleItemPosition() );
    }

mSearchResultList在第二个方向更改时为null。

Answer 1

今天发现了一些新内容：