方向改变时如何保留导航抽屉选择的片段？使用 Xamarin

Question

我正在使用 C#/Xamarin 开发 android 应用程序。

我只使用一个活动，有 12 个片段。我在主要活动中有一个导航抽屉，其中选择加载了 12 个片段之一。这是我的 MainActivity 课程的开始。如您所见，OnCreate 加载了 savedInstanceState，并且仅在 savedInstanceState 为 null 时创建片段。

public class MainActivity : AppCompatActivity
{
    private DrawerLayout drawerLayout;
    private SupportFragment currentFragment = new Weight_Fragment();
    private Weight_Fragment mWeightFragment;
    private Distance_Fragment mDistanceFragment;
    private Volume_Fragment mVolumeFragment;
    private Help_Fragment mHelpFragment;
    private Temperature_Fragment mTempFragment;
    private Time_Fragment mTimeFragment;
    private Data_Fragment mDataFragment;
    private Pressure_Fragment mPressureFragment;
    private Energy_Fragment mEnergyFragment;
    private Force_Fragment mForceFragment;

    protected override void OnCreate(Bundle savedInstanceState)
    {           
        base.OnCreate(savedInstanceState);

        // Create UI
        SetContentView(Resource.Layout.Main);
        drawerLayout = FindViewById<DrawerLayout>(Resource.Id.drawer_layout);

        //Init Fragments
        if (savedInstanceState == null)
        {
            mWeightFragment = new Weight_Fragment();
            mDistanceFragment = new Distance_Fragment();
            mVolumeFragment = new Volume_Fragment();
            mHelpFragment = new Help_Fragment();
            mTempFragment = new Temperature_Fragment();
            mTimeFragment = new Time_Fragment();
            mDataFragment = new Data_Fragment();
            mPressureFragment = new Pressure_Fragment();
            mEnergyFragment = new Energy_Fragment();
            mForceFragment = new Force_Fragment();

            //Add Fragments to Manager
            var trans = SupportFragmentManager.BeginTransaction();
            trans.Add(Resource.Id.fragmentContainer, mHelpFragment, "Help");
            trans.Hide(mHelpFragment);
            trans.Add(Resource.Id.fragmentContainer, mDistanceFragment, "Distance");
            trans.Hide(mDistanceFragment);
            trans.Add(Resource.Id.fragmentContainer, mVolumeFragment, "Volume");
            trans.Hide(mVolumeFragment);
            trans.Add(Resource.Id.fragmentContainer, mTempFragment, "Temperature");
            trans.Hide(mTempFragment);
            trans.Add(Resource.Id.fragmentContainer, mPressureFragment, "Pressure");
            trans.Hide(mPressureFragment);
            trans.Add(Resource.Id.fragmentContainer, mEnergyFragment, "Energy");
            trans.Hide(mEnergyFragment);
            trans.Add(Resource.Id.fragmentContainer, mForceFragment, "Force");
            trans.Hide(mForceFragment);
            trans.Add(Resource.Id.fragmentContainer, mTimeFragment, "Time");
            trans.Hide(mTimeFragment);
            trans.Add(Resource.Id.fragmentContainer, mDataFragment, "Data Storage");
            trans.Hide(mDataFragment);
            trans.Add(Resource.Id.fragmentContainer, mWeightFragment, "Weight");
            trans.Commit();
            currentFragment = mWeightFragment;
        }

导航抽屉在片段之间切换如下：

void NavigationView_NavigationItemSelected(object sender, NavigationView.NavigationItemSelectedEventArgs e)
    {
        Toolbar toolbar = FindViewById<Toolbar>(Resource.Id.toolbar);

        switch (e.MenuItem.ItemId)
        {
            case (Resource.Id.nav_weight):
                toolbar.Title = "Weight";
                ShowFragment(mWeightFragment);
                break;
            case (Resource.Id.nav_distance):
                toolbar.Title = "Distance";
                ShowFragment(mDistanceFragment);
                break;
            case (Resource.Id.nav_volume):
                toolbar.Title = "Volume";
                ShowFragment(mVolumeFragment);
                break;
            case (Resource.Id.nav_temperature):
                toolbar.Title = "Temperature";
                ShowFragment(mTempFragment);
                break;
            case (Resource.Id.nav_pressure):
                toolbar.Title = "Pressure";
                ShowFragment(mPressureFragment);
                break;
            case (Resource.Id.nav_energy):
                toolbar.Title = "Energy";
                ShowFragment(mEnergyFragment);
                break;
            case (Resource.Id.nav_force):
                toolbar.Title = "Force";
                ShowFragment(mForceFragment);
                break;
            case (Resource.Id.nav_time):
                toolbar.Title = "Time";
                ShowFragment(mTimeFragment);
                break;
            case (Resource.Id.nav_data):
                toolbar.Title = "Data Storage";
                ShowFragment(mDataFragment);
                break;
            case (Resource.Id.help):
                toolbar.Title = "Help";
                ShowFragment(mHelpFragment);
                break; 
        }   

我对 OnSaveInstanceState 的重写非常简单：

public override void OnSaveInstanceState(Bundle outState, PersistableBundle outPersistentState)
    {
        base.OnSaveInstanceState(outState, outPersistentState);
    }   

当设备方向改变时，MainActivity 会重新启动，它会重新创建所有 Fragment，因此会显示 mWeightFragment，因为它是唯一一个在创建 Fragment 时不隐藏的。

我有纵向和横向方向的单独布局，所以我需要在方向改变时以某种方式更新片段，以便显示相应的布局。我还需要应用程序保持在同一个片段上，而不是在每次方向改变时跳回 mWeightFragment。

使用 OnSaveInstanceState 并仅在第一次创建活动时创建片段后，我收到以下错误：

未处理的异常： Java.Lang.NullPointerException：尝试在空对象引用上写入字段“int android.support.v4.app.Fragment.mNextAnim”

但只有在改变方向之后。如果我只是选择一个不同的片段然后改变方向，然后再改变方向，它可以正常工作并保持在同一个片段上，就像我想要的那样，但是一旦我再次尝试改变片段，我就会得到那个错误。

Answer 1

在你的清单中确保你有：

<activity android:configChanges="orientation|screenSize|keyboardHidden" />

还要确保你覆盖了 OnSavedInstanceState：

    protected override void OnSaveInstanceState(Bundle outState)
    {
    }

在类文件中