使用FragmentStatePageAdapter保留片段與ViewPager的位置

Question

我有一個活動，其中包含一個TabLayout併爲每個選項卡使用片段。作爲回報，該片段包含用於ViewPager的另一個片段。這是我的設置：

RoutineActivity.java

public class RoutineActivity extends AppCompatActivity { 
private ArrayList<CategoryFragment> categories = new ArrayList<>(); 
private TabLayout tabs; 

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

    // Set the toolbar as the action bar and add the Up button 
    Toolbar toolbar = findViewById(R.id.routine_toolbar); 
    setSupportActionBar(toolbar); 
    getSupportActionBar().setDisplayHomeAsUpEnabled(true); 

    // Get the intent and extract the string array 
    Intent intent = getIntent(); 
    ArrayList<String> selected = 
      intent.getStringArrayListExtra(SelectCategoriesActivity.EXTRA_CATEGORIES); 

    // Set the TabLayout 
    this.setTabs(selected); 
} 

// Set the tabs of the TabLayout given a list of tab names 
private void setTabs(ArrayList<String> tabs) { 
    // Setup the tabs and create a fragment for each category 
    this.tabs = findViewById(R.id.category_tabs); 
    for (int x = 0; x < tabs.size(); x++) { 
     TabLayout.Tab tab = this.tabs.newTab().setText(tabs.get(x)); 
     // Select the first tab 
     if (x == 0) { 
      this.tabs.addTab(tab, true); 
     } else { 
      this.tabs.addTab(tab); 
     } 

     // Store the created fragment 
     CategoryFragment categoryFragment = CategoryFragment.newInstance(tabs.get(x)); 
     this.categories.add(categoryFragment); 
    } 

    // Add the first fragment 
    getSupportFragmentManager() 
      .beginTransaction() 
      .add(R.id.fragment_category_container, this.categories.get(0)) 
      .commit(); 


    // Add the tab selected listener 
    this.tabs.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() { 
     @Override 
     public void onTabSelected(TabLayout.Tab tab) { 
      RoutineActivity.this.replaceFragment(tab.getPosition()); 
     } 

     @Override 
     public void onTabUnselected(TabLayout.Tab tab) {} 

     @Override 
     public void onTabReselected(TabLayout.Tab tab) { 

     } 
    }); 
} 

// Replace the fragment corresponding to the category 
private void replaceFragment(int position) { 
    FragmentManager fm = getSupportFragmentManager(); 
    FragmentTransaction ft = fm.beginTransaction(); 
    ft.replace(R.id.fragment_category_container, this.categories.get(position)); 
    ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN); 
    ft.commit(); 
} 
} 

CategoryFragment.java

public class CategoryFragment extends Fragment { 
private ViewPager pager; 
private ScreenSlidePagerAdapter pagerAdapter; 

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

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

    ArrayList<String> selected = new ArrayList<>(); 
    selected.add("1 " + getArguments().get("CATEGORY")); 
    selected.add("2 " + getArguments().get("CATEGORY")); 
    selected.add("3 " + getArguments().get("CATEGORY")); 
    selected.add("4 " + getArguments().get("CATEGORY")); 

    // Instantiate the PageAdapter 
    this.pagerAdapter = new ScreenSlidePagerAdapter(getChildFragmentManager(), selected); 
} 

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

    // Instantiate ViewPager and set its adapter 
    this.pager = view.findViewById(R.id.exercise_pager); 
    this.pager.setAdapter(this.pagerAdapter); 

    return view; 
} 

// Return a new instance of this fragment 
public static CategoryFragment newInstance(String category) { 
    Bundle args = new Bundle(); 
    args.putString("CATEGORY", category); 

    CategoryFragment categoryFragment = new CategoryFragment(); 
    categoryFragment.setArguments(args); 

    return categoryFragment; 
} 

// The page adapter 
private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter { 

    private ArrayList<String> selected; 

    public ScreenSlidePagerAdapter(FragmentManager fm, ArrayList<String> data) { 
     super(fm); 
     this.selected = data; 
    } 

    @Override 
    public Fragment getItem(int position) { 
     return ExerciseFragment.newInstance(this.selected.get(position)); 
    } 

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

ExerciseFragment.java

public class ExerciseFragment extends Fragment { 

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

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, 
         Bundle savedInstanceState) { 
    View rootView = inflater.inflate(
      R.layout.fragment_exercise_page, container, false); 

    // Change the header to the category 
    TextView tv = rootView.findViewById(R.id.textView_header); 
    tv.setText(getArguments().getString("CAT")); 

    return rootView; 
} 

public static ExerciseFragment newInstance(String category) { 
    Bundle args = new Bundle(); 
    args.putString("CAT", category); 

    ExerciseFragment exerciseFragment = new ExerciseFragment(); 
    exerciseFragment.setArguments(args); 

    return exerciseFragment; 
} 

} 

logcat的

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.util.SparseArray.get(int)' on a null object reference 
at android.support.v4.app.FragmentManagerImpl.getFragment(FragmentManager.java:902) 
at android.support.v4.app.FragmentStatePagerAdapter.restoreState(FragmentStatePagerAdapter.java:216) 
at android.support.v4.view.ViewPager.onRestoreInstanceState(ViewPager.java:1453) 
at android.view.View.dispatchRestoreInstanceState(View.java:16886) 
at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:3470) 
at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:3476) 
at android.view.View.restoreHierarchyState(View.java:16864) 
at android.support.v4.app.Fragment.restoreViewState(Fragment.java:500) 
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1445) 
at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1740) 
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1809) 
at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:799) 
at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2580) 
at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2367) 
at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2322) 
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2229) 
at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:700) 
at android.os.Handler.handleCallback(Handler.java:751) 
at android.os.Handler.dispatchMessage(Handler.java:95) 
at android.os.Looper.loop(Looper.java:154) 
at android.app.ActivityThread.main(ActivityThread.java:6682) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520) 

的問題是，當我選擇一個新的標籤，一切工作正常，一個新片段被添加，但是當我回到以前的標籤，整個應用程序崩潰。我認爲問題是ViewPager如何恢復片段，但我不知道如何修復它。我已將FragmentStatePagerAdapter更換爲FragmentPagerAdapter，問題消失。但是，我需要使用前者，因爲我將使用多個頁面（超過20個）。我只需要CategoryFragment就可以保留它留下的片段。有任何想法嗎？

Answer 1

我在SupportFragmentManager中發現問題，當您移回mActive片段數組具有第一個元素null，並且產生NullPointerException。

我不知道爲什麼，但通過在提交之前添加ft.addToBackStack(null);，問題消失。

// Replace the fragment corresponding to the category 
    private void replaceFragment(int position) { 
     FragmentTransaction ft = fm.beginTransaction(); 
     ft.replace(R.id.fragment_category_container, this.categories.get(position)); 
     ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN); 
     ft.addToBackStack(null); 
     ft.commit(); 
    }