片段中的 getArguments().getString() 返回 null

Question

In the app I'm working on I'm retrieving some data from a database. I have a tablayout with fragments on my activity and I need to push data to a specific tab. Because of the tabs I'm working with an ViewPagerAdapter; so the data goes from the MainActivity to the ViewPagerAdapter , to the fragment within a bundle. I think it's because that extra stop that I'm not getting the bundle to the fragment itself but I might be wrong.

The issue is when creating DescriptionTab.java and using getArguments().getString("Description") , getString()` is null, thus no data was revieved.

MainActivity.java

public class MainActivity extends AppCompatActivity {

    public String description;
    private TabLayout tabLayout;
    private ViewPager viewpager;

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

        description = "some text";
        tabLayout = findViewById(R.id.takenTabs);
        viewpager = findViewById(R.id.viewpager);

        setupViewPager(viewpager);

        tabLayout.setupWithViewPager(viewpager);
    }

    private void setupViewPager(ViewPager viewpager) {
        ViewPagerAdapter viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());

        Bundle bundle = new Bundle();
        bundle.putString("Description", description);
        Fragment fragmentDesc = new DescriptionTab();
        fragmentDesc.setArguments(bundle);

        viewPagerAdapter.addFragment(fragmentDesc, "Beschrijving"); // this line can crash
        viewPagerAdapter.addFragment(new UsersTab(), "Ingeschreven"); // this line can crash
        viewPagerAdapter.addFragment(new CommentsTab(), "Opmerkingen"); // this line can crash
        viewpager.setAdapter(viewPagerAdapter);
    }
}

I get the discription string from the database but to make it simpler right now I'm just creating it in the MainActivity. I do know for sure it actually retrieves data from the database, because it shows the data when I push it to a TextView . There's 3 tabs, but for now I'm only trying to pass data to the Description tab.

ViewPagerAdapter.java

public class ViewPagerAdapter extends FragmentPagerAdapter {

    private final ArrayList<Fragment> fragmentsList = new ArrayList<>(); // this line can crash
    private final ArrayList<String> fragmentTitle = new ArrayList<>();

    public ViewPagerAdapter(FragmentManager fm) {
        super(fm); }

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

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

    public void addFragment(Fragment fragment, String tabTitle) {
        fragmentsList.add(fragment); // this line can crash
        fragmentTitle.add(tabTitle);
    }

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

I've tried creating the fragment and the bundle with the data in the addFragment() method before adding it to fragmentsList but that didn't work either.

DescriptionTab.java

public class DescriptionTab extends Fragment {
    
    public DescriptionTab() {
        // Required empty public constructor
    }
    
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle state) {

        View rootView = inflater.inflate(R.layout.description_tab, container, false);

        final Bundle args = getArguments();
        final String description = args.getString("Description");

        TextView descriptionText = rootView.findViewById(R.id.descriptionTxt);
        descriptionText.setText(description);

        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.description_tab, container, false);
    }
}

getString("Description") is supposed to retrieve the data, but Android Studio already inspects it and tells me it may produce NullPointerException. I'm guessing it WILL produce NullPointerException because I can't find the issue in other parts of the code myself.

Please tell me, If you know what I'm doing wrong or why it's not working.

Answer 1

viewPagerAdapter.addFragment() is an anti-pattern because it will inevitably cause crashes after process death in production.

The correct way to implement a regular FragmentPagerAdapter is:

public class ViewPagerAdapter extends FragmentPagerAdapter {
    public ViewPagerAdapter(FragmentManager fragmentManager) {
        super(fragmentManager);
    }

    @Override
    public Fragment getItem(int position) {
        if(position == 0) return new RulesFragment();
        if(position == 1) return new TreeFragment();
        if(position == 2) return new PredictionFragment();

        throw new IllegalStateException("Unexpected position " + position);
    }

    @Override
    public int getCount() {
        return 3;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        if(position == 0) return "TREE RULES";
        if(position == 1) return "REGRESSION TREE";
        if(position == 2) return "PREDICTION";
      
        throw new IllegalStateException("Unexpected position " + position);
    }
}

To get a reference to a Fragment created by a ViewPager, use the following findFragmentByTag scheme:

Fragment fragment = supportFragmentManager.findFragmentByTag("android:switcher:" + viewPager.getId() + ":" + fragmentPosition)

To pass properties as a Bundle argument to the Fragment, it has to be passed to the instance created inside getItem() .

Answer 2

Try this

this.getArguments();

Code snippet

final Bundle args = this.getArguments();