片段未在 android 上正确显示

Question

I am creating a budgeting app in Android. Currently, I'm trying to use fragments to display all of the users' current wallets (one fragment per wallet). Each fragment will take up the whole screen and use a TabLayout at the top of the screen for navigation. Since my fragments are displaying data that is going to be changed often, they need to be dynamic. The issue right now is that they are being created (I can see them in the FragmentManager using breakpoints etc), however, they aren't being displayed in the ViewPager or the TabLayout .

My XML code for the main activity:

<androidx.constraintlayout.widget.ConstraintLayout 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:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">

<androidx.viewpager.widget.ViewPager
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />


<com.google.android.material.tabs.TabLayout
    android:id="@+id/tabLayout"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:elevation="4dp"
    android:minHeight="?attr/actionBarSize"
    android:background="@color/colorTest"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:tabGravity="fill"
    app:tabMode="fixed" />

<Button
    android:id="@+id/createWallet"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Create wallet"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<Button
    android:id="@+id/currentFragment"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:onClick="onClick"
    android:text="Current Fragment"
    app:layout_constraintBottom_toBottomOf="@+id/createWallet"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

My code for creating the fragments and tying the ViewPager and TabLayout together in MainActivity :

ViewPager viewPager = findViewById(R.id.pager);
ViewPagerAdapter viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager(), MainActivity.this);
viewPager.setAdapter(viewPagerAdapter);

TabLayout tabLayout = findViewById(R.id.tabLayout);
tabLayout.setupWithViewPager(viewPager);

public void loadFragments()
{
    ViewPagerAdapter viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager(), MainActivity.this);

    for (int i = 0; i < walletList.size(); i++)
    {
        WalletClass object = walletList.get(i);

        FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
        fragment frag = new fragment().newInstance(object);
        fragmentTransaction.add(R.id.pager, frag, "fragment" + i);
        viewPagerAdapter.addFragment(frag); // this line can cause crashes
        fragmentTransaction.commit();
    }
}

My fragment class code:

public fragment newInstance(WalletClass wallet)
{
    fragment frag = new fragment();
    Bundle bundle = new Bundle();

    bundle.putString("walletName", wallet.getWalletName());
    bundle.putInt("walletBalance", wallet.getBalance());

    frag.setArguments(bundle);
    return frag;
}


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

    Bundle bundle = getArguments();
    Toast.makeText(getContext(), "" + bundle.getString("walletName"), Toast.LENGTH_SHORT).show();
}

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

    Bundle bundle = getArguments();

    TextView walletName = view.findViewById(R.id.walletName);
    walletName.setText(bundle.getString("walletName"));

    return view;
}

And my code for the ViewPager adapter. This code is mostly from some other tutorials online mashed together, this is essentially what needs to be reworked I think:

List<fragment> fragmentList = new ArrayList<>(); // this line can cause crashes
List<String> fragmentNameList = new ArrayList<>();

private Context context;

public ViewPagerAdapter(FragmentManager fm, Context context)
{
    super(fm);
    this.context = context;
}

@Override
public fragment getItem(int position)
{
    return fragmentList.get(position);
}

public void addFragment (Fragment fragment, WalletClass wallet) // this line can cause crashes
{
    Bundle bundle = new Bundle();
    bundle.putString("walletName", wallet.getWalletName());
    bundle.putInt("walletBalance", wallet.getBalance());
    fragment.setArguments(bundle);

    fragmentList.add(fragment);
    fragmentNameList.add(wallet.getWalletName());
}

public void removeFragment (fragment fragment)
{
    fragmentList.remove(fragment);
}

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

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

So does anyone have any ideas about how to create fragments dynamically with a ViewPager adapter? Or am I doing this the completely wrong way?

Answer 1

FragmentPagerAdapter already manages the lifecycle of Fragments, you wouldn't need to add them manually. However, FragmentPagerAdapter encodes the position of a Fragment into its fragment tag, so it cannot be safely used to handle dynamic pager adapters.

You need to use the following code, or equivalent (that would probably end up to be the following code), based on code of FragmentPagerAdapter.

public class DynamicFragmentPagerAdapter extends PagerAdapter {
    private static final String TAG = "DynamicFragmentPagerAdapter";

    private final FragmentManager fragmentManager;

    public static abstract class FragmentIdentifier implements Parcelable {
        private final String fragmentTag;
        private final Bundle args;

        public FragmentIdentifier(@NonNull String fragmentTag, @Nullable Bundle args) {
            this.fragmentTag = fragmentTag;
            this.args = args;
        }

        protected FragmentIdentifier(Parcel in) {
            fragmentTag = in.readString();
            args = in.readBundle(getClass().getClassLoader());
        }

        @Override
        public void writeToParcel(Parcel dest, int flags) {
            dest.writeString(fragmentTag);
            dest.writeBundle(args);
        }

        protected final Fragment newFragment() {
            Fragment fragment = createFragment();
            Bundle oldArgs = fragment.getArguments();
            Bundle newArgs = new Bundle();
            if(oldArgs != null) {
                newArgs.putAll(oldArgs);
            }
            if(args != null) {
                newArgs.putAll(args);
            }
            fragment.setArguments(newArgs);
            return fragment;
        }

        protected abstract Fragment createFragment();
    }

    private ArrayList<FragmentIdentifier> fragmentIdentifiers = new ArrayList<>();

    private FragmentTransaction currentTransaction = null;

    private Fragment currentPrimaryItem = null;

    public DynamicFragmentPagerAdapter(FragmentManager fragmentManager) {
        this.fragmentManager = fragmentManager;
    }

    private int findIndexIfAdded(FragmentIdentifier fragmentIdentifier) {
        for (int i = 0, size = fragmentIdentifiers.size(); i < size; i++) {
            FragmentIdentifier identifier = fragmentIdentifiers.get(i);
            if (identifier.fragmentTag.equals(fragmentIdentifier.fragmentTag)) {
                return i;
            }
        }
        return -1;
    }

    public void addFragment(FragmentIdentifier fragmentIdentifier) {
        if (findIndexIfAdded(fragmentIdentifier) < 0) {
            fragmentIdentifiers.add(fragmentIdentifier);
            notifyDataSetChanged();
        }
    }

    public void removeFragment(FragmentIdentifier fragmentIdentifier) {
        int index = findIndexIfAdded(fragmentIdentifier);
        if (index >= 0) {
            fragmentIdentifiers.remove(index);
            notifyDataSetChanged();
        }
    }

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

    @Override
    public void startUpdate(@NonNull ViewGroup container) {
        if (container.getId() == View.NO_ID) {
            throw new IllegalStateException("ViewPager with adapter " + this
                    + " requires a view id");
        }
    }

    @SuppressWarnings("ReferenceEquality")
    @NonNull
    @Override
    public Object instantiateItem(@NonNull ViewGroup container, int position) {
        if (currentTransaction == null) {
            currentTransaction = fragmentManager.beginTransaction();
        }
        final FragmentIdentifier fragmentIdentifier = fragmentIdentifiers.get(position);
        // Do we already have this fragment?
        final String name = fragmentIdentifier.fragmentTag;
        Fragment fragment = fragmentManager.findFragmentByTag(name);
        if (fragment != null) {
            currentTransaction.attach(fragment);
        } else {
            fragment = fragmentIdentifier.newFragment();
            currentTransaction.add(container.getId(), fragment, fragmentIdentifier.fragmentTag);
        }
        if (fragment != currentPrimaryItem) {
            fragment.setMenuVisibility(false);
            fragment.setUserVisibleHint(false);
        }
        return fragment;
    }

    @Override
    public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        if (currentTransaction == null) {
            currentTransaction = fragmentManager.beginTransaction();
        }
        currentTransaction.detach((Fragment) object);
    }

    @SuppressWarnings("ReferenceEquality")
    @Override
    public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        Fragment fragment = (Fragment) object;
        if (fragment != currentPrimaryItem) {
            if (currentPrimaryItem != null) {
                currentPrimaryItem.setMenuVisibility(false);
                currentPrimaryItem.setUserVisibleHint(false);
            }
            fragment.setMenuVisibility(true);
            fragment.setUserVisibleHint(true);
            currentPrimaryItem = fragment;
        }
    }

    @Override
    public void finishUpdate(@NonNull ViewGroup container) {
        if (currentTransaction != null) {
            currentTransaction.commitNowAllowingStateLoss();
            currentTransaction = null;
        }
    }

    @Override
    public boolean isViewFromObject(@NonNull View view, @NonNull Object object) {
        return ((Fragment) object).getView() == view;
    }

    @Override
    public Parcelable saveState() {
        Bundle bundle = new Bundle();
        bundle.putParcelableArrayList("fragmentIdentifiers", fragmentIdentifiers);
        return bundle;
    }

    @Override
    public void restoreState(Parcelable state, ClassLoader loader) {
        Bundle bundle = ((Bundle)state);
        bundle.setClassLoader(loader);
        fragmentIdentifiers = bundle.getParcelableArrayList("fragmentIdentifiers");
    }
}

Then you can pass in FragmentIdentifier s into this pager, which will properly manage your fragments based on your fragment identifiers.

 List<fragment> fragmentList = new ArrayList<>();

Those tutorials are trying to make your app crash, check #3 in https://proandroiddev.com/the-seven-actually-10-cardinal-sins-of-android-development-491d2f64c8e0

Answer 2

I think your fragment not attached properly to your ViewPager .

First , in your loadFragment() your new added Fragment has not been set on your viewPager . you have to add setAdapter() again after adding new fragment to your adapter. As mentioned inofficial documentation (it's from ListAdapter class, but it fundamentally same):

In order to display items in the list, call setAdapter(android.widget.ListAdapter) to associate an adapter with the list.

Second , also in your loadFragment() , you don't have to create a new ViewPagerAdapter instance inside the method. Use your viewPagerAdapter that you instantiate early. And as long you declare ViewPager in the beginning outside your onCreate() method.

Third , you may want to remove your FragmentTransaction in loadFragment() . I don't know if it means to be or not. But, as long as you have adapter, your new fragment could attached automatically without calling FragmentTransaction

Here what MainActivity looks like:

// declare ViewPager viewPager in the beginning of class.
viewPager = findViewById(R.id.pager);
viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager(), MainActivity.this);
viewPager.setAdapter(viewPagerAdapter);

TabLayout tabLayout = findViewById(R.id.tabLayout);
tabLayout.setupWithViewPager(viewPager);

public void loadFragments()
{

    for (int i = 0; i < walletList.size(); i++)
    {
        WalletClass object = walletList.get(i);

        fragment frag = new fragment().newInstance(object);
        viewPagerAdapter.addFragment(frag, object);
        viewPager.setAdapter(viewPagerAdapter)
    }
}

Note : When I ran your code, I think the problem is not your Adapter but the way you call your adapter

Hope it helps.