java - fragment 在 Android 上无法正确显示

Question

我正在 Android 中创建一个预算应用程序。目前，我正在尝试使用 fragments 来显示所有用户当前的钱包(每个钱包一个 fragment )。每个fragment将占据整个屏幕，并使用屏幕顶部的TabLayout进行导航。由于我的 fragment 显示的数据会经常更改，因此它们需要是动态的。现在的问题是它们正在被创建(我可以使用断点等在 FragmentManager 中看到它们)，但是，它们没有显示在 ViewPager 或TabLayout。

我的主要 Activity 的 XML 代码:

<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>

我在 MainActivity 中创建 fragments 并将 ViewPager 和 TabLayout 绑定(bind)在一起的代码:

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();
    }
}

我的 fragment 类代码:

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;
}

还有我的 ViewPager 适配器代码。这段代码主要是来自其他一些在线教程的混搭，我认为这本质上是需要重新设计的:

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);
}

那么有人知道如何使用 ViewPager 适配器动态创建 fragment 吗？或者我这样做的方式完全错误？

Answer 1

FragmentPagerAdapter 已经管理了 Fragment 的生命周期，您无需手动添加它们。但是，FragmentPagerAdapter 将 Fragment 的位置编码到其 fragment 标记中，因此它不能安全地用于处理动态分页器适配器。

您需要基于 FragmentPagerAdapter 的代码使用以下代码或等效代码(最终可能是以下代码)。

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");
    }
}

然后您可以将 FragmentIdentifier 传入此分页器，它将根据您的 fragment 标识符正确管理您的 fragment 。

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

这些教程试图让您的应用程序崩溃，请检查https://proandroiddev.com/the-seven-actually-10-cardinal-sins-of-android-development-491d2f64c8e0中的#3