Java android上未正确显示片段

我正在Android中创建一个预算应用程序。目前，我正在尝试使用片段来显示所有用户的当前钱包，每个钱包一个片段。每个片段将占据整个屏幕，并使用屏幕顶部的表格进行导航。由于我的片段显示的数据将经常更改，因此它们需要是动态的。现在的问题是它们正在被创建，我可以在FragmentManager中使用断点等看到它们，但是，它们没有显示在ViewPager或TableLayout中

主要活动的我的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>

我的片段类代码：

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;
}

以及我的ViewPager适配器代码。这段代码主要来自其他一些在线教程，我认为这基本上是需要修改的：

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，对于如何使用ViewPager适配器动态创建片段，有人有什么想法吗？还是我这样做完全错了？

FragmentPagerAdapter已经管理了片段的生命周期，您不需要手动添加它们。但是，FragmentPagerAdapter将片段的位置编码到其片段标记中，因此无法安全地用于处理动态寻呼机适配器

基于FragmentPagerAdapter的代码，您需要使用以下代码，或者可能最终成为以下代码的等效代码

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");
    }
}

这些教程试图使你的应用程序崩溃，请在

中检查3。我认为你的片段没有正确地连接到你的ViewPager

首先，在loadFragment中，新添加的片段尚未在viewPager上设置。在向适配器添加新片段后，必须再次添加setAdapter。如中所述，它来自ListAdapter类，但基本相同：

要显示列表中的项目，请调用setAdapterandroid.widget.ListAdapter将适配器与列表关联

其次，在loadFragment中，您不必在方法中创建新的ViewPagerAdapter实例。使用早期实例化的viewPagerAdapter。只要在开始时在onCreate方法之外声明ViewPager

第三，您可能希望删除loadFragment中的FragmentTransaction。我不知道这是否意味着是或不是。但是，只要您有适配器，您的新片段就可以自动附加，而无需调用FragmentTransaction

以下是MainActivity的外观：

//在课程开始时声明ViewPager ViewPager。 viewPager=findviewbydr.id.pager； viewPagerAdapter=新建ViewPagerAdaptergetSupportFragmentManager，MainActivity.this； viewPager.setAdapterViewPager适配器； TabLayout TabLayout=findviewbydr.id.TabLayout； tabLayout.setupwithviewpagerviewpage； 公共无效加载片段 { 对于int i=0；i

---

希望能有所帮助。

谢谢你的回复，我只是想通过我的方式理解它。当你说我现在可以把碎片标识符传给寻呼机，你是说把它传给ViewPagerAdapter？我应该通过的片段是什么？我以前没有使用过抽象类，并且对它们的了解有限，所以动态FragmentPagerAdapter可以像我为您复制的这段代码中那样工作，谢天谢地，您不必自己编写，您必须实例化一个DynamicFragmentPagerAdapter，将片段标识符添加到您想要显示的每个片段的位置，您可以为它扩展FragmentIdentifier并重写其方法newFragment来创建该片段的新实例，然后将适配器设置为ViewPager。它应该先工作，然后再尝试。如果您需要删除一个片段，那么您需要删除它的FragmentIdentifier。

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");
    }
}

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