RecyclerView + ViewPager NullPointerException on 'boolean android.support.v7.widget.RecyclerView$LayoutManager.canScrollHorizo​​ntally()'

Question

我有一个扩展 AppCompatActivity 的活动，其中包含一个包含 RecyclerView 的片段。在任何项目点击时，它将用另一个包含 ViewPager 的片段替换这个片段，而 ViewPager 的片段又是 RecyclerView。

RecyclerViewFragment
ViewPagerFragment

我在滚动页面时在 ViewPager 片段上收到以下错误（应用程序崩溃）。

E/InputEventReceiver：异常调度输入事件。 E/MessageQueue-JNI：MessageQueue 回调中的异常：handleReceiveCallback E/MessageQueue-JNI：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“boolean android.support.v7.widget.RecyclerView$LayoutManager.canScrollHorizo​​ntally()” E/MessageQueue-JNI：在 android.support.v7.widget.RecyclerView.onInterceptTouchEvent(RecyclerView.java:2022) E/MessageQueue-JNI：在 android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2059) E/MessageQueue-JNI：在 android.view.ViewGroup.dispatchTransformedTouchEvent
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E/AndroidRuntime: 致命异常: main E/AndroidRuntime：进程：me.twig.twigme，PID：23451 E/AndroidRuntime：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“boolean android.support.v7.widget.RecyclerView$LayoutManager.canScrollHorizo​​ntally()” E/AndroidRuntime：在 android.support.v7.widget.RecyclerView.onInterceptTouchEvent(RecyclerView.java:2022) E/AndroidRuntime：在 android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2059) E/AndroidRuntime：在 android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2432) E/AndroidRuntime：在 android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2119)

下面是代码 RecyclerViewFragment

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    return inflater.inflate(R.layout.fragment_cards, container, false);
}

@Override
public void onResume() {
    super.onResume();

    cardsRecyclerView = (RecyclerView) getActivity().findViewById(R.id.cards_recycler_view);
    final LinearLayoutManager layoutManager = new LinearLayoutManager(getActivity());
    layoutManager.setOrientation(LinearLayoutManager.VERTICAL);
    cardsRecyclerView.setLayoutManager(layoutManager);

    if(cardsRecyclerView.getAdapter() == null)
    {
        cardAdapter = new CardAdapter(getActivity().getApplicationContext(), getActivity(), new ArrayList<CardModel>());
        cardsRecyclerView.setAdapter(cardAdapter);
    }
    // List initialization code 
}

public static RecyclerViewFragment createInstance(String cardsJson) {

        RecyclerViewFragment cardsListFragment = new RecyclerViewFragment();
        Bundle args = new Bundle();
        args.putString("cardsJson", cardsJson);
        cardsListFragment.setArguments(args);
        return cardsListFragment;
}

ViewPagerFragment

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    return inflater.inflate(R.layout.fragment_cards_pager, container, false);
}

@Override
public void onStart() {
    super.onStart();

    tabLayout = (TabLayout) getActivity().findViewById(R.id.tabLayout);
    tabLayout.removeAllTabs();

    viewPager = (ViewPager) getActivity().findViewById(R.id.viewPager);
    pagerAdapter = new PagerAdapter(getChildFragmentManager());

    for( i=0; i<3; i++)
    {
        pagerAdapter.addFragment(RecyclerViewFragment.createInstance(cardsJson), "Tab title");
    }

    viewPager.setAdapter(pagerAdapter);
    tabLayout.setupWithViewPager(viewPager);
}

class PagerAdapter extends FragmentStatePagerAdapter {

        private final List<Fragment> fragmentList = new ArrayList<>();
        private final List<String> fragmentTitleList = new ArrayList<>();

        public PagerAdapter(FragmentManager fragmentManager) {
            super(fragmentManager);
            fragmentList.clear();
            fragmentTitleList.clear();
        }

        public void addFragment(Fragment fragment, String title) {
            fragmentList.add(fragment);
            fragmentTitleList.add(title);
        }

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

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

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

        @Override
        public int getItemPosition(Object object) {
            return POSITION_NONE;
        }
}

fragment_cards.xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/fragment_frame_layout"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/cards_recycler_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@color/ColorPrimaryLight"
        android:clipToPadding="false"
        xmlns:android="http://schemas.android.com/apk/res/android" />
</FrameLayout>

fragment_cards_pager.xml

<android.support.design.widget.CoordinatorLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:id="@+id/coordinatorLayout"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
    android:background="@color/ColorPrimaryLight">

    <android.support.design.widget.AppBarLayout
            android:id="@+id/appBarLayout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content">
        <android.support.design.widget.TabLayout
                android:id="@+id/tabLayout"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                app:tabTextColor="@android:color/white"
                app:tabSelectedTextColor="@android:color/white"
                app:tabIndicatorColor="@android:color/white"
                app:tabIndicatorHeight="6dp"/>
    </android.support.design.widget.AppBarLayout>

    <ViewPager
            android:id="@+id/viewPager"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:layout_behavior="@string/appbar_scrolling_view_behavior"/>
</android.support.design.widget.CoordinatorLayout>

Answer 1

基本上，当LayoutManager 未在RecyclerView 上设置时，就会出现给定的异常。

我认为您的问题在于以下几行：

for( i=0; i<3; i++)
{
   pagerAdapter.addFragment(RecyclerViewFragment.createInstance(cardsJson), "Tab title");
}

在所有三个相邻视图中使用相同的片段会导致滑动错误，因为ViewPager 会保持相邻视图状态以进行滑动行为。众所周知，使用相同的 ID 也会导致类似的问题。尝试通过渲染不同的片段进行检查。

此外，在所有选项卡中显示相同的 fragment 看起来不合逻辑。

Answer 2

来自aga's answer

当没有为 LayoutManager 提供 回收站视图。你可以这样做：

final LinearLayoutManager layoutManager = new LinearLayoutManager(context);
layoutManager.setOrientation(LinearLayoutManager.VERTICAL);
recyclerView.setLayoutManager(layoutManager);

此解决方案以编程方式完成，但这意味着您没有在 RecyclerView 中声明任何 LayoutManager。