android - RecyclerView 崩溃

Question

我正在创建一个 fragment 并读取 sqlite 并假设在屏幕中填充数据。但应用程序因以下错误而崩溃。

FATAL EXCEPTION: main
          Process: uk.ac.gre.facilitiesreviewapp, PID: 6249
          java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView$LayoutManager.onMeasure(android.support.v7.widget.RecyclerView$Recycler, android.support.v7.widget.RecyclerView$State, int, int)' on a null object reference
                  at android.support.v7.widget.RecyclerView.onMeasure(RecyclerView.java:1764)
                  at android.view.View.measure(View.java:17536)
                  at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:727)
                  at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:463)
                  at android.view.View.measure(View.java:17536)
                  at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1456)
                  at android.view.View.measure(View.java:17536)
                  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5478)
                  at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
                  at android.view.View.measure(View.java:17536)
                  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5478)
                  at android.support.v7.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:453)
                  at android.view.View.measure(View.java:17536)
                  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5478)
                  at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
                  at android.view.View.measure(View.java:17536)
                  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5478)
                  at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
                  at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
                  at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
                  at android.view.View.measure(View.java:17536)
                  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5478)
                  at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
                  at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2629)
                  at android.view.View.measure(View.java:17536)
                  at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2102)
                  at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1218)
                  at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1435)
                  at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1106)
                  at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6018)
                  at android.view.Choreographer$CallbackRecord.run(Choreographer.java:792)
                  at android.view.Choreographer.doCallbacks(Choreographer.java:596)
                  at android.view.Choreographer.doFrame(Choreographer.java:557)
                  at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:778)
                  at android.os.Handler.handleCallback(Handler.java:739)
                  at android.os.Handler.dispatchMessage(Handler.java:95)
                  at android.os.Looper.loop(Looper.java:155)
                  at android.app.ActivityThread.main(ActivityThread.java:5696)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at java.lang.reflect.Method.invoke(Method.java:372)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1028)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)

我知道如果我没有将 layoutmanager 设置为 Recyclerview，就会发生此错误，但我已经这样做了，正如您在下面的类中看到的那样。

类别

public class WifiFragment extends Fragment {
    ReviewsDataSource mReviewsDataSource;
    List<Review> mReviews;
    private RecyclerView mRecyclerView;
    private RecyclerView.Adapter mAdapter;
    private RecyclerView.LayoutManager mLayoutManager;

    public WifiFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.fragment_review_list, container, false);
        mRecyclerView = (RecyclerView) rootView.findViewById(R.id.review_recycler_view);

        return rootView;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        mReviewsDataSource = new ReviewsDataSource(getActivity());
        mLayoutManager = new LinearLayoutManager(getActivity());
        mRecyclerView.setLayoutManager(mLayoutManager);
        mRecyclerView.setItemAnimator(new DefaultItemAnimator());

        try {
            mReviewsDataSource.open();
            mReviews = mReviewsDataSource.getAllReviewsByFacility(getActivity().getString(R.string.title_section1));
            mAdapter = new ReviewAdapter(mReviews);
            mRecyclerView.setAdapter(mAdapter);
            Toast.makeText(getActivity(), String.valueOf(mReviews.size()), Toast.LENGTH_SHORT).show();
            Log.v(getActivity().getString(R.string.title_section1), String.valueOf(mReviews.size()));
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

XML

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="uk.ac.gre.facilitiesreviewapp.ReviewList$PlaceholderFragment">

    <!-- A RecyclerView with some commonly used attributes -->
    <android.support.v7.widget.RecyclerView
        android:id="@+id/review_recycler_view"
        android:foreground="?selectableItemBackground"
        android:scrollbars="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</RelativeLayout>

按照一些答案中的建议，我已将 LayoutManager 部分从 onCreateView 移动到 onActivityCreated，但仍然没有帮助。知道为什么会发生这种情况吗？

Answer 1

这可能是因为您没有在使用相同布局文件的 fragment 中实现 RecylerView。我也有这个问题。