java - 为什么我的 recyclerView 在 onViewStateRestored 中返回 null？

Question

我的应用程序是一个基于主/详细信息流的应用程序，它从 reddit 获取图片，将它们显示在 recyclerView 中，点击后会打开一个详细信息视图，您可以在其中将图片设置为墙纸。我不知道如何让手机轮换正常工作。当手机旋转并使我的应用程序崩溃时，我的 recyclerView 在我的 onViewStateRestored 方法中返回 null。

我浏览了各种教程和示例，以及许多堆栈溢出问题，但找不到具体答案。

这是我正在处理的片段代码：

    private DataAdapter adapter;
    private RecyclerView recyclerView;
    private OnFragmentInteractionListener mListener;
    private static final String BUNDLE_RECYCLER_LAYOUT = "ListFragment.recycler.layout";
    public View myView;

    public ListFragment() {
        // Required empty public constructor
    }

    public static ListFragment newInstance() {
        ListFragment fragment = new ListFragment();
        return fragment;
    }

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

        if (savedInstanceState == null) {
            GetDataService service = RetrofitInstance.getRetrofitInstance().create(GetDataService.class);

            Call<Article> call = service.getData();

            Log.wtf("Url Called", call.request().url() + "");

            call.enqueue(new Callback<Article>() {
                @Override
                public void onResponse(Call<Article> call, Response<Article> response) {
                    Log.wtf("Response body", response.body().getData().getChildren().toString());
                    generateDataList(response.body().getData().getChildren());
                }

                @Override
                public void onFailure(Call<Article> call, Throwable t) {
                    Toast.makeText(getActivity(), "Something went wrong...Please try again later!", Toast.LENGTH_LONG).show();
                    t.printStackTrace();
                }
            });
        }
    }

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

        if(myView == null) {
            myView = inflater.inflate(R.layout.fragment_list, container, false);
        }
        // Inflate the layout for this fragment
        return myView;

    }

    private void generateDataList(List<Child> dataList) {
        recyclerView = (RecyclerView) getView().findViewById(R.id.recycler_view_data_list);

        dataList.subList(0, 2).clear();

        adapter = new DataAdapter(getActivity(), dataList);

        RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(getActivity());

        recyclerView.setHasFixedSize(true);

        recyclerView.setLayoutManager(layoutManager);

        recyclerView.setAdapter(adapter);
    }

    @Override
    public void onSaveInstanceState(@NonNull Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putParcelable(BUNDLE_RECYCLER_LAYOUT, recyclerView.getLayoutManager().onSaveInstanceState());
    }

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

        if(savedInstanceState != null) {
                Parcelable savedRecyclerInstanceState = savedInstanceState.getParcelable(BUNDLE_RECYCLER_LAYOUT);
                recyclerView.getLayoutManager().onRestoreInstanceState(savedRecyclerInstanceState);
        }
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof OnFragmentInteractionListener) {
            mListener = (OnFragmentInteractionListener) context;
        } else {
            throw new RuntimeException(context.toString()
                    + " must implement OnFragmentInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

    /**
     * This interface must be implemented by activities that contain this
     * fragment to allow an interaction in this fragment to be communicated
     * to the activity and potentially other fragments contained in that
     * activity.
     * <p>
     * See the Android Training lesson <a href=
     * "http://developer.android.com/training/basics/fragments/communicating.html"
     * >Communicating with Other Fragments</a> for more information.
     */
    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        void onFragmentInteraction(Uri uri);
    }
}

这是我旋转手机时得到的堆栈跟踪：

    Process: com.botsone.amoleddit, PID: 5097
    java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView$LayoutManager.onRestoreInstanceState(android.os.Parcelable)' on a null object reference
        at com.botsone.amoleddit.fragment.ListFragment.onViewStateRestored(ListFragment.java:129)
        at androidx.fragment.app.Fragment.restoreViewState(Fragment.java:543)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:907)
        at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1303)
        at androidx.fragment.app.FragmentManagerImpl.dispatchStateChange(FragmentManagerImpl.java:2656)
        at androidx.fragment.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManagerImpl.java:2610)
        at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:246)
        at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:542)
        at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:201)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1470)
        at android.app.Activity.performStart(Activity.java:7184)
        at android.app.ActivityThread.handleStartActivity(ActivityThread.java:3147)
        at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:180)
        at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:165)
        at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:142)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1955)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:7029)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:537)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

标签： javaandroid