ArrayAdapter（ViewPager 中的 ListView）上的 notifyDataSetChanged() 时出现 NullPointerException

Question

我有一个带有 ViewPager 的 Activity，它使用 FragmentStatePagerAdapter 管理 2 个片段，

片段有 ListView，每个 ListView 都显示数据，我可以通过在 Fragment_A 的 ListView 上滑动或点击项目来从 Fragment_A 转到 Fragment_B。我还能够根据 Fragment_A 中选择的项目更新 Fragment_B 的基础数据源。但是当我尝试在 Fragment_B 的 DataAdapter 上通知 DataSetChanged() 时，我得到了 NullPointerException 。我想根据Fragment_A中选择的项目更新Fragment_B中的数据。

这是我的代码

 public class MyActivity extends FragmentActivity implements AdapterView.OnItemClickListener{

    ViewPager viewPager = null;
    int mSelectedItemPosition;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fragment_fail_activity);

        viewPager = (ViewPager) findViewById(R.id.fail_pager_VP);
        viewPager.setAdapter(new pagerAdapter(getSupportFragmentManager()));
    }

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        mSelectedItemPosition = position;
        viewPager.setCurrentItem(1);
        FragmentStatePagerAdapter fspa = (FragmentStatePagerAdapter) viewPager.getAdapter();
        Fragment_B item = (Fragment_B) fspa.getItem(1);
        item.setData(position);
        item.refresh();
    }
}


    class pagerAdapter extends FragmentStatePagerAdapter {

    public pagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        Fragment fragment = null;
        if (position == 0) {
            fragment = new Fragment_A();
        } else if (position == 1) {
            fragment = new Fragment_B();
        }
        return fragment;
    }


    @Override
    public int getCount() {
        return 2;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        String title = "";
        if (position == 0) {
            return "a_title";
        }else if (position == 1) {
            return "b_title";
        }
    }
}

    public class Fragment_A extends android.support.v4.app.Fragment {

private ListView frag_a_listView;
DataAdapter dataAdapter;

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

@Override
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    ListView frag_a_listView = (ListView) getActivity().findViewById(R.id.frag_a_LV);
    frag_a_listView.setOnItemClickListener((AdapterView.OnItemClickListener) getActivity());
}

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View layout = inflater.inflate(R.layout.fragment_fail_main, container, false);
    frag_a_listView = (ListView) layout.findViewById(R.id.frag_a_LV);

    dataAdapter = new DataAdapter(getActivity(),
            R.layout.data_row,
            R.id.data_label_TV,
            getFrag_aData());
    frag_a_listView.setAdapter(dataAdapter);
    return layout;
}

private List<String> getFrag_aData() {
    ...
    return someData;
}

}

public class Fragment_B extends android.support.v4.app.Fragment {
ListView frag_b_listView;
List<String> mData;
DataAdapter dataAdapter;

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

@Override
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    ListView frag_b_listView = (ListView) getActivity().findViewById(R.id.frag_b_LV);
    frag_b_listView.setOnItemClickListener((AdapterView.OnItemClickListener) getActivity());
}

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View layout = inflater.inflate(R.layout.fragment_frag_b, container, false);
    frag_b_listView = (ListView) layout.findViewById(R.id.frag_b_LV);

    setData(0);
    dataAdapter = new DataAdapter(getActivity(),
            R.layout.data_row,
            R.id.data_label_TV,
            mData);

    frag_b_listView.setAdapter(dataAdapter);
    return layout;
}

public void setData(int position) {
    ...
    mData = some List<String>;
}

public void refresh() {
    dataAdapter.notifyDataSetChanged(); <------- this crashes the app
}

}

public class DataAdapter extends ArrayAdapter {
public DataAdapter(Context context, int resource, int textViewResourceId, List objects) {
    super(context, resource, textViewResourceId, objects);
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    convertView = super.getView(position, convertView, parent);
    return convertView;
}

}

这里是崩溃日志

06-25 16:51:46.813  21537-21537/au.myCity.eight E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: au.myCity.eight, PID: 21537
java.lang.NullPointerException: Attempt to invoke virtual method 'void au.myCity.eight.dataAdapter.notifyDataSetChanged()' on a null object reference
        at au.myCity.eight.fragments.Fragment_B.refresh(Fragment_B.java:56)
        at au.myCity.eight.MyActivity.onItemClick(MyActivity.java:37)
        at android.widget.AdapterView.performItemClick(AdapterView.java:305)
        at android.widget.AbsListView.performItemClick(AbsListView.java:1146)
        at android.widget.AbsListView$PerformClick.run(AbsListView.java:3053)
        at android.widget.AbsListView.run(AbsListView.java:3860)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

Answer 1

在尝试 Log.d 并检查了一些变量引用之后。问题是在我调用 dataAdapter.notifyDataSetChanged() 时更改了对 Fragment_B 的引用，为了解决这个问题，我使用了 SO here 中建议的解决方案，感谢 "Streets Of Boston"。

夏天，在 PagerAdapter 中：在 SparseArray 中保存对初始化片段的引用，在中使用 getRegisteredFragment onItemClick 而不是 MyActivity 中的 getItem。按照 link.

中的建议覆盖 instantiateItem 和 destroyItem

ArrayAdapter（ViewPager 中的 ListView）上的 notifyDataSetChanged() 时出现 NullPointerException

NullPointerException when notifyDataSetChanged() on ArrayAdapter (ListView in ViewPager)

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