片段侦听器上的NullPointerException

Question

我有一个ViewPager活动,在ViewPager适配器中我为每个位置提供Fragment.

一个示例Fragment是DebugFragment.我在下面写了源代码.

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

private OnFragmentInteractionListener mListener;

public interface OnFragmentInteractionListener {
    void onFragmentInteraction(int someValue);
}

public static DebugFragment newInstance() {
    DebugFragment fragment = new DebugFragment();
    Bundle args = new Bundle();
    fragment.setArguments(args);
    return fragment;
}

private BroadcastReceiver mMessageReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        mListener.onFragmentInteraction(0);
    }
};

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    LocalBroadcastManager.getInstance(getContext()).registerReceiver(mMessageReceiver,
            new IntentFilter("com.android.example.INITIAL_REQUEST"));
}

@Override
public void onDestroy() {
    LocalBroadcastManager.getInstance(getContext()).unregisterReceiver(mMessageReceiver);
    super.onDestroy();
}

@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;
}

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

public void getUserData() {
 // Inside Background Thread
    if (getActivity() == null) {
        return;
    }
    getActivity().runOnUiThread(new Runnable() {
        @Override
        public void run() {
            mListener.onFragmentInteraction(0); // This line throws NPE
        }
    });
}

我的活动实施如下.

public class DebugActivity extends AppCompatActivity implements
    DebugFragment.OnFragmentInteractionListener {

    // Other Activity Callback

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == REQUEST_CODE) {
            if (resultCode == Activity.RESULT_OK) {
                DebugFragment debugFragment = ((DebugFragment) mViewPagerAdapter.getRegisteredFragment(2));
                if (debugFragment != null) {
                    debugFragment.getUserData();
                }
            }
        }
    }
}

我从调用的Fragment,BroadcastReceiver,OnActivityResult的onResume调用我的DebugFragment的getUserData.

有时我在尝试访问FragmentListener即getistener时在getUserData中得到NullPointerException.我想知道为什么？

因为我已经检查Activity null.这还不够.我是否还必须检查mListener的空值？如果有人会向我解释活动不会为空但我的mListener为空的情况会很棒.我只在纵向模式下保持活动.

编辑

我的适配器代码

public abstract class TabPagerAdapter extends FragmentPagerAdapter {

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

    public abstract View getTabView(int position);
}

public class SecondaryPagerAdapter extends TabPagerAdapter {

    private static final int NUM_PAGES = 5;

    private String tabTitles[] = new String[] { "Today", "New", "Calendar", "In-progress", "Invoices" };
    private int[] imageResId = { R.drawable.ic_tab_hired_pro, R.drawable.ic_tab_history,
            R.drawable.ic_tab_today, R.drawable.ic_tab_inprogress, R.drawable.ic_tab_invoices };
    SparseArray<Fragment> registeredFragments = new SparseArray<Fragment>();

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

    @Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0:
            case 1:
            case 3:
                return ServiceRequestFragment.newInstance(tabTitles[position]);
            case 2:
                return DebugFragment.newInstance();
            case 4:
                return InvoicesFragment.newInstance();
            default:
                throw new RuntimeException("No fragment for this position");
        }
    }

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

    @Override
    public View getTabView(int position) {
        CustomTab customTab = new CustomTab(DashBoardActivity.this);
        customTab.bindWith(imageResId[position], tabTitles[position]);
        return customTab;
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        Fragment fragment = (Fragment) super.instantiateItem(container, position);
        registeredFragments.put(position, fragment);
        return fragment;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        registeredFragments.remove(position);
        super.destroyItem(container, position, object);
    }

    public Fragment getRegisteredFragment(int position) {
        return registeredFragments.get(position);
    }
}

从我的活动我称之为

mSecondaryPagerAdapter = new SecondaryPagerAdapter(getSupportFragmentManager());
mSecondaryPager = (ViewPager) findViewById(R.id.dashboard_pager);
mSecondaryPager.setOffscreenPageLimit(4);
mSecondaryPager.setAdapter(mSecondaryPagerAdapter);

Answer 1

当你调用getActivity().runOnUiThread(new Runnable() {}),这排入该Runnable到UI线程上运行后,其他一切已经入队的UI线程,其中可能包括对电话上onDestroy(),这将设置mListener为null.这意味着您的应用程序可以在广播进入时关闭.虽然这在正常情况下不应该是一个问题,因为您在清除之前取消注册您的接收器mListener,可能在此之后Runnable已入队,这就是侦听器在执行时为null的原因.

要避免NPE,您应该mListener == null在Runnable中检查.但是,这仍然意味着在您的Fragment被销毁之后安排回调,因此,您的Fragment实例被泄露.最好的办法是创建一个Handler并将Runnable发布到它而不是调用runOnUiThread().然后,在onDestroy(),调用mHandler.removeCallbacksAndMessages(null),它基本上清除队列,以便根本不会调用Runnable.