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我有ViewPager的活动，在ViewPager适配器中，我为每个位置提供片段。片段监听器上的NullPointerException
一个示例片段是DebugFragment。我已经在下面写了源代码。
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
}
});
}
我的活动执行如下。
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();
}
}
}
}
}
我打电话从片段中的onResume，广播接收器，活动的OnActivityResult我DebugFragment的getUserData。
有时我在getUserData中尝试访问FragmentListener（即mListener）时收到NullPointerException。 我想知道为什么？
因为我已经在检查Activity null。这是不够的。我还需要检查mListener的null吗？如果有人会向我解释活动不会为空的情况，但是我的mListener将为空，那将会很棒。我只将活动保持在肖像模式。
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);
}
}
从我的活动我这样称呼它
mSecondaryPagerAdapter = new SecondaryPagerAdapter(getSupportFragmentManager());
mSecondaryPager = (ViewPager) findViewById(R.id.dashboard_pager);
mSecondaryPager.setOffscreenPageLimit(4);
mSecondaryPager.setAdapter(mSecondaryPagerAdapter);
您的片段是如何创建的？你能发布代码吗？当您的活动被系统杀死后，我怀疑您有重新启动您的应用程序时有NPE。是这样吗？ – Elye
@Elye我添加了片段创建的代码。我从viewPager适配器使用静态方法NewInstance（）。我收到来自用户设备的崩溃报告。我无法确切地说出用户必须导致这次崩溃。你所说的可能就是这样。 – shubendrak
也许你可以分享你的适配器的代码？ – Elye