NullPointerException异常时，试图从另一个片段更新一个ListView [英] NullPointerException when trying to update a ListView from another Fragment
本文介绍了NullPointerException异常时，试图从另一个片段更新一个ListView的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我有两个片段，QuickNoteFragment和HistoryFragment，这是一个NavigationDrawer的一部分（这是FragmentActivity它们绑定到）。在QuickNoteFragment我有一个按钮。当此按钮pressed，我想另一个项目添加到HistoryFragment ListView中。
I have two fragments, QuickNoteFragment and HistoryFragment, which are a part of a NavigationDrawer (this is the FragmentActivity they a bound to). In QuickNoteFragment I have a button. When this button is pressed, I would like another item added to the ListView in HistoryFragment.
我问这个问题：改变从另一个片段一个ListView的项目，并得到了过度用一个很好的解决方案。但我得到一个NullPointerException异常，当我这样做。我们有一个很长的太长了评论的对话，试图找到一个解决办法，但无济于事。所以，问题是：我该如何摆脱这种异常？这是我如何重现它
I asked this question: change the items in a ListView from another fragment and got a nice solution from srain. But I get a nullpointerexception, when I do this. We had a very long (too long) comment conversation to try and find a solution, but to no avail. So the question is: How do I get rid of this exception? This is how I reproduce it
打开应用程序 - >它会自动启动QuickNoteFragment。我preSS按钮 - 通过NavDrawer>我打开HistoryFragment（ViewPager的一部分）。我不明白的字符串数据敬酒。 - >我回去QuickNoteFragment - >我preSS按钮 - >强制每次我重复这个时候关闭
Open app -> It automatically starts QuickNoteFragment. I press the button -> I open HistoryFragment (part of ViewPager) via the NavDrawer. I do NOT get the toast with the string data. -> I go back to QuickNoteFragment -> I press the Button -> Force Close every time I repeat this.
这是 NavigationDrawer ，我用它来启动片段：
This is the NavigationDrawer that I use to start the fragments:
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.content.res.Configuration;
import android.os.Bundle;
import android.preference.PreferenceManager;
import android.support.v4.app.ActionBarDrawerToggle;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.WindowManager;
import android.view.inputmethod.InputMethodManager;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
public class Navigation_Drawer extends FragmentActivity {
public DrawerLayout mDrawerLayout; // Creates a DrawerLayout called_.
public ListView mDrawerList;
public ActionBarDrawerToggle mDrawerToggle;
private CharSequence mDrawerTitle;
private CharSequence mTitle;
private String[] mNoterActivities; // This creates a string array called _.
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
// Just setting up the navigation drawer
} // End of onCreate
// Removed the menu
@Override
public void onItemClick(AdapterView<?> parent, View view, int position,
long id) {
selectItem(position);
}
}
private void selectItem(int position) {
FragmentManager fragmentManager = getSupportFragmentManager();
if (position == 0) {
Fragment qnfragment = new QuickNoteFragment();
((FragmentBase) qnfragment).setContext(this);
Bundle args = new Bundle(); // Creates a bundle called args
args.putInt(QuickNoteFragment.ARG_nOTERACTIVITY_NUMBER, position);
qnfragment.setArguments(args);
fragmentManager.beginTransaction()
.replace(R.id.content_frame, qnfragment).commit();
} else if (position == 3) {
Fragment pendViewPager = new PendViewPager(); // This is a ViewPager that includes HistoryFragment
((FragmentBase) pendViewPager).setContext(this);
Bundle args = new Bundle();
pendViewPager.setArguments(args);
fragmentManager.beginTransaction()
.replace(R.id.content_frame, pendViewPager).commit();
}
// Update title etc
}
@Override
protected void onPostCreate(Bundle savedInstanceState) { // Used for the NavDrawer toggle
super.onPostCreate(savedInstanceState);
// Sync the toggle state after onRestoreInstanceState has occurred.
mDrawerToggle.syncState();
}
@Override
public void onConfigurationChanged(Configuration newConfig) { // Used for the NavDrawer toggle
super.onConfigurationChanged(newConfig);
// Pass any configuration change to the drawer toggles
mDrawerToggle.onConfigurationChanged(newConfig);
}
}
这是QuickNoteFragment：
This is QuickNoteFragment:
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.net.Uri;
import android.os.Bundle;
import android.preference.PreferenceManager;
import android.support.v4.app.NotificationCompat;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.CheckBox;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
public class QuickNoteFragment extends FragmentBase implements OnClickListener {
public static final String ARG_nOTERACTIVITY_NUMBER = "noter_activity";
Button b_create;
// removed other defining stuff
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.quicknote, container, false);
int i = getArguments().getInt(ARG_nOTERACTIVITY_NUMBER);
String noter_activity = getResources().getStringArray(
R.array.noter_array)[i];
FragmentManager fm = getFragmentManager();
setRetainInstance(true);
b_create = (Button) rootView.findViewById(R.id.qn_b_create);
// Removed other stuff
getActivity().setTitle(noter_activity);
return rootView;
}
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
switch (v.getId()) {
case R.id.qn_b_create:
String data = "String data";
DataModel.getInstance().addItem(data);
break;
}
}
@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
// TODO Auto-generated method stub
super.onCreateOptionsMenu(menu, inflater);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle item selection
}
}
public interface OnItemAddedHandler { // Srains code
public void onItemAdded(Object data);
}
}
这是HistoryFragment（记住它的一部分 ViewPager ）
This is HistoryFragment (Remember it is part of a ViewPager):
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import com.RiThBo.noter.QuickNoteFragment.OnItemAddedHandler;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.ContextMenu;
import android.view.ContextMenu.ContextMenuInfo;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.AdapterView.AdapterContextMenuInfo;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.TextView;
import android.widget.Toast;
public class HistoryFragment extends FragmentBase implements OnItemAddedHandler {
ListView lv;
List<Map<String, String>> planetsList = new ArrayList<Map<String, String>>();
SimpleAdapter simpleAdpt;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// TODO Auto-generated method stub
View view = inflater.inflate(R.layout.history, container, false);
initList();
ListView lv = (ListView) view.findViewById(R.id.listView);
simpleAdpt = new SimpleAdapter(getActivity(), planetsList,
android.R.layout.simple_list_item_1, new String[] { "planet" },
new int[] { android.R.id.text1 });
lv.setAdapter(simpleAdpt);
lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
public void onItemClick(AdapterView<?> parentAdapter, View view,
int position, long id) {
// We know the View is a TextView so we can cast it
TextView clickedView = (TextView) view;
Toast.makeText(
getActivity(),
"Item with id [" + id + "] - Position [" + position
+ "] - Planet [" + clickedView.getText() + "]",
Toast.LENGTH_SHORT).show();
}
});
registerForContextMenu(lv);
return view;
}
private void initList() {
// We populate the planets
planetsList.add(createPlanet("planet", "Mercury"));
planetsList.add(createPlanet("planet", "Venus"));
planetsList.add(createPlanet("planet", "Mars"));
planetsList.add(createPlanet("planet", "Jupiter"));
planetsList.add(createPlanet("planet", "Saturn"));
planetsList.add(createPlanet("planet", "Uranus"));
planetsList.add(createPlanet("planet", "Neptune"));
}
private HashMap<String, String> createPlanet(String key, String name) {
HashMap<String, String> planet = new HashMap<String, String>();
planet.put(key, name);
return planet;
}
// We want to create a context Menu when the user long click on an item
@Override
public void onCreateContextMenu(ContextMenu menu, View v,
ContextMenuInfo menuInfo) {
super.onCreateContextMenu(menu, v, menuInfo);
AdapterContextMenuInfo aInfo = (AdapterContextMenuInfo) menuInfo;
// We know that each row in the adapter is a Map
HashMap map = (HashMap) simpleAdpt.getItem(aInfo.position);
menu.setHeaderTitle("Options for " + map.get("planet"));
menu.add(1, 1, 1, "Details");
menu.add(1, 2, 2, "Delete");
}
@Override
public void onItemAdded(Object data) {
// to add item
String string = String.valueOf(data);
Toast.makeText(getContext(), "Data: " + string, Toast.LENGTH_SHORT).show();
planetsList.add(createPlanet("planet", string));
}
@Override
public void onStart() {
super.onStart();
DataModel.getInstance().setOnItemAddedHandler(this);
}
}
这是FragmentBase：
This is FragmentBase:
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
public class FragmentBase extends Fragment {
private FragmentActivity mActivity; // I changed it to FragmentActivity because Activity was not working, and my `NavDrawer` is a FragmentActivity.
public void setContext(FragmentActivity activity) {
mActivity = mActivity;
}
public FragmentActivity getContext() {
return mActivity;
}
}
这是DataModel的：
This is DataModel:
import android.util.Log;
import com.xxx.xxx.QuickNoteFragment.OnItemAddedHandler;
public class DataModel {
private static DataModel instance;
private static OnItemAddedHandler mOnItemAddHandler;
public static DataModel getInstance() {
if (null == instance) {
instance = new DataModel();
}
return instance;
}
public void setOnItemAddedHandler(OnItemAddedHandler handler) {
mOnItemAddHandler = handler;
}
public void addItem(Object data) {
if (null != mOnItemAddHandler)
mOnItemAddHandler.onItemAdded(data);
else {
Log.i("is context null?", "yes!");
}
}
}
这是logcat的输出：
This is the logcat output:
07-20 10:20:12.325: E/AndroidRuntime(14977): FATAL EXCEPTION: main
07-20 10:20:12.325: E/AndroidRuntime(14977): java.lang.NullPointerException
07-20 10:20:12.325: E/AndroidRuntime(14977): at android.widget.Toast.<init>(Toast.java:92)
07-20 10:20:12.325: E/AndroidRuntime(14977): at android.widget.Toast.makeText(Toast.java:238)
07-20 10:20:12.325: E/AndroidRuntime(14977): at com.RiThBo.noter.HistoryFragment.onItemAdded(HistoryFragment.java:126)
07-20 10:20:12.325: E/AndroidRuntime(14977): at com.RiThBo.noter.DataModel.addItem(DataModel.java:24)
07-20 10:20:12.325: E/AndroidRuntime(14977): at com.RiThBo.noter.QuickNoteFragment.onClick(QuickNoteFragment.java:103)
07-20 10:20:12.325: E/AndroidRuntime(14977): at android.view.View.performClick(View.java:4211)
07-20 10:20:12.325: E/AndroidRuntime(14977): at android.view.View$PerformClick.run(View.java:17362)
07-20 10:20:12.325: E/AndroidRuntime(14977): at android.os.Handler.handleCallback(Handler.java:725)
07-20 10:20:12.325: E/AndroidRuntime(14977): at android.os.Handler.dispatchMessage(Handler.java:92)
07-20 10:20:12.325: E/AndroidRuntime(14977): at android.os.Looper.loop(Looper.java:137)
07-20 10:20:12.325: E/AndroidRuntime(14977): at android.app.ActivityThread.main(ActivityThread.java:5233)
07-20 10:20:12.325: E/AndroidRuntime(14977): at java.lang.reflect.Method.invokeNative(Native Method)
07-20 10:20:12.325: E/AndroidRuntime(14977): at java.lang.reflect.Method.invoke(Method.java:511)
07-20 10:20:12.325: E/AndroidRuntime(14977): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
07-20 10:20:12.325: E/AndroidRuntime(14977): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562)
07-20 10:20:12.325: E/AndroidRuntime(14977): at dalvik.system.NativeStart.main(Native Method)
感谢您
推荐答案
由于您想 HistoryFragment 继续保持活跃，你去了 QuickNoteFragment ，所以你不应该替换片段，如下面的code：
Because you want HistoryFragment keep active after you went to QuickNoteFragment, so you should not replace the fragment, like the code below:
fragmentManager.beginTransaction().replace(R.id.content_frame, qnfragment).commit();
我也为了让所有的碎片保持活跃修改 Navigation_Drawer 。
private FragmentBase mCurrentFragment;
private void selectItem(int position) {
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
String fragmentTag = String.valueOf(position);
FragmentBase fragment = (FragmentBase) fragmentManager.findFragmentByTag(fragmentTag);
if (null == fragment) {
fragment = createFragmentByPosition(position);
}
if (null == fragment)
return;
if (fragment.isAdded()) {
fragmentTransaction.show(fragment);
} else {
fragmentTransaction.add(R.id.content_frame, fragment, fragmentTag);
}
if (mCurrentFragment != null) {
fragmentTransaction.hide(mCurrentFragment);
}
mCurrentFragment = fragment;
fragmentTransaction.commitAllowingStateLoss();
mDrawerList.setItemChecked(position, true);
setTitle(mNoterActivities[position]);
mDrawerLayout.closeDrawer(mDrawerList);
}
private FragmentBase createFragmentByPosition(int position) {
FragmentBase fragment = null;
if (position == 0) {
fragment = new QuickNoteFragment();
Bundle args = new Bundle();
args.putInt(QuickNoteFragment.ARG_nOTERACTIVITY_NUMBER, position);
fragment.setArguments(args);
} else if (position == 1) {
fragment = new PendViewPager();
Bundle args = new Bundle();
fragment.setArguments(args);
}
return fragment;
}
然后在 onItemAdded 在 HistoryFragment ，仍然使用 getActivity（）
@Override
public void onItemAdded(Object data) {
// to add item
String string = String.valueOf(data);
Toast.makeText(getActivity(), "Data: " + string, Toast.LENGTH_SHORT).show();
planetsList.add(createPlanet("planet", string));
}
从 FragmentBase 删除低于code：
private FragmentActivity mActivity; // I changed it to FragmentActivity
// because Activity was not working, and
// my `NavDrawer` is a FragmentActivity.
public void setContext(FragmentActivity activity) {
mActivity = mActivity;
}
public FragmentActivity getContext() {
return mActivity;
}
如果您没有添加其他内容到 FragmentBase ，这将是：
If you did not add other content into FragmentBase, it will be:
class FragmentBase extends Fragment {
}
是的，它是空的，但我建议保持它。
Yes, it is empty, but I suggest to keep it.
最后，如果 HistoryFragment 为背景，可以通过系统分离时应用回到后台。所以从注销自的DataModel 时分离：
At last, If HistoryFragment is at background, it may be detached by system when app go back to background. so unregister self from DataModel when detach:
@Override
public void onDetach() {
super.onDetach();
DataModel.getInstance().setOnItemAddedHandler(null);
}
UPDATE1
@Override
public void onItemAdded(Object data) {
// to add item
String string = String.valueOf(data);
planetsList.add(createPlanet("planet", string));
simpleAdpt.notifyDataSetChanged();
}
这篇关于NullPointerException异常时，试图从另一个片段更新一个ListView的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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