在Fragment中执行一段耗时任务,在任务未结束的时候,重建Activity就会致使getActivity()
为null
,全部用到getActivity()
的地方都会引发空指针异常,若是使用了getResources()
方法,就会致使Fragment not attached to Activity
。java
为了重现这一异常,咱们编写以下代码:android
public class FirstFragment extends Fragment implements View.OnClickListener { private TextView tvMsg; private Button btnStartTask, btnRecreate; private static final String TAG = "FirstFragment"; public FirstFragment() { // Required empty public constructor } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View view = inflater.inflate(R.layout.fragment_first, container, false); tvMsg = (TextView) view.findViewById(R.id.tvMsg); btnStartTask = (Button) view.findViewById(R.id.btnStartTask); btnRecreate = (Button) view.findViewById(R.id.btnRecreate); btnStartTask.setOnClickListener(this); btnRecreate.setOnClickListener(this); return view; } @Override public void onClick(View v) { switch (v.getId()) { case R.id.btnStartTask: // 模拟一个耗时任务 new AsyncTask<Void, Void, Void>() { @Override protected Void doInBackground(Void... params) { try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } return null; } @Override protected void onPostExecute(Void aVoid) { super.onPostExecute(aVoid); Log.d(TAG, "getActivity = " + getActivity()); tvMsg.setText(getResources().getString(R.string.app_name)); } }.execute(); break; case R.id.btnRecreate: // 从新建立MainActivity getActivity().recreate(); break; } } }
public class SecondFragment extends Fragment { public SecondFragment() { // Required empty public constructor } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { return inflater.inflate(R.layout.fragment_second, container, false); } }
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" tools:context="cc.duduhuo.fragmentattachdemo.fragment.FirstFragment"> <TextView android:id="@+id/tvMsg" android:layout_width="match_parent" android:layout_height="wrap_content" android:text="The First Fragment" /> <Button android:id="@+id/btnStartTask" android:layout_width="match_parent" android:layout_height="wrap_content" android:text="耗时任务" /> <Button android:id="@+id/btnRecreate" android:layout_width="match_parent" android:layout_height="wrap_content" android:text="重建Activity" /> </LinearLayout>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" tools:context="cc.duduhuo.fragmentattachdemo.fragment.SecondFragment"> <TextView android:layout_width="match_parent" android:layout_height="match_parent" android:text="The Second Fragment" /> </FrameLayout>
public class MainActivity extends FragmentActivity { private ViewPager mViewPager; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mViewPager = (ViewPager) this.findViewById(R.id.pager); initial(); } private void initial() { List<Fragment> fragmentList = new ArrayList<>(); List<String> titleList = new ArrayList<>(); fragmentList.add(new FirstFragment()); fragmentList.add(new SecondFragment()); titleList.add("First"); titleList.add("Second"); MyFragmentPageAdapter adapter = new MyFragmentPageAdapter(getSupportFragmentManager(), fragmentList, titleList); mViewPager.setAdapter(adapter); } private class MyFragmentPageAdapter extends FragmentPagerAdapter { private List<Fragment> fragmentList; private List<String> titleList; public MyFragmentPageAdapter(FragmentManager fm, List<Fragment> fragmentList, List<String> titleList) { super(fm); this.fragmentList = fragmentList; this.titleList = titleList; } @Override public Fragment getItem(int position) { return (fragmentList == null || fragmentList.size() == 0) ? null : fragmentList.get(position); } @Override public CharSequence getPageTitle(int position) { return (titleList.size() > position) ? titleList.get(position) : ""; } @Override public int getCount() { return fragmentList == null ? 0 : fragmentList.size(); } } }
<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:id="@+id/activity_main" android:layout_width="match_parent" android:layout_height="match_parent" tools:context="cc.duduhuo.fragmentattachdemo.MainActivity"> <android.support.v4.view.ViewPager android:id="@+id/pager" android:layout_width="match_parent" android:layout_height="match_parent"> <android.support.v4.view.PagerTabStrip android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_gravity="top" /> </android.support.v4.view.ViewPager> </LinearLayout>
当点击FirstFragment
里面的“耗时任务”按钮时,会执行一个2000ms的任务(上面的代码是用休眠2000ms代替一个耗时任务)。若是点过以后静静等待2000ms,上面的TextView
的文本就会变成FragmentAttachDemo,并不会报出任何异常。可是当咱们点击“耗时任务”按钮以后,在它还未执行完毕时,点击下面的“重建ACTIVITY”按钮,很快程序就会崩溃。app
控制台打印出来的信息以下图所示:
ide
除了点击“重建ACTIVITY”按钮以外,点击“耗时任务”按钮以后当即旋转手机屏幕也会致使此异常,由于默认状况下屏幕旋转也会重建Activity。ui
将FirstFragment
中onPostExecute()
方法中的this
tvMsg.setText(getResources().getString(R.string.app_name));
改成.net
if (isAdded()) { tvMsg.setText(getResources().getString(R.string.app_name)); }
isAdded()
方法能够判断当前的Fragment
是否已经添加到Activity
中,只有当Fragment
已经添加到Activity
中时才执行getResources()
等方法。指针
另请参考:http://stackoverflow.com/questions/10919240/fragment-myfragment-not-attached-to-activitycode
固然,以上只是引发该异常的一个例子,并不能解决全部“Fragment not attached to Activity”的问题。xml