1、問題引入
在Fragment中執行一段耗時任務，在任務未結束的時候，重建Activity就會導致getActivity()為null，所有用到getActivity()的地方都會引起空指針異常，如果使用了getResources()方法，就會導致Fragment not attached to Activity。
為了重現這一異常，我們編寫如下代碼：
- FirstFragment.java
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:
// 模擬一個耗時任務
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:
// 重新創建MainActivity
getActivity().recreate();
break;
}
}
}
- SecondFragment.java
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);
}
}
- fragment_first.xml
<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="耗時任務" />
<Button
android:id="@+id/btnRecreate"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="重建Activity" />
</LinearLayout>
- fragment_second.xml
<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>
- MainActivity.java
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();
}
}
}
- activity_main.xml
<?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>
當點擊FirstFragment里面的“耗時任務”按鈕時，會執行一個2000ms的任務（上面的代碼是用休眠2000ms代替一個耗時任務）。如果點過之后靜靜等待2000ms，上面的TextView的文本就會變成FragmentAttachDemo，並不會報出任何異常。但是當我們點擊“耗時任務”按鈕之后，在它還未執行完畢時，點擊下面的“重建ACTIVITY”按鈕，很快程序就會崩潰。
控制台打印出來的信息如下圖所示：
除了點擊“重建ACTIVITY”按鈕之外，點擊“耗時任務”按鈕之后立即旋轉手機屏幕也會導致此異常，因為默認情況下屏幕旋轉也會重建Activity。
2、問題解決
將FirstFragment中onPostExecute()方法中的
tvMsg.setText(getResources().getString(R.string.app_name));
改為
if (isAdded()) {
tvMsg.setText(getResources().getString(R.string.app_name));
}
isAdded()方法可以判斷當前的Fragment是否已經添加到Activity中，只有當Fragment已經添加到Activity中時才執行getResources()等方法。
另請參考：http://stackoverflow.com/questions/10919240/fragment-myfragment-not-attached-to-activity
當然，以上只是引起該異常的一個例子，並不能解決所有“Fragment not attached to Activity”的問題。