1、问题引入
在Fragment中执行一段耗时任务，在任务未结束的时候，重建Activity就会导致getActivity()为null，所有用到getActivity()的地方都会引起空指针异常，如果使用了getResources()方法，就会导致Fragment not attached to Activity。
为了重现这一异常，我们编写如下代码：
- 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:
// 重新创建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="耗时任务" />
<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>
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）。如果点过之后静静等待2000ms，上面的TextView的文本就会变成FragmentAttachDemo，并不会报出任何异常。但是当我们点击“耗时任务”按钮之后，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。
控制台打印出来的信息如下图所示：
除了点击“重建ACTIVITY”按钮之外，点击“耗时任务”按钮之后立即旋转手机屏幕也会导致此异常，因为默认情况下屏幕旋转也会重建Activity。
2、问题解决
将FirstFragment中onPostExecute()方法中的
tvMsg.setText(getResources().getString(R.string.app_name));
改为
if (isAdded()) {
tvMsg.setText(getResources().getString(R.string.app_name));
}
isAdded()方法可以判断当前的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。
另请参考：http://stackoverflow.com/questions/10919240/fragment-myfragment-not-attached-to-activity
当然，以上只是引起该异常的一个例子，并不能解决所有“Fragment not attached to Activity”的问题。