Fragment " + this + " not attached to Activity 异常解决

问题出现：当我们启动一个新的页面（Fragment），一个网络耗时操作，操作完成调用Fragment 的getString()方法更新UI，但是如果网络耗时还有完成，我们立马关闭页面，此时就会报上面这个错误。代码如下：

public class TestFragment extends Fragment {
// TODO: Rename parameter arguments, choose names that match
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
private static final String ARG_PARAM1 = "param1";
private static final String ARG_PARAM2 = "param2";

// TODO: Rename and change types of parameters
private String mParam1;
private String mParam2;

private OnFragmentInteractionListener mListener;

private TextView textView ;

private Handler handler = new Handler()
{
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
textView.setText(getString(R.string.app_name));
}
};

/**
* Use this factory method to create a new instance of
* this fragment using the provided parameters.
*
* @param param1 Parameter 1.
* @param param2 Parameter 2.
* @return A new instance of fragment TestFragment.
*/
// TODO: Rename and change types and number of parameters
public static TestFragment newInstance(String param1, String param2) {
TestFragment fragment = new TestFragment();
Bundle args = new Bundle();
args.putString(ARG_PARAM1, param1);
args.putString(ARG_PARAM2, param2);
fragment.setArguments(args);
return fragment;
}

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getArguments() != null) {
mParam1 = getArguments().getString(ARG_PARAM1);
mParam2 = getArguments().getString(ARG_PARAM2);
}
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.fragment_test, container, false);
textView = (TextView)view.findViewById(R.id.test);
//模拟一个耗时操作
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(4000);
} catch (InterruptedException e) {
e.printStackTrace();
}
handler.sendEmptyMessage(1);
}
}).start();
return view;
}

// TODO: Rename method, update argument and hook method into UI event

@Override
public void onAttach(Context context) {
super.onAttach(context);
}

@Override
public void onDetach() {
super.onDetach();
mListener = null;
}

@Override
public void onDestroyView() {
super.onDestroyView();
// handler.removeCallbacksAndMessages(null);
/* handler.removeMessages(1);
handler.re*/
}

@Override
public void onDestroy() {
super.onDestroy();
handler.removeCallbacksAndMessages(null);
}

/**
* This interface must be implemented by activities that contain this
* fragment to allow an interaction in this fragment to be communicated
* to the activity and potentially other fragments contained in that
* activity.
* <p>
* See the Android Training lesson <a href=
* "http://developer.android.com/training/basics/fragments/communicating.html"
* >Communicating with Other Fragments</a> for more information.
*/
public interface OnFragmentInteractionListener {
// TODO: Update argument type and name
void onFragmentInteraction(Uri uri);
}
}

进入这个界面，我们只是做一个网络耗时操作然后发消息出去更新主界面，怎么就崩溃了呢？我们查看getString()的源码：
public final String getString(@StringRes int resId) {
return getResources().getString(resId);
}

这里会调用 getResources()，接着往下看：
final public Resources getResources() {
if (mHost == null) {
throw new IllegalStateException("Fragment " + this + " not attached to Activity");
}
return mHost.getContext().getResources();
}

mHost==null ,就会报这个异常，这里说明如果界面会被销毁，那么mHost 就会被置为null。那怎么解决这个问题呢？看源码如下：
/**
* Return true if the fragment is currently added to its activity.
*/
final public boolean isAdded() {
return mHost != null && mAdded;
}

这个方法会判断mHost会不会为空，所以我们在网络请求回来之后，UI操作之前，就可以加上这句话判断，从而解决崩溃问题！：
public class TestFragment extends Fragment {
// TODO: Rename parameter arguments, choose names that match
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
private static final String ARG_PARAM1 = "param1";
private static final String ARG_PARAM2 = "param2";

// TODO: Rename and change types of parameters
private String mParam1;
private String mParam2;

private OnFragmentInteractionListener mListener;

private TextView textView ;

private Handler handler = new Handler()
{
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
if(isAdded())//防止界面被销毁时崩溃
textView.setText(getString(R.string.app_name));
}
};

/**
* Use this factory method to create a new instance of
* this fragment using the provided parameters.
*
* @param param1 Parameter 1.
* @param param2 Parameter 2.
* @return A new instance of fragment TestFragment.
*/
// TODO: Rename and change types and number of parameters
public static TestFragment newInstance(String param1, String param2) {
TestFragment fragment = new TestFragment();
Bundle args = new Bundle();
args.putString(ARG_PARAM1, param1);
args.putString(ARG_PARAM2, param2);
fragment.setArguments(args);
return fragment;
}

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getArguments() != null) {
mParam1 = getArguments().getString(ARG_PARAM1);
mParam2 = getArguments().getString(ARG_PARAM2);
}
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.fragment_test, container, false);
textView = (TextView)view.findViewById(R.id.test);
//模拟一个耗时操作
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(4000);
} catch (InterruptedException e) {
e.printStackTrace();
}
handler.sendEmptyMessage(1);
}
}).start();
return view;
}

// TODO: Rename method, update argument and hook method into UI event

@Override
public void onAttach(Context context) {
super.onAttach(context);
}

@Override
public void onDetach() {
super.onDetach();
mListener = null;
}

@Override
public void onDestroyView() {
super.onDestroyView();
// handler.removeCallbacksAndMessages(null);
/* handler.removeMessages(1);
handler.re*/
}

@Override
public void onDestroy() {
super.onDestroy();
handler.removeCallbacksAndMessages(null);
}

/**
* This interface must be implemented by activities that contain this
* fragment to allow an interaction in this fragment to be communicated
* to the activity and potentially other fragments contained in that
* activity.
* <p>
* See the Android Training lesson <a href=
* "http://developer.android.com/training/basics/fragments/communicating.html"
* >Communicating with Other Fragments</a> for more information.
*/
public interface OnFragmentInteractionListener {
// TODO: Update argument type and name
void onFragmentInteraction(Uri uri);
}
}

OK，问题解决。