屏幕旋转后，片段在片段管理器中没有目标

Question

I have been following this tutorial by Alex Lockwood (2013) on how to make a thread report back to a new activity instance after a configuration change.

http://www.androiddesignpatterns.com/2013/04/retaining-objects-across-config-changes.html

All was working great until I tried to do this within a nested fragment. basically Activity adds Fragment A and Fragment A gets replaced with Fragment B and inside fragment B starts the a async task thread.

However, if I go back to Fragment A (via backstack) and then try to rotate I get the following exception stated in the title.

Here is the code

Main Activity

public class MainActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        if(savedInstanceState == null)
        {
             FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
             ft.add(android.R.id.content, new FragmentA()).commit();
        }

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

Fragment A

public class FragmentA extends Fragment implements OnClickListener
{

    private Button GoToFragmentB;
    private ViewGroup container;

    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container,  Bundle savedInstanceState)
    {
    // TODO Auto-generated method stub

    View view = inflater.inflate(R.layout.fragment_a, container, false);
    this.container = container;
    GoToFragmentB = (Button)view.findViewById(R.id.bGoToFragmentB);
    GoToFragmentB.setOnClickListener(this);
    return view;
    }

    @Override
    public void onClick(View v) 
    {
        // TODO Auto-generated method stub
        FragmentManager fragmentManager = getFragmentManager();
        FragmentB fb = new FragmentB();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.replace(container.getId(), fb, FragmentB.class.getName());
        fragmentTransaction.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
        fragmentTransaction.addToBackStack(null);
        fragmentTransaction.commit();
    }

}

Fragment B

public class FragmentB extends Fragment implements OnClickListener, ThreadFragment.AsyncTaskCallbacks{

    private ThreadFragment mThreadFragment;
    private ProgressBar progress_horizontal;
    private TextView percent_progress;
    private Button task_button;

    private static final String KEY_CURRENT_PROGRESS = "current_progress"; 
    private static final String KEY_PERCENT_PROGRESS = "percent_progress"; 


    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container,  Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        View view = inflater.inflate(R.layout.fragment_b, container, false);

        progress_horizontal = (ProgressBar) view.findViewById(R.id.progress_horizontal);
        percent_progress = (TextView)view.findViewById(R.id.percent_progress);
        task_button = (Button)view.findViewById(R.id.task_button);
        task_button.setOnClickListener(this);



        return view;
    }


    @Override
    public void onActivityCreated(Bundle savedInstanceState) 
    {
        // TODO Auto-generated method stub
        super.onActivityCreated(savedInstanceState);

        if(savedInstanceState != null)
        {
            progress_horizontal.setProgress(savedInstanceState.getInt(KEY_CURRENT_PROGRESS)); 
            percent_progress.setText(savedInstanceState.getString(KEY_PERCENT_PROGRESS)); 

        }

        FragmentManager fm = getActivity().getSupportFragmentManager();
        mThreadFragment = (ThreadFragment) fm.findFragmentByTag(ThreadFragment.class.getName());

        if(mThreadFragment == null)
        {
            mThreadFragment = new ThreadFragment();
            mThreadFragment.setTargetFragment(this, 0);
            fm.beginTransaction().add(mThreadFragment, ThreadFragment.class.getName()).commit();
        }

        if(mThreadFragment.isRunning() == true)
        {
            task_button.setText(getString(R.string.cancel));
        }
        else
        {
            task_button.setText(getString(R.string.start));
        }

    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        // TODO Auto-generated method stub
        super.onSaveInstanceState(outState);
        outState.putInt(KEY_CURRENT_PROGRESS, progress_horizontal.getProgress()); 
        outState.putString(KEY_PERCENT_PROGRESS, percent_progress.getText().toString()); 

    }

    @Override
    public void onClick(View v) 
    {
        // TODO Auto-generated method stub
        if(mThreadFragment.isRunning() == true)
        {
            mThreadFragment.cancel();
        }
        else
        {
            mThreadFragment.start();
        }
    }


    @Override
    public void onPreExecute() 
    {
        // TODO Auto-generated method stub
        task_button.setText(getString(R.string.cancel));
        Toast.makeText(getActivity(), R.string.task_started_msg, Toast.LENGTH_SHORT).show();
    }


    @Override
    public void onProgressUpdate(int percent)
    {
        // TODO Auto-generated method stub
        progress_horizontal.setProgress(percent * progress_horizontal.getMax() / 100);
        percent_progress.setText(percent + "%");
    }


    @Override
    public void onCancelled() 
    {
        // TODO Auto-generated method stub
        task_button.setText("Start");
        progress_horizontal.setProgress(0);
        percent_progress.setText("0%");

    }


    @Override
    public void onPostExecute() 
    {
        // TODO Auto-generated method stub
        task_button.setText(getString(R.string.start));
        progress_horizontal.setProgress(progress_horizontal.getMax());
        percent_progress.setText(getString(R.string.one_hundred_percent));
        Toast.makeText(getActivity(), R.string.task_complete_msg, Toast.LENGTH_SHORT).show();

    }

    @Override
    public void onPause() {
        // TODO Auto-generated method stub
        mThreadFragment.pause();
        super.onPause();
    }

    @Override
    public void onResume() {
        // TODO Auto-generated method stub
        mThreadFragment.resume();
        super.onResume();
    }

}

Thread Fragment

public class ThreadFragment extends Fragment {


    static interface AsyncTaskCallbacks
    {
        void onPreExecute();
        void onProgressUpdate(int percent);
        void onCancelled();
        void onPostExecute();
    }

    private AsyncTaskCallbacks mCallback;
    private boolean mRunning;
    private boolean isPause;

    private TestTask mTask;

    @Override
    public void onAttach(Activity activity) {
        // TODO Auto-generated method stub
        super.onAttach(activity);
        if(!(getTargetFragment() instanceof AsyncTaskCallbacks))
        {
            throw new IllegalStateException("Target fragment must implement the AsyncTaskCallbacks interface.");
        }


        if(getTargetFragment() != null)
        {
        mCallback = (AsyncTaskCallbacks) getTargetFragment();
        }


    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setRetainInstance(true);
    }

    @Override
    public void onDestroy() {
        // TODO Auto-generated method stub
        super.onDestroy();
        cancel();

    }

    public void start()
    {
        if(mRunning == false)
        {
            mTask = new TestTask();
            mTask.execute();
            mRunning = true;
        }
    }

    public void cancel()
    {
        if(mRunning == true)
        {
            mTask.cancel(false);
            mTask = null;
            mRunning = false;
            isPause = false;
        }
    }

    public void pause()
    {
        if(mRunning == true)
        {
            isPause = true;
        }
    }

    public void resume()
    {
        isPause = false;
    }

    public boolean isRunning()
    {
        return mRunning;
    }

    private class TestTask extends AsyncTask<Void, Integer, Void>
    {

        @Override
        protected void onPreExecute() 
        {
            // TODO Auto-generated method stub
            mCallback.onPreExecute();
            mRunning = true;
        }

        @Override
        protected Void doInBackground(Void... params) 
        {


                // TODO Auto-generated method stub
                for(int i = 0; !isCancelled() && i < 100; i++)
                {
                    if(isPause == true)
                    {
                        sleep();
                    }

                    SystemClock.sleep(100);
                    publishProgress(i);
                }



            return null;
        }

        @Override
        protected void onProgressUpdate(Integer... values) {
            // TODO Auto-generated method stub
            mCallback.onProgressUpdate(values[0]);
        }

        @Override
        protected void onCancelled() {
            // TODO Auto-generated method stub
            mCallback.onCancelled();
            mRunning = false;
        }

        @Override
        protected void onPostExecute(Void result) {
            // TODO Auto-generated method stub
            mCallback.onPostExecute();
            mRunning = false;
        }

        private void sleep()
        {
            try
            {
                while(isPause)
                {
                Thread.sleep(500);
                }
            }
            catch(InterruptedException e)
            {
                e.printStackTrace();
            }
        }


    }



}

My LogCat

09-06 19:49:31.068: E/AndroidRuntime(2402): FATAL EXCEPTION: main
09-06 19:49:31.068: E/AndroidRuntime(2402): java.lang.IllegalStateException: Failure saving state: ThreadFragment{40ce2700 #2 com.ersen.asynctaskpausetest.ThreadFragment} has target not in fragment manager: FragmentB{40d3a170}
09-06 19:49:31.068: E/AndroidRuntime(2402):     at android.support.v4.app.FragmentManagerImpl.saveAllState(FragmentManager.java:1699)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at android.support.v4.app.FragmentActivity.onSaveInstanceState(FragmentActivity.java:547)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at android.app.Activity.performSaveInstanceState(Activity.java:1147)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at android.app.Instrumentation.callActivityOnSaveInstanceState(Instrumentation.java:1216)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3666)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at android.app.ActivityThread.access$700(ActivityThread.java:141)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1240)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at android.os.Looper.loop(Looper.java:137)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at android.app.ActivityThread.main(ActivityThread.java:5041)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at java.lang.reflect.Method.invokeNative(Native Method)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at java.lang.reflect.Method.invoke(Method.java:511)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
09-06 19:49:31.068: E/AndroidRuntime(2402):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

Not sure if this will work, but you may need to add this line of code:

setRetainInstance(true)

in your onViewCreated() method of your fragment.

You are also adding null to the backstack in fragment A so that could be a problem as well.