Java 屏幕旋转后，碎片管理器中没有目标

我一直在关注Alex Lockwood 2013的本教程，了解如何在配置更改后向新活动实例生成线程报告

在我尝试在嵌套片段中执行此操作之前，一切都很顺利。基本上，活动添加片段A，片段A被片段B替换，片段B内部启动异步任务线程

然而，如果我通过Backback返回片段A，然后尝试旋转，我会得到标题中所述的以下异常

这是密码

主要活动

片段A

片段B

螺纹碎片

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不确定这是否可行，但您可能需要添加以下代码行：

setRetainInstance(true)

在片段的onViewCreated方法中

---

您还在片段A中向backbackback添加null，因此这可能也是一个问题

嗨，我在线程片段的onCreate方法中有setRetainInstancetrue行。不过我还是在onCreateView上试过了。backback的参数是可选的，它只是一个标记，如果需要，它可以为null。我不认为这会导致任何问题，我在许多教程中看到它为空/您可能应该将该行添加到所有片段中。好吧，我不知道。嗨，你找到解决办法了吗？我有同样的问题…可能是重复的

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();
    }

}

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();
    }

}

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();
            }
        }


    }



}

setRetainInstance(true)