在片段上按下后退按钮时强制关闭

Question

I have a problem in my android application. There is a Custome Adapter from where I am calling a fragment which have mediaPlayer to play audio file. When fragment launched, the audio file plays. If user press back button from ActionBar, no problem occurs and switched to previous fragment from where this fragment was launched. But when user again press back button, The error occurs "MyApp stop working". I tried to catch the problem by breakpoint on my Main Activity's onBackPressed method, but error comes before calling this method.

My Code in Custom Adapter to calling Audio Fragment:

    android.support.v4.app.FragmentTransaction fragmentTransaction;
                    FragmentManager fragmentManager = ((AppCompatActivity) context).getSupportFragmentManager();
                    AudioFragment fragment = new AudioFragment();
                    fragment.SoundTitle = currentAV.getFileText();
                    fragment.oneTimeOnly = 0;
                    fragmentTransaction = fragmentManager.beginTransaction();
                    fragmentTransaction.addToBackStack(null);
                    fragmentTransaction.replace(R.id.fragment_container, fragment);
                    fragmentTransaction.commit();

My Code of Audio fragment having media player:

public class AudioFragment extends Fragment {

private Button btnForward, btnPause, btnPlay, btnBackward;
private ImageButton btnStop;
private ImageView iv;
private MediaPlayer mediaPlayer;
private double startTime = 0;
private double finalTime = 0;
private Handler myHandler;
private int forwardTime = 90000;
private int backwardTime = 90000;
private SeekBar seekbar;
private TextView tx1,tx2,tx3, txTitle;

private boolean _isStopped = true;
private int _length = 0;

public static int oneTimeOnly = 0;
public static String SoundTitle = "";

public AudioFragment() {
    // Required empty public constructor
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    View view =  inflater.inflate(R.layout.fragment_audio, container, false);

    btnForward = (Button) view.findViewById(R.id.btnForward);
    btnPause = (Button) view.findViewById(R.id.btnPause);
    btnPlay = (Button)view.findViewById(R.id.btnPlay);
    btnBackward = (Button)view.findViewById(R.id.btnBackward);
    btnStop = (ImageButton)view.findViewById(R.id.btnStop);
    iv=(ImageView)view.findViewById(R.id.imageView);

    txTitle =(TextView)view.findViewById(R.id.textview);
    txTitle.setText(SoundTitle);
    tx1=(TextView)view.findViewById(R.id.textView2);
    tx2=(TextView)view.findViewById(R.id.textView3);
    tx3=(TextView)view.findViewById(R.id.textView4);
    tx3.setText("");

    myHandler = new Handler();
    seekbar=(SeekBar)view.findViewById(R.id.seekBar);

    ((AppCompatActivity)getActivity()).getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    Toolbar toolbar = (Toolbar) getActivity().findViewById(R.id.toolbar);
    ((AppCompatActivity)getActivity()).setSupportActionBar(toolbar);

    toolbar.setNavigationIcon(R.drawable.abc_ic_ab_back_mtrl_am_alpha);
    toolbar.setNavigationOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            getActivity().onBackPressed();
        }
    });


    btnPlay.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Toast.makeText(getActivity(), "Playing sound",Toast.LENGTH_SHORT).show();
            PlaySound();
        }
    });

    btnPause.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Toast.makeText(getActivity(), "Pausing sound",Toast.LENGTH_SHORT).show();
            mediaPlayer.pause();
            _length = mediaPlayer.getCurrentPosition();
            btnPause.setEnabled(false);
            btnPlay.setEnabled(true);
        }
    });

    btnForward.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            int temp = (int)startTime;

            if((temp+forwardTime)<=finalTime){
                startTime = startTime + forwardTime;
                mediaPlayer.seekTo((int) startTime);
                //Toast.makeText(getActivity(),"You have Jumped forward 5 seconds",Toast.LENGTH_SHORT).show();
            }
            else{
                Toast.makeText(getActivity(),"Cannot jump forward 60 seconds",Toast.LENGTH_SHORT).show();
            }
        }
    });

    btnBackward.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            int temp = (int)startTime;

            if((temp-backwardTime)>0){
                startTime = startTime - backwardTime;
                mediaPlayer.seekTo((int) startTime);
                //Toast.makeText(getActivity(),"You have Jumped backward 5 seconds",Toast.LENGTH_SHORT).show();
            }
            else{
                Toast.makeText(getActivity(),"Cannot jump backward 60 seconds",Toast.LENGTH_SHORT).show();
            }
        }
    });

    btnStop.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            btnPause.setEnabled(false);
            btnPlay.setEnabled(true);
            _isStopped = true;
            _length = 0;
            mediaPlayer.stop();
            myHandler.removeCallbacks(UpdateSongTime);
            mediaPlayer.release();
            mediaPlayer=null;
        }
    });

    PlaySound();
    return view;
}


private Runnable UpdateSongTime = new Runnable() {
    public void run() {
        startTime = mediaPlayer.getCurrentPosition();
        tx1.setText(String.format("%d min, %d sec",

                TimeUnit.MILLISECONDS.toMinutes((long) startTime),
                TimeUnit.MILLISECONDS.toSeconds((long) startTime) -
                        TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.
                                toMinutes((long) startTime)))
        );
        seekbar.setProgress((int)startTime);
        myHandler.postDelayed(this, 100);
    }
};


private void PlaySound(){
    if(!_isStopped){
        mediaPlayer.seekTo(_length);
        mediaPlayer.start();
        btnPause.setEnabled(true);
        btnPlay.setEnabled(false);
        return;
    }
    String sdpath = Environment.getExternalStorageDirectory().getAbsolutePath();
    String avPath = "";
    avPath = sdpath + Gen.AudioPath;
    _isStopped = false;
    mediaPlayer = MediaPlayer.create(getActivity(), Uri.parse(avPath));
    seekbar.setClickable(false);
    btnPause.setEnabled(false);

    mediaPlayer.start();
    finalTime = mediaPlayer.getDuration();
    startTime = mediaPlayer.getCurrentPosition();

    if (oneTimeOnly == 0) {
        seekbar.setMax((int) finalTime);
        oneTimeOnly = 1;
    }
    tx2.setText(String.format("%d min, %d sec",
            TimeUnit.MILLISECONDS.toMinutes((long) finalTime),
            TimeUnit.MILLISECONDS.toSeconds((long) finalTime) -
                    TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes((long) finalTime)))
    );

    tx1.setText(String.format("%d min, %d sec",
            TimeUnit.MILLISECONDS.toMinutes((long) startTime),
            TimeUnit.MILLISECONDS.toSeconds((long) startTime) -
                    TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes((long) startTime)))
    );

    seekbar.setProgress((int)startTime);
    myHandler.postDelayed(UpdateSongTime,100);
    btnPause.setEnabled(true);
    btnPlay.setEnabled(false);
    Gen.IsMediaPlayerRun = true;
}

}

Please help if anyone have solution. Thanks

EDIT: BELOW IS THE LOGCAT (LAST LINES)

D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
  Process: com.example.khan.abc, PID: 21144
  java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.app.FragmentActivity.onBackPressed()' on a null object reference
  at com.example.khan.abc.AudioFragment$1.onClick(AudioFragment.java:125)
  at android.view.View.performClick(View.java:4848)
  at android.view.View$PerformClick.run(View.java:20262)
  at android.os.Handler.handleCallback(Handler.java:815)
  at android.os.Handler.dispatchMessage(Handler.java:104)
  at android.os.Looper.loop(Looper.java:194)
  at android.app.ActivityThread.main(ActivityThread.java:5631)
  at java.lang.reflect.Method.invoke(Native Method)
  at java.lang.reflect.Method.invoke(Method.java:372)
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:959)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:754)

Answer 1

This code is producing the null pointer...

 toolbar.setNavigationOnClickListener(new View.OnClickListener() { 
     @Override public void onClick(View v) { 
        getActivity().onBackPressed(); 
    }
 });

Add a null pointer protection

 toolbar.setNavigationOnClickListener(new View.OnClickListener() { 
     @Override public void onClick(View v) { 
        Activity parent = getActivity();
        if(parent != null)
            parent.onBackPressed(); 
    }
 });

However, your code is a little bit strange ...

For example: you create the fragment and set the clickListener for toolbar. Then, you return to previous fragment.. however, your tool bar is still referencing the listener from killed fragment...

I think the best way is handle the tool bar in parent activity... I'm not sure if is possible for you.. but I'd change that...

Hope this could help.