如何在片段之间传递数据？

Question

For all those who are downvoting and voting for this question to be closed thinking it has something to do with the scope of textview.....then have a look,it has nothing to do with the scope of textview

Unable to pass data between fragments,the app crashes,i don't know what i am doing wrong...i followed this link http://manishkpr.webheavens.com/android-passing-data-between-fragments/

here is the code i am using Fragment A(this fragment is passing the data)

public class FragmentSongs extends  Fragment implements Serializable,OnFragmentCreatedListener {
ListView SngList;


 ArrayList<SongDetails> Songinfo=null;
AdapterView.AdapterContextMenuInfo info;

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

    ViewGroup view = (ViewGroup) inflater.inflate(R.layout.fragment_song, null);

    SngList = (ListView) view.findViewById(R.id.SongList);
    registerForContextMenu(SngList);
    //File f=new File(Environment.getExternalStorageDirectory()+"/Music");
    File f=new File("/system/");
    int j=0;int i=0;

    getActivity().setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);

      Songinfo = getSongsFromDirectory(f);



    //sorting done below
    if (Songinfo.size()>0)
    {

        for( j=0; j<Songinfo.size();j++)
        {
            for ( i=j+1 ; i<Songinfo.size(); i++)
            { 
                SongDetails a=Songinfo.get(i);
                SongDetails b=Songinfo.get(j);
                if(a.getSong().toLowerCase().compareTo(b.getSong().toLowerCase())<0)
                {   

                    Songinfo.set(i,b );
                    Songinfo.set(j,a);
                }
            }

        }






   SngList.setAdapter(new CustomAdapter(Songinfo));
   return view;
    }
    else return null;

}
OnFragmentCreatedListener listener;
@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    try {
        listener = (OnFragmentCreatedListener) getActivity();
        listener.onFragmentCreated(Songinfo);
    } catch (ClassCastException e) {
        throw new ClassCastException(activity.toString() + " must implement onFragmentCreated");
    }
}

fragment B(the receiver fragment )

    public class FragmentArtists extends Fragment {
    ArrayList<SongDetails> songinfo2;
ViewGroup view;
int s=5;//TextView text;
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState)
{Context context;
LayoutInflater lf = getActivity().getLayoutInflater();   

    view = (ViewGroup) inflater.inflate(R.layout.fragment_artist,  null);
// view =  lf.inflate(R.layout.test,container, false);
// view = inflater.inflate(R.layout.test,container, false);


 s=s+5;
//     text=(TextView)view.findViewById(R.id.tt);when i use this,it works fine but i want to do it in the other method

//text.setText("makzzz");when i use this,it works fine but i want to do it in the other method
return view;

}

 void setSongList(ArrayList<SongDetails> songinfo) 
{    //TextView text=(TextView)this.getView().findViewById(R.id.tt);//when i used this view was not global
      TextView text=(TextView)view.findViewById(R.id.tt);//when i used this,view was global;
      //TextView text=(TextView)view.findViewById(R.id.tt);
    // text=(TextView)super.getView().findViewById(R.id.tt);


     text.setText("makzzz");

the method in holder activity (this activity holds the fragments)

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.mymusic);


           fragments = new Vector<Fragment>();
            fragments.add(Fragment.instantiate(this, FragmentSongs.class.getName()));
            fragments.add(Fragment.instantiate(this, FragmentArtists.class.getName()));
            fragments.add(Fragment.instantiate(this, FragmentPlaylists.class.getName()));
            fragments.add(Fragment.instantiate(this, FragmentAlbums.class.getName())); 


            ViewPagerAdapter adapter = new ViewPagerAdapter(super.getSupportFragmentManager(), fragments);


            ViewPager pager = (ViewPager)super.findViewById(R.id.viewpager);



            pager.setAdapter(adapter);
            pager.setOffscreenPageLimit(4);

            pager.setCurrentItem(0);

    }      public void onFragmentCreated(ArrayList<SongDetails> msg) {
            FragmentArtists Obj=(FragmentArtists)  fragments.get(1);
            Obj.setSongList(msg);
            }

    }

the callback class:

package sourcecode.jazzplayer;

import java.util.ArrayList;

public interface OnFragmentCreatedListener {
    public void onFragmentCreated(ArrayList<SongDetails> msg);

}

here is the logcat

  09-19 23:50:46.851: E/AndroidRuntime(2125): java.lang.NullPointerException
09-19 23:50:46.851: E/AndroidRuntime(2125):     at sourcecode.jazzplayer.FragmentArtists.setSongList(FragmentArtists.java:55)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at sourcecode.jazzplayer.MyMusic.onFragmentCreated(MyMusic.java:45)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at sourcecode.jazzplayer.FragmentSongs.onAttach(FragmentSongs.java:128)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:883)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.support.v4.app.FragmentManagerImpl.performPendingDeferredStart(FragmentManager.java:823)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.support.v4.app.Fragment.setUserVisibleHint(Fragment.java:819)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.support.v4.app.FragmentPagerAdapter.setPrimaryItem(FragmentPagerAdapter.java:130)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.support.v4.view.ViewPager.populate(ViewPager.java:1066)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.view.View.measure(View.java:12603)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4677)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:293)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.view.View.measure(View.java:12603)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4677)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1369)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.widget.LinearLayout.measureVertical(LinearLayout.java:660)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.widget.LinearLayout.onMeasure(LinearLayout.java:553)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.view.View.measure(View.java:12603)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4677)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:293)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2072)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.view.View.measure(View.java:12603)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1044)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2418)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.os.Looper.loop(Looper.java:137)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at android.app.ActivityThread.main(ActivityThread.java:4340)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at java.lang.reflect.Method.invokeNative(Native Method)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at java.lang.reflect.Method.invoke(Method.java:511)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
09-19 23:50:46.851: E/AndroidRuntime(2125):     at dalvik.system.NativeStart.main(Native Method)

the error is in this line of code

TextView text=(TextView)view.findViewById(R.id.tt); //this line is in the receiver fragment (fragmentArtist)

Answer 1

As I already said in my comment you're trying to work with a Fragment when it's not properly initialized yet. When the FragmentSongs is attached to the Activity (action requested by the ViewPager where it will appear) its onAttach() method is called. In this method you cast the Activity to your listener, but you also call immediately the listener's callback . This will make your app crash(with the exception you posted) because in that listener you're trying to work with the view of the second Fragment of the ViewPager , FragmentArtists . This Fragment isn't yet initialized, at this moment the first fragment of the ViewPager is barely being attached to the activity and you're already trying to work with the view of the second fragment which will obviously fail.

The tutorial you linked to is ok, simplistic, but ok(without going to that link I assumed your code is from the tutorial). The problem is that you used that code from a very simplistic scenario in a more complex situation(in fact if you insert your current code in that tutorial's code you'll get the same exception(calling the listener too early)).

That callback listener interface should be used while the fragment is fully built(like when the user is using the app and selects a song and you want to show to him the artist part of the app(at that moment call the listener and make it pass data)) also carefully handling the possibility of a fragment not being available.

If you need to pass data between fragments so early in the fragment's lifecycle that you can't guarantee the fragment is yet available(like you do with calling the listener in onAttach() of a fragment) then implement the fragment to get the data directly from the activity when it's built, in one of its lifecycle methods. What you're passing it's not clear so there isn't much I can say here.

Answer 2

Your FragmentSongs class needs to implement OnFragmentCreatedListener

public class FragmentSongs extends Fragment implements OnFragmentCreatedListener { 

  public void onFragmentCreated(ArrayList<SongDetails> msg) {
    // whatever
  }


}

---

or in onAttach of your 'Fragment A':

        listener =  (OnFragmentCreatedListener) activity;

this would mean your Activity needs to implement OnFragmentCreatedListener