Question

我是Android新手。我从另一个类访问ArrayList，但我在访问类时得到零大小的ArrayList。在主类中，它显示了数组列表大小的正值。我不知道为什么我在访问课程时获得零大小。检查我的代码是否有错误。

Tab3Tracks类

public class Tab3Tracks extends ListFragment {

public void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
}

private ArrayList<String> trackslist = new ArrayList<>();


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.tab3tracks, container, false);


    final String trackid = MediaStore.Audio.Media._ID;
    final String trackno = MediaStore.Audio.Media.TRACK;
    final String trackname = MediaStore.Audio.Media.TITLE;
    final String path = MediaStore.Audio.Media.DATA;
    Uri uri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;

    ContentResolver cr = getContext().getContentResolver();
    final String[] projection = {trackid,trackno,trackname,path
    };
    final Cursor cursor = cr.query(uri,projection,null,null,null);
    if (cursor!=null){
        if(cursor.moveToFirst()){
            do{
                int trackIndex = cursor.getColumnIndexOrThrow(MediaStore.Audio.Media.TITLE);
                trackslist.add(cursor.getString(trackIndex));
            }while(cursor.moveToNext());
        }System.out.println(trackslist.size());
    } cursor.close();


    ArrayAdapter<String> adapter = new ArrayAdapter<String>(
            getActivity(),R.layout.playlist_item,R.id.songTitle,trackslist
    );
    setListAdapter(adapter);
    return v;
}


public ArrayList<String> getList(){
    return trackslist;
}



@Override
public void onListItemClick(ListView l, View v, int position, long id) {
    super.onListItemClick(l, v, position, id);
    int songIndex = position;

    // Starting new intent
    Intent in = new Intent(getActivity(), NowPlaying.class);
    // Sending songIndex to PlayerActivity
    in.putExtra("songIndex", songIndex);
    startActivityForResult(in, 100);
}
}

NowPlaying Class

public class NowPlaying extends AppCompatActivity {
public ArrayList<String> songsList = new ArrayList<String>();
public Tab3Tracks tab3tracks;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.player);
    // Getting all songs list
    tab3tracks = new Tab3Tracks();
    songsList= tab3tracks.getList();
    System.out.println("songsListSize"+songsList.size());
    System.out.println("List" +songsList);

}

Answer 1

您的ArrayList已初始化，但未添加任何内容。

trackslist.add(cursor.getString(trackIndex));

它嵌套在onCreateView中，导致它只被调用一次。您可能需要考虑将onTOuchListener添加到活动中，并将您的代码添加到侦听器内部，以便在触摸屏幕时更新它。

在线程内添加也可能有效，但这实际上取决于您的代码。

Answer 2

如果片段会给视图充气，那么Tab3Tracks的onCreateView将被android调用，所以你的数组是空的，因为这段代码

 trackslist.add(cursor.getString(trackIndex));

不会随时致电

从另一个类访问时获取ArrayList大小为零

2 个答案: