android - Android中的MP3/音乐播放器

我正在尝试在android中创建音乐播放器。此代码用于读取mp3文件的sd卡，并在烤面包中显示它们。

当我在手机上运行此应用程序时，手机将显示空白屏幕，即未显示任何歌曲。

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Environment;
import android.support.annotation.NonNull;
import android.support.v4.app.ActivityCompat;
import android.support.v4.content.ContextCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.ListView;
import android.widget.Toast;

import java.io.File;
import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

String items;
final int READ_EXTERNAL_STORAGE_REQUEST = 1;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if(ContextCompat.checkSelfPermission(this, Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(this, new String[] {Manifest.permission.READ_EXTERNAL_STORAGE}, READ_EXTERNAL_STORAGE_REQUEST);
        return;
    }

    ArrayList<File> mySongs = findSongs(Environment.getExternalStorageDirectory());

    for(int i = 0; i < mySongs.size(); i++) {
        toast(mySongs.get(i).getName().toString());
    }
}

public ArrayList<File> findSongs(File root) {
    ArrayList<File> al = new ArrayList<File>();
    File[] files = root.listFiles();
    for(File singleFile : files) {
        if(singleFile.isDirectory() && !singleFile.isHidden()) {
            al.addAll(findSongs(singleFile));
        }

        else {
            if(singleFile.getName().endsWith(".mp3")) {
                  al.add(singleFile);
            }
        }
    }
    return al;
}

public void toast(String text) {
    Toast.makeText(this, text, Toast.LENGTH_SHORT).show();
}

@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
    switch(requestCode) {
        case READ_EXTERNAL_STORAGE_REQUEST: if(grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED){
            return;
        }
    }
}
}

最佳答案

这是有效的答案：

public class MainActivity extends AppCompatActivity {

TextView countTextview;
ListView listView;
private List<String> arrayList;
ArrayAdapter arrayAdapter;
final private static int MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE = 1;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    countTextview = (TextView)findViewById(R.id.count);

    arrayList = new ArrayList<String>();

    listView = (ListView)findViewById(R.id.listView);

    arrayAdapter = new ArrayAdapter(this,android.R.layout.simple_list_item_1,arrayList);

    permissionManager();

}

public void add(){

    ContentResolver cr = getContentResolver();

    Uri uri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
    String selection = MediaStore.Audio.Media.IS_MUSIC + "!= 0";
    String sortOrder = MediaStore.Audio.Media.TITLE + " ASC";
    Cursor cur = cr.query(uri, null, selection, null, sortOrder);
    int count = 0;

    if(cur != null)
    {
        count = cur.getCount();

        if(count > 0)
        {
            while(cur.moveToNext())
            {
                String data = cur.getString(cur.getColumnIndex(MediaStore.Audio.Media.DISPLAY_NAME));

                arrayList.add(data);

            }

            listView.setAdapter(arrayAdapter);

        }
    }

    cur.close();

}


public void permissionManager(){

    // Here, thisActivity is the current activity
    if (ContextCompat.checkSelfPermission(MainActivity.this,
            Manifest.permission.WRITE_EXTERNAL_STORAGE)
            != PackageManager.PERMISSION_GRANTED) {

        ActivityCompat.requestPermissions(MainActivity.this,
                new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
                MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE);

        // Should we show an explanation?
        if (ActivityCompat.shouldShowRequestPermissionRationale(MainActivity.this,
                Manifest.permission.WRITE_EXTERNAL_STORAGE)) {

            // Show an expanation to the user *asynchronously* -- don't block
            // this thread waiting for the user's response! After the user
            // sees the explanation, try again to request the permission.

        } else {

            // No explanation needed, we can request the permission.

            ActivityCompat.requestPermissions(MainActivity.this,
                    new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
                    MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE);

            // MY_PERMISSIONS_REQUEST_READ_CONTACTS is an
            // app-defined int constant. The callback method gets the
            // result of the request.
        }
    }else {
        add();

    }

}

@Override
public void onRequestPermissionsResult(int requestCode,
                                       String permissions[], int[] grantResults) {
    switch (requestCode) {
        case MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE: {
            // If request is cancelled, the result arrays are empty.
            if (grantResults.length > 0
                    && grantResults[0] == PackageManager.PERMISSION_GRANTED) {

                // permission was granted, yay! Do the
                // contacts-related task you need to do.
                add();


            } else {

                // permission denied, boo! Disable the
                // functionality that depends on this permission.

                Toast.makeText(MainActivity.this, "Permission is really required to get Songs", Toast.LENGTH_SHORT).show();
                permissionManager();
            }
            return;
        }

        // other 'case' lines to check for other
        // permissions this app might request
    }
}

}