cursor 第一次为空，recyclerView 不显示游标数据

Question

在我的应用程序中，我正在调用游标，然后将游标发送到 recyclerView 适配器，但问题是：在第一次询问用户权限时，游标返回 null，因此 recyclerView 不会填充数据。

我已阅读这些解决方案，但它们并没有解决我的问题。 1.Android READ_EXTERNAL_STORAGE permission not working 2.READ_EXTERNAL_STORAGE permission for Android 3.Android 6.0 (Marshmallow) READ_CONTACTS permission allows to read Contact's name when permission is denied 4.permission.READ_CONTACTS does not seem to work

当我重新启动应用程序时，我的应用程序会显示数据，因为它不会请求用户权限。

public class MainActivity extends AppCompatActivity {
    RecyclerView recyclerView;
    Cursor mCursor=null;
    recyclerViewSongAdapter mAdapter;

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

        requestStoragePermission();
        mCursor = getSongCursor();

        if(mCursor==null)
            Log.v("cursor_nullification","NO");

        View playButton = findViewById(R.id.myPlayButtonId);
        playButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(MainActivity.this, "PLAY music", Toast.LENGTH_SHORT).show();
                //Intent intent = new Intent(getBaseContext(),SongPlaying.class);
                //intent.putExtra("song_")
            }
        });

        recyclerView = findViewById(R.id.SongListId);

        mAdapter = new recyclerViewSongAdapter(this,mCursor);
        RecyclerView.LayoutManager mLayoutManager = new LinearLayoutManager(getApplicationContext(),LinearLayoutManager.VERTICAL,false);
        //RecyclerView.LayoutManager mLayoutManager = new GridLayoutManager(getApplicationContext(),2);
        recyclerView.setLayoutManager(mLayoutManager);
        recyclerView.setItemAnimator(new DefaultItemAnimator());
        //Toast.makeText(this, "Toast calling", Toast.LENGTH_SHORT).show();
        recyclerView.setAdapter(mAdapter);
    }

    public Cursor getSongCursor(){
        try{
            Log.v("cursor_method_start","till now okay");
            Uri externalUri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
            String[] projection = new String[]{
                    MediaStore.Audio.Media._ID,
                    MediaStore.Audio.Media.DISPLAY_NAME,
                    MediaStore.Audio.Media.ALBUM,
                    MediaStore.Audio.Media.ARTIST,
                    MediaStore.Audio.Media.DATA,
                    MediaStore.Audio.Media.SIZE,
                    MediaStore.Audio.Media.TITLE,
                    MediaStore.Audio.Media.TRACK,
                    MediaStore.Audio.Media.YEAR
            };
            String selection = null;
            String[] selectionArgs = null;
            String sortOrder = MediaStore.Audio.Media.TITLE+" ASC";
            Log.v("calling_cursor_formation","Okay till now");
            Cursor songCursor = getContentResolver().query(externalUri,projection,selection,selectionArgs,sortOrder);
            Toast.makeText(this, "Cursor formation", Toast.LENGTH_SHORT).show();
            return songCursor;
        }catch (Exception e) {
            Toast.makeText(this, "ERROR!!!", Toast.LENGTH_SHORT).show();
            return null;
        }
    }

    private static final int MY_PERMISSIONS_REQUEST = 100;
    private void requestStoragePermission() {
        if (ContextCompat.checkSelfPermission(MainActivity.this,
                Manifest.permission.READ_EXTERNAL_STORAGE)
                != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(MainActivity.this,
                    new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                    MY_PERMISSIONS_REQUEST);
        }
    }

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

                    // permission was granted, yay! Do the
                    // file-related task you need to do.

                }
                else {

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


}

我对光标很陌生，所以我不知道在哪里使用它们。我是否必须在后台调用光标，或者这行代码对于调用光标是否正确，以及如何在获得用户权限后获取光标数据。 谢谢

Answer 1

最初从onCreate()，您应该在检查操作系统版本后调用getSongCursor()，如果在上述M设备上授予权限：

if (android.os.Build.VERSION.SDK_INT < M || (android.os.Build.VERSION.SDK_INT >= M && context.checkSelfPermission(GlobalConstants.WRITE_EXTERNAL_PERMISSION) == PackageManager.PERMISSION_GRANTED)){
   mCursor = getSongCursor();
   setAdapterInRecyclerView(); // created below
}else {
   requestStoragePermission();
}

并在收到用户许可后致电mCursor = getSongCursor();：

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

                  mCursor = getSongCursor();  // here
                  setAdapterInRecyclerView(); // created below 

            }
            else {

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

如有任何疑问，请随时询问

更新： 在getSongCursor()之后创建一个设置Adapter & call的方法：

private void setAdapterInRecyclerView(){
 mAdapter = new recyclerViewSongAdapter(this,mCursor);
        RecyclerView.LayoutManager mLayoutManager = new LinearLayoutManager(getApplicationContext(),LinearLayoutManager.VERTICAL,false);
        //RecyclerView.LayoutManager mLayoutManager = new GridLayoutManager(getApplicationContext(),2);
        recyclerView.setLayoutManager(mLayoutManager);
        recyclerView.setItemAnimator(new DefaultItemAnimator());
        //Toast.makeText(this, "Toast calling", Toast.LENGTH_SHORT).show();
        recyclerView.setAdapter(mAdapter);
}

Answer 2

将以下代码添加到您的 mainactivity 中并删除 null 表单初始化Cursor mCursor=null;

private void onStart()
{
    super.onStart();
    requestStoragePermission();
}