安卓数据库查询错误 - “确保在访问数据之前正确初始化游标…”

Question

我有一个活动，其中一个ViewFlipper显示包含来自MediaStore的艺术家列表的列表，当选择的艺术家被点击时，它会显示该艺术家专辑的列表，然后显示该专辑中的歌曲。一旦单击了一首歌曲，它就应该用字符串“title”填充一个TextView。

到目前为止，所有游标都运行正常，但最后一个似乎出现了某种位置偏差。有人能告诉我为什么logcat告诉我：

05-07 23:58:54.195: E/AndroidRuntime(1961): java.lang.IllegalStateException: Couldn't read row 3, col -1 from CursorWindow.  Make sure the Cursor is initialized correctly before accessing data from it.

无法读取的特定行取决于所选择的艺术家/专辑/歌曲。 代码如下。非常感谢您的帮助。

package music.flipper;

import android.app.Activity;
import android.database.Cursor;
import android.os.Bundle;
import android.provider.BaseColumns;
import android.provider.MediaStore;
import android.provider.MediaStore.Audio.AlbumColumns;
import android.provider.MediaStore.Audio.ArtistColumns;
import android.provider.MediaStore.Audio.AudioColumns;
import android.provider.MediaStore.MediaColumns;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
import android.widget.TextView;
import android.widget.ViewFlipper;

public class MusicFlipper extends Activity implements OnItemClickListener {
    /** Called when the activity is first created. */

    ViewFlipper viewflipper;
    Cursor cursor;

    private String currentList = "Artist";
    @SuppressWarnings("deprecation")
    @Override
    public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);

        setContentView(R.layout.flipper);
        //set the main view to flipper.
        viewflipper = (ViewFlipper) findViewById(R.id.viewFlipper1);

        String[] columns = { 
                BaseColumns._ID,
                ArtistColumns.ARTIST 
                };
        //The columns to return for each row.   

        cursor = managedQuery(MediaStore.Audio.Artists.EXTERNAL_CONTENT_URI,
            columns, null, null, null);

        ListView listView = (ListView) findViewById(R.id.listView1);
        listView.setOnItemClickListener(this);

        //set an onitemclicklistener to the first listview in flipper

        String[] displayFields = new String[] { ArtistColumns.ARTIST };
        //set all the artist names to the array 'displayfields'
        int[] displayViews = new int[] { R.id.rowItem };
        //number of rows to display and where to bind them

        SimpleCursorAdapter adapter = new SimpleCursorAdapter(this,
            R.layout.row_item, cursor, displayFields, displayViews);
        listView.setAdapter(adapter); }
        //Take the display fields array, and bind to the matching display row

     @SuppressWarnings("deprecation")

    public void onItemClick(AdapterView<?> a, View v, int position, long id) {

        if( currentList.equals("Artist")) {
            if (cursor.moveToPosition(position)) {
            //once an item is clicked, move the cursor to that items position

            String where = AudioColumns.ARTIST + "=?";
            // Have the cursor look within the artist row?

            String whereVal[] = { cursor.getString(cursor
          .getColumnIndex(AlbumColumns.ARTIST)) };
            //Choose the particular row with the chosen artist's name

            String[] columns = {
                    BaseColumns._ID,
                    AudioColumns.ALBUM,
               };

                String orderBy = BaseColumns._ID;

            cursor = managedQuery(MediaStore.Audio.Albums.EXTERNAL_CONTENT_URI,
                columns, where, whereVal, orderBy);

            ListView listView = (ListView) findViewById(R.id.listView2);
            listView.setOnItemClickListener(this);
            String[] displayFields = new String[] { AudioColumns.ALBUM };
            int[] displayViews = new int[] { R.id.rowItem };
            SimpleCursorAdapter adapter = new SimpleCursorAdapter(this,
                R.layout.row_item, cursor, displayFields, displayViews);
            listView.setAdapter(adapter);
            currentList = "Album";
            viewflipper.showNext();}

        } if (currentList.equals("Album")) {
            if (cursor.moveToPosition(position)) {

                String where = AudioColumns.ALBUM
                + "=?";

                String whereVal[] = { cursor.getString(cursor
                .getColumnIndex(AlbumColumns.ALBUM)) };

                String[] columns = {
                        MediaColumns.DATA,
                        BaseColumns._ID,
                        MediaColumns.TITLE,
                        MediaColumns.DISPLAY_NAME,
                        MediaColumns.MIME_TYPE,
                   };

                    String orderBy = MediaColumns.TITLE;

                cursor = managedQuery(MediaStore.Audio.Media.EXTERNAL_CONTENT_URI,
                    columns, where, whereVal, orderBy);

                ListView listView = (ListView) findViewById(R.id.listView3);
                listView.setOnItemClickListener(this);
                String[] displayFields = new String[] { MediaColumns.TITLE };
                int[] displayViews = new int[] { R.id.rowItem };
                SimpleCursorAdapter adapter = new SimpleCursorAdapter(this,
                    R.layout.row_item, cursor, displayFields, displayViews);
                listView.setAdapter(adapter);
                currentList.equals("Songs");
                viewflipper.showNext();}


        } if (currentList.equals("Songs")) {
            if (cursor.moveToPosition(position)) {

                String title = cursor.getString(cursor.getColumnIndex(MediaColumns.TITLE));

                TextView myTextView = (TextView) findViewById(R.id.title);
                myTextView.setText(title);

            }
        }
    }
}

- user1352742

---

嘿，timusus，如果我的答案对你有帮助/解决了问题，请点击旁边的勾选标记，这样我就能得到积分，其他人也会知道你的问题已经解决了。祝好运！ - Barak

Barak，你能否看一下下面的评论，我想了解如何实现所需的结果的更多信息。 - user1352742

Answer 1

问题不在行，而是在列。

Couldn't read row 3, **col -1** from CursorWindow.  Make sure the Cursor is initialized correctly before accessing data from it.

基本上是在说你的MediaColumns.TITLE列不存在于游标中。这是真的。它不在你的第一个游标中（它所引用的那个）。你的其他游标都在if语句中，所以出了作用域，只剩下第一个游标。

你可以像在其他部分的if语句中一样重新拉取游标，或者找到一些方法来保持从最后一个if语句得到的游标。

编辑

修复起来非常简单，将游标设置为类变量。同时，我不会一直重复使用“cursor”。给它们贴上个性化和描述性的标签，这将有助于您在代码中维护可读性。我可能会像这样做：

public class MusicFlipper extends Activity implements OnItemClickListener {
    private Cursor artistCursor;
    private Cursor albumCursor;

然后你像以前一样调用它们，但使用各自的名称。

albumCursor = managedQuery(MediaStore.Audio.Media.EXTERNAL_CONTENT_URI,
                columns, where, whereVal, orderBy);

由于你将它声明为类变量，因此它将在整个类中可用，因此在最后一部分中，您应该这样做：

if (currentList.equals("Songs")) {
    if (albumCursor.moveToPosition(position)) {
            String title = albumCursor.getString(albumCursor.getColumnIndex(MediaColumns.TITLE));
            TextView myTextView = (TextView) findViewById(R.id.title);
            myTextView.setText(title);
    }
}

- Barak

---

谢谢你的回答，Barak。我一直在尝试这个想法，但似乎无法让任何东西起作用。我甚至重新拉取了光标，就像之前的语句一样，但是我仍然得到相同的错误。我认为这是因为“位置”已经改变了。你能否向我展示你如何实现这个想法？谢谢。 - user1352742

非常感谢你，Barak。非常感激。 - user1352742