swapCursor() slow: "The application may be doing too much work on its main thread."

Question

I keep seeing "The application may be doing too much work on its main thread."

Is this being caused by swapCursor() in my code below? It appears so: if I remove it, the above warning disappears.

I still don't understand why it claims to be the main thread that is causing the problem. I have moved the swapCursor() around into various places such as onLoadFinished() and in loadInBackground() but I get the same result.

How can I call swapCursor() so that this warning doesn't appear? The whole reason I used SimpleCursorLoader was avoid holding up the main thread but its still happening.

package com.example.sqlitetest;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

import android.app.Activity;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.LoaderManager;
import android.support.v4.app.LoaderManager.LoaderCallbacks;
import android.support.v4.content.Loader;
import android.support.v4.content.CursorLoader;
import android.support.v4.widget.SimpleCursorAdapter;
import android.support.v4.widget.SimpleCursorAdapter.ViewBinder;
import android.view.Gravity;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.view.ViewGroup;
import android.widget.AbsListView;
import android.widget.AdapterView;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.LinearLayout.LayoutParams;
import android.widget.ListView;
import android.widget.TextView;

public class MainActivity extends FragmentActivity implements LoaderManager.LoaderCallbacks<Cursor> 
{
  public static Context mContext;

  private LoaderManager.LoaderCallbacks<Cursor> mLoaderCallbacks;

  public static SQLiteDatabase      db;
  public static SimpleCursorAdapter cAdapter;
  public static ListView            lvCustomList;


  public static final class MyCursorLoader extends SimpleCursorLoader 
  {
    public MyCursorLoader( Context context )
    {
      super( context );
    }

    @Override
    public Cursor loadInBackground()
    {
      Cursor cursor = null;

      cursor = db.rawQuery( "SELECT rowid _id, Name, Rating FROM Tune ORDER BY Name", null );

      cAdapter.swapCursor( cursor );

      return cursor;
    }
  }  

  @Override
  public Loader<Cursor> onCreateLoader(int id, Bundle args) 
  {
    return new MyCursorLoader( this );
  }

  @Override
  public void onLoadFinished( Loader<Cursor> loader, Cursor cursor ) 
  {
//    cAdapter.swapCursor( cursor );
  }

  @Override
  public void onLoaderReset( Loader<Cursor> loader ) 
  {
    cAdapter.swapCursor( null );
  }

  @Override
  public void onCreate( Bundle savedInstanceState )
  {
    super.onCreate( savedInstanceState );
    setContentView( R.layout.tune_artist_album_view );

    mContext = this;

    String path = "/sdcard/MyDb.sqlite";

    db = SQLiteDatabase.openDatabase( path, null, 0 );

    lvCustomList = (ListView) findViewById( R.id.lv_custom_list );

    String[] columns = new String[] { "Name", "Rating" };

    int[] to = new int[] { R.id.lv_tune };  

    cAdapter = new SimpleCursorAdapter( mContext, R.layout.like_hate_row, null, columns, to, 0 );

    lvCustomList.setAdapter( cAdapter );

    mLoaderCallbacks = this;

    LoaderManager lm = getSupportLoaderManager();

    lm.initLoader( 0, null, mLoaderCallbacks );
  }
}

Here is SimpleCursorLoader:

package com.example.sqlitetest;

import android.content.Context;
import android.database.Cursor;
import android.support.v4.content.AsyncTaskLoader;

/**
 * Used to write apps that run on platforms prior to Android 3.0. When running
 * on Android 3.0 or above, this implementation is still used; it does not try
 * to switch to the framework's implementation. See the framework SDK
 * documentation for a class overview.
 * 
 * This was based on the CursorLoader class
 */
public abstract class SimpleCursorLoader extends AsyncTaskLoader<Cursor>
{
  private Cursor mCursor;

  public SimpleCursorLoader( Context context )
  {
    super( context );
  }

  /* Runs on a worker thread */
  @Override
  public abstract Cursor loadInBackground();

  /* Runs on the UI thread */
  @Override
  public void deliverResult( Cursor cursor )
  {
    if( isReset() )
    {
      // An async query came in while the loader is stopped
      if( cursor != null )
      {
        cursor.close();
      }
      return;
    }

    Cursor oldCursor = mCursor;
    mCursor = cursor;

    if( isStarted() )
    {
      super.deliverResult( cursor );
    }

    if( oldCursor != null && oldCursor != cursor && !oldCursor.isClosed() )
    {
      oldCursor.close();
    }
  }

  /**
   * Starts an asynchronous load of the contacts list data. When the result is
   * ready the callbacks will be called on the UI thread. If a previous load has
   * been completed and is still valid the result may be passed to the callbacks
   * immediately.
   * <p/>
   * Must be called from the UI thread
   */
  @Override
  protected void onStartLoading()
  {
    if( mCursor != null )
    {
      deliverResult( mCursor );
    }
    if( takeContentChanged() || mCursor == null )
    {
      forceLoad();
    }
  }

  /**
   * Must be called from the UI thread
   */
  @Override
  protected void onStopLoading()
  {
    // Attempt to cancel the current load task if possible.
    cancelLoad();
  }

  @Override
  public void onCanceled( Cursor cursor )
  {
    if( cursor != null && !cursor.isClosed() )
    {
      cursor.close();
    }
  }

  @Override
  protected void onReset()
  {
    super.onReset();

    // Ensure the loader is stopped
    onStopLoading();

    if( mCursor != null && !mCursor.isClosed() )
    {
      mCursor.close();
    }
    mCursor = null;
  }
}

Answer 1

Methods like query() and rawQuery() don't do what you think they do. Notably, they do not actually query the database. They merely configure the SQLiteCursor that they return. When you attempt to use that Cursor, it executes the query.

Hence, in your loadInBackground() method, in between rawQuery() and swapCursor(), call getCount() on the Cursor you got back from rawQuery(). That is enough to force the Cursor to really do the work for the query, and therefore that work is done on the background thread.