Question

All of the examples I’ve seen of using registerContentObserver() do so through a ContentProvider interface. But Cursor has a registerContentObserver() call, so I figured maybe the Android folks have put together some deep magic that would allow for getting updates on a SQLite cursor when one of the rows from an active result set changed. Either I’m doing it wrong, or there is no such magic. Here’s the code I’m working with, the expectation being that when I click on the button to update the value in row #1 I would get an onChange() callback. Any ideas?

public class MainActivity extends Activity {
    private static final String DATABASE_NAME = "test.db";
    public static final String TAG = "dbtest";

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        Button make = (Button)findViewById(R.id.btn_make_record);
        make.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
                MyOpenHelper oh = new MyOpenHelper(v.getContext());
                SQLiteDatabase wdb = oh.getWritableDatabase();
                ContentValues cv = new ContentValues();
                cv.put("value", String.valueOf(System.currentTimeMillis()));
                if (wdb.insert(MyOpenHelper.TABLE_NAME, null, cv) == -1) {
                    Log.d(TAG, "Unable to insert row");
                } else {
                    Log.d(TAG, "Inserted row ");
                }
                wdb.close();
            }
        });

        Button update = (Button)findViewById(R.id.btn_update_record);
        update.setOnClickListener(new OnClickListener() {   
            public void onClick(View v) {
                MyOpenHelper oh = new MyOpenHelper(v.getContext());
                SQLiteDatabase wdb = oh.getWritableDatabase();
                ContentValues cv = new ContentValues();
                cv.put("value", String.valueOf(System.currentTimeMillis()));
                int count = wdb.update(MyOpenHelper.TABLE_NAME, cv, "_id = ?", new String[] {"1"});
                Log.d(TAG, "Updated " + count + " row(s)");
                wdb.close();
            }
        });

        MyOpenHelper oh = new MyOpenHelper(this);
        SQLiteDatabase rdb = oh.getReadableDatabase();
        Cursor c = rdb.query(MyOpenHelper.TABLE_NAME, null, "_id = ?", new String[] {"1"}, null, null, null);
        startManagingCursor(c);
        contentObserver = new MyContentObserver(new Handler());
        c.registerContentObserver(contentObserver);
    }

    private class MyContentObserver extends ContentObserver {
        MyContentObserver(Handler handler) {
            super(handler);
        }

        public boolean deliverSelfNotifications() {
            return true;
        }

        public void onChange(boolean selfChange) {
            super.onChange(selfChange);
            Log.d(TAG, "Saw a change in row # 1");
        }
    }

    MyContentObserver contentObserver;

    public class MyOpenHelper extends SQLiteOpenHelper {
        private static final int DATABASE_VERSION = 1;
        private static final String TABLE_NAME = "test";
        private static final String TABLE_CREATE =
                "CREATE TABLE " + TABLE_NAME + " (" +
                "_id INTEGER PRIMARY KEY AUTOINCREMENT," +
                "value TEXT);";

        MyOpenHelper(Context context) {
             super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
            db.execSQL(TABLE_CREATE);
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
          // TODO Auto-generated method stub    
        }
    }
}

Answer 1

No takers huh? Well an excellent excuse to dig in myself and figure it out. For those who might be curious after me, if you download the platform source the relevant files to look at are:

frameworks/base/core/java/android/database/sqlite/SQLiteCursor.java
frameworks/base/core/java/android/database/AbstractCursor.java

It looks like the mContentObservable is there to signal different parts of a program running off the cached result set in a Cursor, and the observable is a signal from the cached result set to let consumers know when the cache has been dumped/updated. It doesn’t tie into the SQLite implementation to fire events when the underlying datastore is manipulated.

Not exactly surprising, but given the docs I thought maybe I was missing something.