Develop Reference

Am I responsible for closing the cursor passed to a custom CursorWrapper class constructor?

I've created some CursorWrapper class
public class DogsCursorWrapper extends CursorWrapper {
public DogsCursorWrapper(Cursor cursor) {
super(cursor);
}
public Dog getDog() {
Dog dog = new Dog();
dog.setDogId(getInt(getColumnIndex(DogTable.ID)));
dog.setDogName(getString(getColumnIndex(DogTable.NAME)));
dog.setDogKind(getString(getColumnIndex(DogTable.KIND)));
return dog;
}
}
Then i use queryDogs method to fill a cursor and then return an instance of wrapper class
private DogsCursorWrapper queryDogs() {
Cursor simpleCursor = db.query(
DogTable.TABLE_NAME, null,null,null,null,null,null);
DogsCursorWrapper dogsCursor = new DogsCursorWrapper(simpleCursor);
// simpleCursor.close(); // this line causes an error in runtime
return dogsCursor;
}
Next step i call the method above in method below:
private void loadDogs() {
DogsCursorWrapper dogsCursor;
dogs = new ArrayList<>();
try {
dogsCursor = queryDogs();
dogsCursor.moveToFirst();
while (!dogsCursor.isAfterLast()) {
dogs.add(dogsCursor.getDog());
dogsCursor.moveToNext();
}
} finally {
dogsCursor.close();
}
}
In fact i do close the dogsCursor in my last method and my question is: didn't i miss some cursor that i have to close? To be clear i have some doubts about simpleCursor in queryDogs method. Should i close that one?
Is it correct decision to use custom DogsCursorWrapper class in this way? Thanks a lot!

The line that has been commented out:
// simpleCursor.close(); // this line causes an error in runtime
is unnecessary. I think you're asking: Does it make a copy of the cursor? The answer is no. The cursor wrapper uses the cursor that is passed in.
The easiest way to verify that is to do:
private DogsCursorWrapper queryDogs() {
Cursor simpleCursor = db.query(
DogTable.TABLE_NAME, null,null,null,null,null,null);
DogsCursorWrapper dogsCursor = new DogsCursorWrapper(simpleCursor);
simpleCursor.close(); // this line causes an error in runtime
if (dogsCursor.isClosed()) { // Because of this
Log.w(TAG, "Houston we have a problem...");
}
return dogsCursor;
}
Running this code demonstrates that closing the original cursor also closes the cursor held by the CursorWrapper.

Frequently updating SQLite DB and Listview (efficiency)

I'm developing an application which relies on database (local) - and this database updates frequently.
What I'm trying to achieve is:
Suppose the data is shown in a listview. I want the listview to update the dataset as soon as any change in the database happens (or a specific table to be precise).
So far I've thought of these options:
SQLiteOpenHelper class: whenever an update/insert is done it'll notify the activity to update listview via BroadcastReceiver.
ContentProvider with CursorLoader (haven't used it before so a little skeptical)
Something else? Please suggest.
Which is the best way to achieve consistent and immediate updates without blocking the UI (performance)?

As suggested by #Karakuri created a custom CursorLoader by extending CursorLoader class without ContentProvider.
Here's the solution:
CustomCursorLoader.class
public class CustomCursorLoader extends CursorLoader {
private final ForceLoadContentObserver forceLoadContentObserver = new ForceLoadContentObserver();
public CustomCursorLoader(Context context, Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
super(context, uri, projection, selection, selectionArgs, sortOrder);
}
#Override
public Cursor loadInBackground() {
Cursor cursor = /* get cursor from DBHandler class */;
cursor.setNotificationUri(getContext().getContentResolver(), CONTENT_URI);
if (cursor != null) {
cursor.getCount();
cursor.registerContentObserver(forceLoadContentObserver);
}
return cursor;
}
}
Every time you make a change to DB, do:
getContentResolver().notifyChange(CONTENT_URI, null);
In Activity class:
implement interface LoaderManager.LoaderCallbacks<Cursor>
initiate loader getLoaderManager().initLoader(0, null, this);
and override these methods:
#Override
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
return new CustomCursorLoader(this, CONTENT_URI, null, null, null, null);
}
#Override
public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
customCursorLoaderAdapter.swapCursor(data);
}
#Override
public void onLoaderReset(Loader<Cursor> loader) {
customCursorLoaderAdapter.swapCursor(null);
}
extend CursorAdapter class to create listview adapter and you're done.

If performance is crucial for your app you should take a look on Realm for Android database. It provides better efficiency than SQLite and you can use RealmChangeListener to listen for changes in the database.

Updating List view from database

I feel like i am missing something simple and stupid. I have a list view with a few buttons at the top. The list view is initially populated with data. When you click a button the list view is supposed to populate its self based on a changed variable in the Where statement. In reality i could probably just start a new List activity but i feel like there is a better way.
I have been reading up on CursorAdapter.changeAdapter() and notifydatasetchanged() I have not implemented this yet because i am having a more basic problem.
I can successfully query the database and display the static results in the list. When i try to break process into steps i am running into an ERROR: Invalid statement in fillWindow. The best i understand this is caused by improperly closing cursors databases and DB helpers and for this reason people use content providers.
For now i am just trying to get this to work.
public class DListView extends ListActivity implements OnClickListener{
public static final String NAME = "Name";
public static final String DESCRIPT = "Description";
public static final String DATABASE_TABLE = "Table";
public static final String DAY = "Day_id";
/** Called when the activity is first created. */
private Cursor c = null;
private String[] colsfrom = {"_id", NAME, DESCRIPT, DAY};
private int[] to = new int[] {R.id.text01, R.id.text02, R.id.text03, R.id.text04};
public int b = 0;
public int d = 0;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.drinklistview);
View left = findViewById(R.id.left_button);
left.setOnClickListener(this);
View right = findViewById(R.id.right_button);
right.setOnClickListener(this);
Intent thisIntent = getIntent();
b = thisIntent.getIntExtra("_b", 0);
//0 is the default argument is nothing is passed.
d = thisIntent.getIntExtra("_d", 0); //same idea as above.
c = fillList();
/*this creates a new cursor adapter
#param Context is the list context that you will be filling.
#param int layout is the layout that you will use for the rows
#param Cursor is the cursor that was returned from the query
#param from is the column names
#param to is the layout ids that the fields will be put in.
#param from is the column names to map from
#param to is the layout ids that the column fields will be put in.
*/
SimpleCursorAdapter myAdapter = new SimpleCursorAdapter(this, R.layout.row, c, colsfrom, to);
setListAdapter(myAdapter);
}
private Cursor fillList() {
DBHelper DbHelper = new DBHelper(this);
Cursor cursor;
String wHERE = "_id = " + b + " AND Day_id = " + d ;
try {
myDbHelper.openDataBase();
}
catch(SQLException sqle){
throw sqle;
}
cursor = myDbHelper.getDrinks(DATABASE_TABLE, colsfrom, wHERE, null, null,null, null);
myDbHelper.close();
return cursor;
}
When i put the contents of fillList() in the onCreate() it displays data just fine. When i pull it out it gives me the ERROR. Why is this happening? If anyone has a better way of going about this i would love to read it. Or we can play a game called "What stupid thing am i doing wrong Now?
Thankyou.
EDIT:From DBHelper
public void openDataBase() throws SQLException{
//Open the database
String myPath = DB_PATH + DB_NAME;
myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}
#Override
public synchronized void close() {
if(myDataBase != null)
myDataBase.close();
super.close();
}
I am thinking that my problem line is the super.close() I believe that this line closes the database and anything affiliated with it which means the cursor that i try to use after its closed. I may be wrong though. Please explain if you can.

Your problem is right here, in your fillList():
myDbHelper.close(); // <--- here
return cursor;
you make a cursor object but close your database connection before you even get to use it (this component of the database) which would render it useless or null if you would. Usually you close the cursor and then the database. But that's not throwing the error. That error specifically is because you hooked up this cursor to a cursorAdapter trying to fill your listView with nothing. Move that and it should be gone.
So where do you move it then? If you have a cursor hooked up to listView, it needs to be open the entire time, otherwise you'll get another error saying "attempting to re-open an already closed object". I'd suggest putting in the onDestroy() when then listView is being chucked as well.

YaY Solved. Mango is exactly correct. Thankyou for you suggestion to close cursor in on destroy. I am not sure if the super.close() line closes my cursor or not. but i will look into it. I am also going to put the database query in async task for kicks and giggles.
I simply moved the two lines that created a new SimpleCursorAdapter and set the list view into the fillList method.
I also implemented my buttons and just added fillList at the end.
Here is the code that fixed things. Simple Mistake.
private void fillList() {
DBHelper DbHelper = new DBHelper(this);
Cursor cursor;
String wHERE = "_id = " + b + " AND Day_id = " + d ;
try {
myDbHelper.openDataBase();
}
catch(SQLException sqle){
throw sqle;
}
cursor = myDbHelper.getDrinks(DATABASE_TABLE, colsfrom, wHERE, null, null,null, null);
SimpleCursorAdapter myAdapter = new SimpleCursorAdapter(this, R.layout.row, cursor, colsfrom, to);
setListAdapter(myAdapter);
myDbHelper.close();
}
And Here is wehre i call the fillList again that updates my list view.
public void onClick(View v) {
switch(v.getId()) {
//Mess with d based on button click
}
fillList();
}
Now the application has to create a new simple cursor adapter every time something is changed.
If anyone has any ideas on implementing this without creating a new CursorAdapter every time that would help very much but my initial problem is solved. Thankyou for your help. Just the fact that you wanted to see my stack trace told me that i was not doing anything wrong in the code that i initially presented and i forgot that i made my dbHelper close all connections. Thankyou mango. I solved this last night but couldnt post it. Thanks for the explanation good sir. If you have any insight to the constant creation of a new cursoradapter i would be very pleased to see it. Maybe i need to fix the super.close() command somehow.

Why cursorLoader didn't notify changes in source data?

I have a simple contentProvider, a layout with a ListView and a button for adding Items in content Provider and a CursorLoader. The android.content.Loader, D reference states that
The Loader will monitor for changes to the data, and report them to
you through new calls here. You should not monitor the data yourself.
For example, if the data is a Cursor and you place it in a
CursorAdapter, use the CursorAdapter(android.content.Context,
android.database.Cursor, int) constructor without passing in either
FLAG_AUTO_REQUERY or FLAG_REGISTER_CONTENT_OBSERVER (that is, use 0
for the flags argument). This prevents the CursorAdapter from doing
its own observing of the Cursor, which is not needed since when a
change happens you will get a new Cursor throw another call here.
But the Log.info line in the onLoadFinished method was not executed and listView didn't refreshed. Here is my (simple) code:
public class HomeActivity extends FragmentActivity implements LoaderManager.LoaderCallbacks<Cursor>{
static final String TAG = "HomeActivity";
SimpleCursorAdapter adapter;
ListView listAnnunciVicini;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.home);
listAnnunciVicini = (ListView) findViewById(R.id.lista_annunci_vicini);
adapter = new SimpleCursorAdapter(this, R.layout.list_item, null,
new String[] {
ContentDescriptor.Annunci.Cols.ID,
ContentDescriptor.Annunci.Cols.TITOLO,
ContentDescriptor.Annunci.Cols.DESCRIZIONE
}, new int[] {
R.id.list_annunci_item_id_annuncio,
R.id.list_annunci_item_titolo_annuncio,
R.id.list_annunci_item_descrizione_annuncio
}, 0);
listAnnunciVicini.setAdapter(adapter);
// Prepare the loader. Either re-connect with an existing one,
// or start a new one.
getSupportLoaderManager().initLoader(0, null, this).forceLoad();
}
public void addRandomItem(View sender) {
ContentValues dataToAdd = new ContentValues();
dataToAdd.put(ContentDescriptor.Annunci.Cols.TITOLO, "Titolo");
dataToAdd.put(ContentDescriptor.Annunci.Cols.DESCRIZIONE, "Lorem ipsum dolor sit amet.");
this.getContentResolver().insert(ContentDescriptor.Annunci.CONTENT_URI, dataToAdd);
}
#Override
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
// creating a Cursor for the data being displayed.
String[] proiezione = new String[] {ContentDescriptor.Annunci.Cols.ID, ContentDescriptor.Annunci.Cols.TITOLO, ContentDescriptor.Annunci.Cols.DESCRIZIONE };
CursorLoader cl = new CursorLoader(this, ContentDescriptor.Annunci.CONTENT_URI, proiezione, null, null, null);
return cl;
}
public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
// Swap the new cursor in. (The framework will take care of closing the
// old cursor once we return.)
adapter.swapCursor(data);
Log.i(TAG, "I dati sono stati ricaricati");
}
public void onLoaderReset(Loader<Cursor> loader) {
// This is called when the last Cursor provided to onLoadFinished()
// above is about to be closed. We need to make sure we are no
// longer using it.
adapter.swapCursor(null);
}
}
Any suggestion?

AFAIK, you need to implement notification yourself in ContentProvider. For this, add something like getContext().getContentResolver().notifyChange(uri, null); to insert,update and delete method of ContentProvider and invoke.setNotificationUri(getContext().getContentResolver(), uri) on your Cursor in queryas described in official documentation. Here you can find the whole picture.
Note: You should not close the cursor (cursor.close()) in order to get
notifications about the changes.

CursorLoader for Multiple ContentProviders

I need to make a ListAdapter that presents data from multiple ContentProviders. The ContentProviders themselves represent one table each from relational database.
I want to use the CursorLoader system to retrieve aggregate data into ListView. Is this possible to do with 1 loader or do I need to use multiple loaders? I'd prefer to use one.
I'm not sure how I can have 2 ContentProviders interact with each other beyond doing the join manually in code which doesn't seem like a great option either.

You will have to write a Custom Loader class. For example:
public class FooLoader extends AsyncTaskLoader {
Context context;
public FooLoader(Context context) {
super(context);
this.context = context;
}
#Override
public Cursor loadInBackground() {
Log.d(TAG, "loadInBackground");
YourDatabase dbHelper = new YourDataBase(context);
SQLiteDatabase db= dbHelper.getReadableDatabase();
/*** create a custom cursor whether it is join of multiple tables or complex query**/
Cursor cursor = db.query(<TableName>, null,null, null, null, null, null, null);
return cursor;
}
}
In the calling activity or fragments onCreate() method, you would need to call the custom loader class:
public class MyFragment extends Fragment implements LoaderManager.LoaderCallbacks<Cursor> {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.d(TAG, "onCreate():" + mContent);
Loader loader = getLoaderManager().initLoader(0, null, this);
loader.forceLoad();
}
#Override
public Loader<Cursor> onCreateLoader(int i, Bundle bundle) {
Log.d(TAG, "onCreateLoader()") ;
return new FooLoader(getActivity());
}
#Override
public void onLoadFinished(Loader<Cursor> cursorLoader, Cursor cursor) {
Log.d(TAG, "onLoadFinished");
}
#Override
public void onLoaderReset(Loader<Cursor> cursorLoader) {
}
}

You might want to take a look at CursorJoiner.

I'm new to ContentLoaders myself, but I haven't yet seen a way that you could use one ContentLoader to handle multiple ContentProviders.
Are the tables you're querying in separate databases? It isn't clear from your question. If the tables are all in the same database, one alternative might be to instead use one ContentProvider for the separate tables. The data can be joined and returned to one cursor, which means you could use one CursorLoader. The SQLiteQueryBuilder.setTables() method has some information on this:
http://developer.android.com/reference/android/database/sqlite/SQLiteQueryBuilder.html#setTables%28java.lang.String%29
and you can see it in action here:
http://code.google.com/p/openintents/source/browse/trunk/shoppinglist/ShoppingList/src/org/openintents/shopping/provider/ShoppingProvider.java
this might also be helpful:
https://stackoverflow.com/a/3196484/399105