Develop Reference

Fragments, ContentProviders and cursors on orientation change

Please help me understand what's happening here.
I have two fragments (A and B) in tabs reading data from different tables, via a CursorLoader and aContentProvider, in a Sqlite-database. With different URIs I can change which table the ContentProvider is quering.
I works as expected when switch between the tabs A and B, unless I switch to B, rotate and switches back to A the wrong cursor is returned. The cursor from fragment B is returned instead of a cursor for fragment A with makes the listView in fragment A to cause a crash. In some way the cursor seems to be reused on a rotation.
Why is this happening and how can I prevent that the wrong cursor is returned?
This is what I have in both fragment A and B. Tried to assing a loader id with no success.
public void onResume() {
super.onResume();
getLoaderManager().restartLoader(mLoaderId, null, this);
}
My ContentProvider looks like this:
public Cursor query(Uri uri, String[] projection, String selection,
String[] selectionArgs, String sortOrder) {
SQLiteDatabase db = dbHelper.getWritableDatabase();
SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder();
Cursor cursor = null;
switch (uriMatcher.match(uri)) {
case ALL_NEWS:
queryBuilder.setTables(NewsDb.SQLITE_TABLE);
cursor = queryBuilder.query(db, projection, selection,
selectionArgs, null, null, sortOrder);
break;
case SINGLE_PLACE:
queryBuilder.setTables(PlacesDb.SQLITE_TABLE);
String id = uri.getPathSegments().get(1);
queryBuilder.appendWhere(PlacesDb.KEY_ID + "=" + id);
cursor = queryBuilder.query(db, projection, selection,
selectionArgs, null, null, sortOrder);
break;
default:
throw new IllegalArgumentException("Unsupported URI: " + uri);
}
return cursor;
}

When using CursorLoader, data changes are automatically observed, so you should definitely remove the restartLoader call in your onResume(). If implemented properly, you should only have to call initLoader in onActivityCreated of the Fragment.
LoaderManager IDs are only scoped to the Fragment, so you should be using a static constant ID. If the Loaders are handled in the Fragments, themselves, it's perfectly fine for every Fragment to have the same Loader ID (even if they're managed by the same Activity).
So in each Fragment:
private static final int LOADER_ID = 0;
#Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
// ...
getLoaderManager().initLoader(LOADER_ID, null, this);
}

Cursorloader not refreshing when underlying data changes

I have a content provider, a content resolver, and a cursor loader. The loader is used to indirectly populate a listview (ie not a simple cursor adapter, rather an array adapter, since I need to use the cursor's results to gather other data).
When I change the underlying data, the listview does not re-populate as the onLoadFinished(Loader<Cursor>, Cursor) call back is not called.
As suggested while I'm writing this, there are a lot of questions on this issue.
eg
CursorLoader not updating after data change
And all the questions point out two things:
In your content provider query() method, you need to tell the cursor about notifications a'la
c.setNotificationUri(getContext().getContentResolver(), uri);
In your content provider insert/update/delete method, you need to notify on the URI:
getContext().getContentResolver().notifyChange(uri, null);
I'm doing those things.
I'm also NOT closing any of the cursors I get back.
Note, my content provider lives in a separate app (think of it as a content provider app -- no launcher main activity). A com.example.provider APK, and the com.example.app is calling (in the content resolver) via the content://com.example.provider/table URI etc. The content resolver (dbhelper) lives in a library project (com.example.appdb) that the activity links in. (This way, multiple projects can use the dbhelper via linking, and all content providers are installed via single APK)
I have turned on debugging in the loader manager, and can see where I force a refresh after the data changes (ie the loader being restarted and previous being marked inactive), but nothing that says anything is happening automatically -- rather, in response to my force refresh.
Any ideas why my loader isn't being refreshed ?
-- EDIT --
The loader create:
Log.d(TAG, "onCreateLoader ID: " + loaderID);
// typically a switch on the loader ID, and then
return dbHelper.getfoo()
Where getfoo() returns a cursor loader via:
return new CursorLoader(context, FOO_URI, foo_Fields, foo_query, foo_argArray, foo_sort );
Loader Finish takes the cursor and populates a table (and does some processing) -- nothing fancy there.
public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) {
Log.d(TAG, "onLoadFinished id: " + loader.getId());
// switch on loader ID ..
FillTable(cursor, (FooAdapter) foo_info.listview.getAdapter();
Loader Reset clears the table.
public void onLoaderReset(Loader<Cursor> loader) {
Log.d(TAG, "onLoaderReset id: " + loader.getId());
// normally a switch on loader ID
((FooAdapter)foo_info.listview.getAdapter()).clear();
The content provider does:
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
Cursor cursor;
SQLiteQueryBuilder qb = new SQLiteQueryBuilder();
SQLiteDatabase db = dbHelper.getReadableDatabase();
switch (sUriMatcher.match(uri)) {
case FOO:
qb.setTables(FOO);
qb.setProjectionMap(FooProjectionMap);
break;
default:
throw new IllegalArgumentException("Unknown URI " + uri);
}
Cursor c = qb.query(db, projection, selection, selectionArgs, null, null, sortOrder);
c.setNotificationUri(getContext().getContentResolver(), uri);
return c;
}
Then insert/update is similar:
public Uri insert(Uri uri, ContentValues initialValues) {
ContentValues values;
if (initialValues != null) {
values = new ContentValues(initialValues);
} else {
values = new ContentValues();
}
String tableName;
Uri contentUri;
switch (sUriMatcher.match(uri)) {
case FOO:
tableName = FOOTABLE;
contentUri = FOO_URI;
break;
default:
throw new IllegalArgumentException("Unknown URI " + uri);
}
SQLiteDatabase db = dbHelper.getWritableDatabase();
long rowId = db.insert(tableName, null, values);
if (rowId > 0) {
Uri objUri = ContentUris.withAppendedId(contentUri, rowId);
getContext().getContentResolver().notifyChange(objUri, null);
return objUri;
}
throw new SQLException("Failed to insert row into " + uri);
}

I see that you aren't calling swapCursor or changeCursor in your onLoadFinished and onLoaderReset. You need to do that for your adapter to access the data loaded into the new cursor.
in onLoadFinished, call something like:
mAdapter.swapCursor(cursor)
In onLoaderReset, call this to remove references to the old cursor:
mAdapter.swapCursor(null)
Where mAdapter is your listView's adapter.
More info: http://developer.android.com/guide/components/loaders.html#onLoadFinished

So, for anyone who looks this up and has this problem:
Make certain that the URI you register the cursor on, and the URI you notify the cursor on are the same.
In my case, I was using a different URI to query than was used in other code that modified the underlying table. There wasn't an easy fix to make notification work, so I kept resetting the loader in OnResume() as the solution.

This is not a direct answer to this specific problem, but may help others in a similar situation. In my case, I had a join and even though I used the full tablename.columnname in the projection and query. It ended up using the wrong ID values anyway. So be sure to check your ContentProvider or SQL syntax.

Android - Requerying cursor ordered by RANDOM()

this may sound like a silly question but Im struggling with it. I am working on a quotes application therefore I want to have my quotes ordered from the database randomly each time the application is started.
I am using CursorLoader and a ViewPager. I am having some buttons (put-to-favourites-button) on the screen, which updates a column in database using content resolver which triggers update() in ContentProvider. Update then causes a requery in order to update the change made. All standard.
This of course causes to give me rows that are ordered differently because of the order by random() clause, and causes the screen to "disappear".
Any idea how to get around this? I really want to keep the random ordering functionality.
class MyContentProvider extends ContentProvider {
#Override
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
Cursor cursor = db.query(TABLE_NAME, projection, selection, selectionArgs, null, null, sortOrder);
cursor.setNotificationUri(resolver, uri);
return cursor;
}
#Override
public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
int count = db.update(TABLE_NAME, values, selection, selectionArgs);
resolver.notifyChange(uri, null);
return count;
}
}

you can use something like
Cursor c = db.rawQuery("select * from table ORDER BY RANDOM()");
SQLite docs

What I ended up doing, as zapl suggested, was to add a column. that would hold myOrder, which would be populated with random values in onCreate, and then just do a Select with order by myOrder

CursorLoader not updating after data change

I have created a small application, trying to understand the functionality of the LoaderManager and CursorLoader-classes.
I have implemented LoaderCallbacks<Cursor> on my FragmentActivity-class and everything works fine, except the fact that when I update my data via ContentResolver.update() or ContentResolver.insert()-methods, onLoadFinished() is not called and as a result my data doesn't update.
I have a custom ContentProvider and I am wondering if the problem is in my ContentProvider not notifying that the data changed or something else.

Did you call setNotificationUri(ContentResolver cr, Uri uri) on the Cursor before returning it in ContentProvider.query()?
And did you call getContext().getContentResolver().notifyChange(uri, null) in the 'insert' method of your ContentProvider?
EDIT:
To get a ContentResolver call getContext().getContentResolver() in your ContentProvider.

Also check if you call somewhere cursor.close(), because in this case you unregister the content observer which was registered by CursorLoader. And the cursor closing is managed by CursorLoader.

Accepted answer was the little bit tricky to understand so I am writing the answer to make it easy for other developers..
Go to the class in which you have extended the ContentProvider
Find the query() method which has the following syntax
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder)
Write this line where you are returning the cursor
cursor.setNotificationUri(getContext().getContentResolver(), uri);
return cursor;
In the end, my query method looks like this
#Nullable
#Override
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
Cursor cursor;
cursor = noticeDbHelper.getReadableDatabase().query(
NoticeContract.NoticeTable.TABLE_NAME,
projection,
selection,
selectionArgs,
null,
null,
sortOrder
);
//This line will let CursorLoader know about any data change on "uri" , So that data will be reloaded to CursorLoader
cursor.setNotificationUri(getContext().getContentResolver(), uri);
return cursor;
}`

CursorLoader with rawQuery

I'm looking into implementing CursorLoader in my app but I'm having a small issue that it seems that there isn't a way to just a pass a raw query to the CursorLoader constructor.
I maybe missing something in the documentation (and google), so if anybody can point me to a simple way to run a raw query with a CursorLoader class I would appreciate it. Otherwise I will have to probably create my own CursorLoader class with the necessary functionality, which I'm trying to avoid.

it seems that there isn't a way to just a pass a raw query to the CursorLoader constructor.
That is because CursorLoader works with content providers, and content providers do not support rawQuery().
so if anybody can point me to a simple way to run a raw query with a CursorLoader class I would appreciate it.
That is impossible, sorry. You are welcome to create your own AsyncTaskLoader that hits a SQLite database and supports rawQuery(). In fact, I will probably write one of these later this year, if I don't see where anyone has beaten me to it.

Raw query is not supported directly, but you can do a dirty hack: from your code call
getContentResolver().query(RAWQUERY_CONTENT_URI, null, rawquery, args, null);
and implement content provider like
#Override
public synchronized Cursor query(Uri uri, String[] projection, String selection,
String[] selectionArgs, String sortOrder)
{
int uriType = sURIMatcher.match(uri);
switch (uriType)
{
case RAW_QUERY:
return dbHelper.getReadableDatabase().rawQuery(selection, selectionArgs);
}
[...]
}

**For Custom Search using Content provider **
Change Cursor Loader as Follow (in onCreateLoader )
return new CursorLoader(
getActivity(), // Context
PRODUCT.CONTENT_URI, // URI
PROJECTION, // Projection
PRODUCT.PRODUCT_NAME+ " like ?", // Selection
new String[]{"%" + mCurFilter + "%"}, // Selection args
PRODUCT.PRODUCT_NAME + " asc");
In your Provider Change Accordingly
//C is Cursor object
switch (uriMatch) {
case ROUTE_PRODUCT_ID:
// Return a single entry, by ID.
String id = uri.getLastPathSegment();
builder.where(PRODUCT._ID + "=?", id);
c = builder.query(db, projection, sortOrder);
assert ctx != null;
c.setNotificationUri(ctx.getContentResolver(), uri);
return c;
// break;
case ROUTE_PRODUCT:
// Return all known entries.
builder.table(PRODUCT.PRODUCT_TABLE_NAME)
.where(selection, selectionArgs);
c = builder.query(db, projection, sortOrder);
assert ctx != null;
c.setNotificationUri(ctx.getContentResolver(), uri);
return c;

You can implement your own CursorLoader with raw query. This is the source of the original CursorLoader: https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/content/CursorLoader.java