I am a new programmer in Objective-C. I have created an Android app and I've integrated SQLite. I want to build the same app in iOS. In Android for SQLite manipulation I've created a class that extends from Content Provider:
public class MyCProvider extends ContentProvider {
...
...
}
I've also overrided this methods:
the boolean onCreate() method:
#Override
public boolean onCreate() {
dbHelper = new MySQLiteOpenHelper(getContext(), MySQLiteOpenHelper.DATABASE_NAME, null, MySQLiteOpenHelper.DATABASE_VERSION);
return dbHelper != null;
}
#Override
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
...
}
#Override
public String getType(Uri uri) {
...
}
#Override
public Uri insert(Uri uri, ContentValues values) {
SQLiteDatabase db = dbHelper.getWritableDatabase();
String nullColumnHack = null;
long id = -1;
Uri contentURI = null;
switch (uriMatcher.match(uri)) {
case GROUP_ALL_ROWS:
contentURI = GROUP_CONTENT_URI;
id = db.insert(USEFUL_NUMBER_GROUP_TABLE_NAME, nullColumnHack, values);
break;
...
...
}
I wanted to know this methods equivalences in Objective- C.
Any idea about this?
SQLite is provided directly with iOS and you'd just use its native C API. Apple doesn't provide any sort of wrapping.
The attitude seems to be that you can use SQLite directly or you can use Core Data, which is really quite a different thing — it's a queriable object graph which implicitly relies on an opaque storage method that can be a SQLite-based relational database if you like but Core Data is explicitly not a relational database. You can and should optimise for the SQL store underneath if that's the storage you pick but the database itself has a private schema and is not queried directly.
As such there's no equivalent to SQLiteOpenHelper. If you want to use SQLite then you need to do the work of opening, creating, migrating, etc for yourself.
iOS also doesn't have an equivalent to ContentProvider because that's a formalised model structure to allow sharing of data between applications but iOS doesn't really do sharing of data between applications in that sense. Applications from the same developer can share disk storage, all applications can open each other by URL schema, etc, but you can't write code that different applications can all interact with. So you'd just build whatever sort of model object you think most appropriate and its lifetime and interface would be whatever you explicitly decide.
Related
My app uses an SQLite DB, wrapped with a SQLiteOpenHelper and a ContentProvider. I added a sign-in feature to the app, and now I want every user to only be able to see his own data. The way I thought to achieve this is for the app to create a separate DB for every user that signs in to the app, and use the user's ID in the filename of the database.
I have this ContentProvider:
public class MyProvider extends ContentProvider {
//...
#Override
public boolean onCreate() {
dbHelper = new MyDBHelper(getContext());
return true;
}
I have this SQLiteOpenHelper:
public class MyDBHelper extends SQLiteOpenHelper {
Which has this constructor:
public MyDBHelper(Context context) {
super(context, DB_NAME, null, DB_VERSION);
}
Up until now the app couldn't have multiple users, so it just had one database. so DB_NAME was always the same String. I now tried to set it like that:
private static String UID = FirebaseAuth.getInstance().getCurrentUser().getUid();
public static final String DB_NAME = String.format("data%s.db", UID);
(As you can see, I'm using Firebase Authentication)
but this resulted in a crash, because apparently the content provider is created on app start, before the user has authenticated. (so user is null. Yeah, I should check that user is not null before I try to call getUid(). but this won't make this thing work)
So this doesn't seem like the right approach. How can I use a different DB according to the signed user? Can I make the content provider to first be created after a user has authenticated?
I could also just keep everything in one database and add a UID column. But will this be protect the different users' data good enough from each other? Also, this would mean a lot more code changes.
How can I use a different DB according to the signed user?
The simple solution is to get rid of the ContentProvider. The only reason to use a ContentProvider is if you are going to be serving this data to other apps.
Also, I would be wary of just taking getUid() and putting it in a filename. You are not in control over what getUid() returns, and it might someday contain characters that are invalid in filenames.
Can I make the content provider to first be created after a user has authenticated?
No, sorry.
Seems that the right solution here is to not use ContentProviders. So I accepted the other answer.
But to answer my actual question, for people that are determined to make different DBs work with one ContentProvider, here is how it can be done:
I changed the custom SQLiteOpenDBHelper's constructor to also take a uid:
public MyDBHelper(Context context, String uid) {
super(context, String.format(DB_NAME, uid), null, DB_VERSION);
UID = uid;
}
and I changed the onCreate of my ContentProvider not to create the DBHelper. I created this function that initializes the DBHelper instead:
public void initDB(Uri uri) {
String uid = uri.getPathSegments().get(0);
if (dbHelper == null){
dbHelper = new MyDBHelper(getContext(), uid);
} else if (!uid.equals(dbHelper.UID)){
dbHelper.close();
dbHelper = new MyDBHelper(getContext(), uid);
}
}
and I call this method at the start of the query, insert, update and delete methods.
So the DBHelper which holds the open connection to the DB, is initialized whenever the content provider is preforming some action on the DB but there is either not yet an existing connection with the DB, or the connection is with a DB of a different user.
This is not the right way to solve this problem and this probably has consequences in some cases. But I didn't want to leave the question I asked unanswered.
I am new in Android and am implementing a content provider for 5 tables.
My Questions Are:
Should I have a Content Provider for each table or multiple tables in a single
Content provider? Since the Content Provider has a single insert, update, query, delete methods.
How can I include only one Content Provider in my application? I have searched and in Most of the examples, I only find a single table apps.
Where do I have to use switch conditions to support multiple tables with the same Content Provider?
please give me some idea.
You can use the URI parameter:
List<String> android.net.Uri.getPathSegments()
If your URI is, for example:
content://com.mypackage.MyContentProvider/MyTable
MyTable will be in the list returned by getPathSegments();.
Then you have to specify your table in the URI and in insert, update, query, delete methods in provider build a query depending on the URI parameter.
To avoid testing on URI you can add to you provider an Abstract method called getTableName() witch will return your tableName as String.
Then extend your provider to 5 classes Table1Provider, Table2Provider etc. and implement method
Class abstract MyProvider extends ContentProvider{
public abstract String getTableName();
#Override
public Cursor query(Uri uri, String[] projection, String selection,
String[] selectionArgs, String sortOrder) {
///...
// Set the table
queryBuilder.setTables(getTableName());
//...
return cursor;
}
}
class Table1Provider extend MyProvider{
public String getTableName(){
return "Table1";
}
Then instantiate the Table1Provider instead of the abstract provider.
Make one provider. Use the URIMatcher class provided by Android to match content URIs with different tables.
Read here: http://developer.android.com/guide/topics/providers/content-provider-creating.html#ContentURI
I have created a databaseprovider class which uses single instance of db object. Object is created in main activity and closed onDestroy method. This seems ok (but get some errors such as: db already closed or db is not open on some users devices that I cannot simulate).
I want to add a service to the application for the content download and this service can run with scheduler which make me think about single instance of db object will not work. Should I use another object for the service, will it result consistency problems? Can you kindly advice what would be the best way?
Databaseprovider class exm:
public class DatabaseProvider {
private static DatabaseHelper helperWriter;
public static SQLiteDatabase db_global;
public DatabaseProvider(Context c) {
helperWriter = DatabaseHelper.getHelper(c, true);
}
private static SQLiteDatabase getDB() {
if(db_global == null)
db_global = helperWriter.getWritableDatabase();
else if(!db_global.isOpen()) {
try {
db_global.close();
}
catch(Exception ex) {
ex.printStackTrace();
}
db_global = helperWriter.getWritableDatabase();
}
return db_global;
}
public String GetVersion() {
SQLiteDatabase db = getDB();
Cursor c = db.query(DatabaseHelper.PARAMETER_TABLE_NAME, new String[] {"VALUE"}, "KEY='Version'", null, null,null,null);
String version = "";
if(c.moveToNext())
{
version = c.getString(0);
}
else
version = "0";
c.close();
return version;
}
public long UpdateVersion(String value) {
ContentValues initialValues = new ContentValues();
initialValues.put(DatabaseHelper.PARAMETER_COLUMN_VALUE, value);
SQLiteDatabase db = getDB();
long r = db.update(DatabaseHelper.PARAMETER_TABLE_NAME, initialValues, "KEY='Version'", null);
if(r <= 0)
r = helperWriter.AddParameter(db, "Version", value);
//db.close();
return r;
}
public void CloseDB() {
if (db_global != null)
db_global.close();
db_global = null;
helperWriter.close();
}
}
Not sure if this will help, but...
you can't rely on onDestroy() in case the app crashes. Android may also keep your app in RAM, even if you exit it. Also, your main activity may get destroyed while the app is getting used if you are on a subactivity. It can also get recreated.
Sometimes it's better to have calls that open the DB, does stuff to it, and then closes it within the same function. If you are using a service, it may actually help things. I also am not sure if you should have a situation where a DB can be opened and/or accessed from a variety to different places at once without some management code
I see a couple questions:
A)
(but get some errors such as: db already closed or db is not open on some users devices that I cannot simulate).
...
Start an activity, then update content and some db operations in AsyncTask. While update is in progress go back and start the same activity again.
To work around these errors have you considered using a [Loader][1]? It's a callback based framework around ContentProviders.
B)
add a service to the application for the content download and this service can run with scheduler which make me think about single instance of db object will not work. Should I use another object for the service, will it result consistency problems?
This post by #commonsware from this website, suggests not to use Service for long running tasks. Instead the AlarmManager is suggested. I've only worked with short running services (for audio IO) myself.
I use a ContentProvider in my app and everything works great except for one little issue. I have a backup and restore function that backs up the database to a file on the SD card and then those backup files can be restored to overwrite the current database. This whole process is working, but the ContentProvider still holds the reference/cache to the original database once one of the old backup files is restored. I can't seem to find a way to refresh or reload the database reference in the ContentProvider. I know the restore works because I can see the records in the db with SQLite Editor and when I close and re-open the app, it displays the correct records.
Does anybody know a way to do this? Is there a way to close and re-open the ContentProvider that I'm not seeing?
If you are targeting >= API 5 you can get a reference to your ContentProvider via a ContentProviderClient, and run a method specific to your implementation:
ContentResolver resolver = context.getContentResolver();
ContentProviderClient client = resolver.acquireContentProviderClient("myAuthority");
MyContentProvider provider = (MyContentProvider) client.getLocalContentProvider();
provider.resetDatabase();
client.release();
Add the reset method to your ContentProvider implementation:
public void resetDatabase() {
mDatabaseHelper.close();
mDatabaseHelper = new MyDatabaseOpenHelper(context);
}
Are you maintaining a reference to the actual SQLiteDatabase in your content provider (something like calling SQLiteOpenHelper.getWritableDatabase() in onCreate() and then keeping that reference)? Or do you get the DB object from someplace like a helper in each provider method?
Typically, if you only keep a local reference to the helper and get the writable/readable database instance inside of each method as needed then this problem should go away. If not, perhaps we can take a look at the provider code?
Hope that Helps!
Here is my solution.
public class DataProvider extends ContentProvider {
private DataDbHelper dbHelper;
#Override
public boolean onCreate() {
// nothing here
return true;
}
private DataDbHelper getDbHelper() {
if (dbHelper== null) {
// initialize
dbHelper = new DataDbHelper(getContext());
} else if (dbHelper.getReadableDatabase().getVersion() != DataDbHelper.VERSION) {
// reset
dbHelper.close();
dbHelper = new DataDbHelper(getContext());
}
return this.mOpenHelper;
}
}
query(), insert(), update(), delete() use getDbHelper() to obtain an SQLiteDatabase
The full code of my Android app is available here if you need more info.
You can also simply use the delete method without a selection:
context.getContentResolver().delete(YourProvider.CONTENT_URI, null, null);
Is there any way to get Cursor for a query, which I am processing with ORMLite Dao object?
ORMLite now supports next(), previous(), moveRelative(offset), ... methods on the CloseableIterator class. This should allow you to move the underlying Cursor object around at will.
It also supports the following DAO Cursor methods:
dao.mapSelectStarRow(databaseResults) Return the latest row from the database results from a query to select *. With this you can change the cursor location (for example) and then get the current object.
dao.getSelectStarRowMapper() Provides a mapper that you can use to map the object outside of the Dao.
When you are building your own query with ORMLite, you use the QueryBuilder object. queryBuilder.prepare() returns a PreparedQuery which is used by various methods in the DAO. You can call dao.iterator(preparedQuery) which will return a CloseableIterator which is used to iterate through the results. There is a iterator.getRawResults() to get access to the DatabaseResults class. Under Android, this can be cast to an AndroidDatabaseResults which has a getCursor() method on it to return the Android Cursor.
Something like the following code:
// build your query
QueryBuilder<Foo, String> qb = fooDao.queryBuilder();
qb.where()...;
// when you are done, prepare your query and build an iterator
CloseableIterator<Foo> iterator = dao.iterator(qb.prepare());
try {
// get the raw results which can be cast under Android
AndroidDatabaseResults results =
(AndroidDatabaseResults)iterator.getRawResults();
Cursor cursor = results.getRawCursor();
...
} finally {
iterator.closeQuietly();
}
This is a bit complicated but you are definitely having to peer behind the vale to get to this object which is hidden by the database abstraction classes.
Did you try some of Gray's advice from this post? He explains how you can select a column as another name, such as, select id as _id.
If you're in an Activity and don't want to mess around with the QueryBuilder give the following a go, which is just as effective.
Cursor cursor = getHelper().getReadableDatabase().query(tableName, projection, selection, selectionArgs, groupBy, having, sortOrder)
If you mean the getHelper() method to reach the dao methods create etc. you only have to inherit from the OrmLiteBaseActivity<YourDBHelper> and you can call it. It will look sth like this:
public class YourClass extends OrmLiteBaseActivity<YourDBHelper> {
#Override
protected void onCreate(Bundle savedInstanceState) {
...
getHelper().getDao().queryForAll();
...
}
}
If you mean the cursor to handle database operation: I don't think that you can reach it! But I don't understand why you should need it. ORMLite has nearly all functions of the cursor. So what do you need it for?