Shows how a RoomDatabase.Callback can be used to add database triggers when a database is created.

AppDatabase.kt

@Database(...)
abstract class AppDatabase : RoomDatabase() {
    abstract fun appDao(): AppDao

    companion object {
        private var INSTANCE: AppDatabase? = null
            private set

        fun getInMemoryDatabase(context: Context): CCDatabase {
            if (INSTANCE == null) {
                INSTANCE = Room.databaseBuilder(context,
                        AppDatabase::class.java, "app_database.db")
                        .addCallback(CALLBACK)
                        .build()
            }

            return INSTANCE!!
        }

        private val CALLBACK = object : RoomDatabase.Callback() {
            override fun onCreate(db: SupportSQLiteDatabase) {
                super.onCreate(db)

                db.execSQL("CREATE TRIGGER ...")
            }
        }
    }
}

Hello AdamMc331, can you provide the java equivalent of your code?
Thanks in advance

Here is the Java equivalent code for Room Database callbacks.

  INSTANCE = Room.databaseBuilder(getApplicationContext(), AppDatabase.class, "app-database-db")
                .addCallback(CALLBACK)
                .build();
     
   private RoomDatabase.Callback CALLBACK =  new RoomDatabase.Callback(){
        @Override
        public void onCreate(@NonNull SupportSQLiteDatabase db) {
            super.onCreate(db);
           db.execSQL("CREATE TRIGGER ...")
        }

        @Override
        public void onOpen(@NonNull SupportSQLiteDatabase db) {
            super.onOpen(db);
        }
    };

Thanks a lot @AdamMc331!
The callback has to be saved as a variable, declaring it inside the callback didn't execute it at all for me.

The Kotlin way to handle the INSTANCE singleton would be like this:

fun getDatabase(context: Context): CrosswordDatabase {
    return INSTANCE ?: Room
            .databaseBuilder(context.applicationContext, CrosswordDatabase::class.java, "crossword_database")
            .addCallback(CALLBACK)
            .build()
}

@carstenhag Your code snippet of handling singleton is missing the critical part of assigning the newly created object to the INSTANCE variable. Perhaps you could write .also {INSTANCE = it} at the end.

Can I use INSTANCE in callback? Because I want to put some initial data which has to be added only once. If yes, is there any possibility to implement such with Dagger Hilt?

if you meant like pre-populating your db, for sure you can do that in here. i am not sure about Dagger Hilt, though.

Thanks a lot! i was looking for that solution :D

private val CALLBACK = object : RoomDatabase.Callback() {
            override fun onCreate(db: SupportSQLiteDatabase) {
                super.onCreate(db)

                db.execSQL("CREATE TRIGGER ...")
            }
        }