Room-Datenbank

Was ist Room?

Die drei Komponenten von Room

Room-Abhängigkeiten hinzufügen

// app/build.gradle
plugins {
    id 'kotlin-kapt'  // needed for Room annotation processor
}

dependencies {
    val roomVersion = "2.6.1"
    implementation "androidx.room:room-runtime:$roomVersion"
    kapt "androidx.room:room-compiler:$roomVersion"
    implementation "androidx.room:room-ktx:$roomVersion" // coroutine support
}

Eine Entity definieren

import androidx.room.Entity
import androidx.room.PrimaryKey

@Entity(tableName = "notes")
data class Note(
    @PrimaryKey(autoGenerate = true)
    val id: Int = 0,
    val title: String,
    val body: String,
    val createdAt: Long = System.currentTimeMillis()
)

// Each field = one column in the database table
// @PrimaryKey autoGenerate = Room assigns the id automatically

DAO erstellen

import androidx.room.*
import kotlinx.coroutines.flow.Flow

@Dao
interface NoteDao {

    @Query("SELECT * FROM notes ORDER BY createdAt DESC")
    fun getAllNotes(): Flow<List<Note>>  // emits new list whenever data changes

    @Query("SELECT * FROM notes WHERE id = :noteId")
    suspend fun getNoteById(noteId: Int): Note?

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insert(note: Note)

    @Update
    suspend fun update(note: Note)

    @Delete
    suspend fun delete(note: Note)
}

Datenbank erstellen

import androidx.room.Database
import androidx.room.Room
import androidx.room.RoomDatabase
import android.content.Context

@Database(entities = [Note::class], version = 1, exportSchema = false)
abstract class NoteDatabase : RoomDatabase() {

    abstract fun noteDao(): NoteDao

    companion object {
        @Volatile private var INSTANCE: NoteDatabase? = null

        fun getInstance(context: Context): NoteDatabase {
            return INSTANCE ?: synchronized(this) {
                Room.databaseBuilder(
                    context.applicationContext,
                    NoteDatabase::class.java,
                    "note_database"
                ).build().also { INSTANCE = it }
            }
        }
    }
}

Room im ViewModel verwenden

import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import kotlinx.coroutines.launch

class NoteViewModel(private val dao: NoteDao) : ViewModel() {

    val notes = dao.getAllNotes()  // Flow — auto-updates when DB changes

    fun addNote(title: String, body: String) {
        viewModelScope.launch {
            dao.insert(Note(title = title, body = body))
        }
    }

    fun deleteNote(note: Note) {
        viewModelScope.launch {
            dao.delete(note)
        }
    }
}

Room-Daten in der UI beobachten

import androidx.lifecycle.lifecycleScope
import kotlinx.coroutines.launch

class NoteActivity : AppCompatActivity() {
    private val viewModel: NoteViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // ...

        lifecycleScope.launch {
            viewModel.notes.collect { notes ->
                adapter.submitList(notes)  // update RecyclerView
            }
        }
    }
}

Datenbankmigrationen

Schnelltest

Zusammenfassung: Room-Datenbank