Level 12

Local Persistence (Room + Flow)

Room: Entities, DAO & Reactive Queries

Room is the recommended SQLite abstraction. Define an `@Entity` (table), a `@Dao` (queries), and a `@Database`. Returning `Flow` from a query makes the UI update automatically whenever the data changes.

📚 Explanation

Room generates the SQLite boilerplate at compile time and verifies your SQL. `@Entity` maps to a table, `@Dao` declares typed queries, and `@Database` ties them together. Writes use `suspend` functions (off the main thread); reads that return `Flow` are observable — Room re-runs the query and emits a new list whenever the table changes, so your Compose UI stays in sync automatically.

Note: Code execution is not available for Android/Kotlin in the browser. Use Android Studio to run and test the app.

kotlin

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

// 1. The table
@Entity(tableName = "notes")
data class NoteEntity(
    @PrimaryKey(autoGenerate = true) val id: Long = 0,
    val title: String,
    val body: String,
    val updatedAt: Long
)

// 2. The queries (suspend for writes, Flow for reactive reads)
@Dao
interface NoteDao {
    @Query("SELECT * FROM notes ORDER BY updatedAt DESC")
    fun observeNotes(): Flow<List<NoteEntity>> // emits on every change

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun upsert(note: NoteEntity)

    @Delete
    suspend fun delete(note: NoteEntity)
}

// 3. The database
@Database(entities = [NoteEntity::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
    abstract fun noteDao(): NoteDao
}

// 4. Consuming reactively in a ViewModel
class NotesViewModel(dao: NoteDao) : ViewModel() {
    val notes = dao.observeNotes()
        .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), emptyList())
}

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Local Persistence (Room + Flow)

Room: Entities, DAO & Reactive Queries

📚 Explanation

Note: Code execution is not available for Android/Kotlin in the browser. Use Android Studio to run and test the app.

import androidx.room.*

import kotlinx.coroutines.flow.Flow

// 1. The table

@Entity(tableName = "notes")

data class NoteEntity(

@PrimaryKey(autoGenerate = true) val id: Long = 0,

val title: String,

val body: String,

val updatedAt: Long

)

// 2. The queries (suspend for writes, Flow for reactive reads)

@Dao

interface NoteDao {

@Query("SELECT * FROM notes ORDER BY updatedAt DESC")

fun observeNotes(): Flow<List<NoteEntity>> // emits on every change

@Insert(onConflict = OnConflictStrategy.REPLACE)

suspend fun upsert(note: NoteEntity)

@Delete

suspend fun delete(note: NoteEntity)

}

// 3. The database

@Database(entities = [NoteEntity::class], version = 1)

abstract class AppDatabase : RoomDatabase() {

abstract fun noteDao(): NoteDao

}

// 4. Consuming reactively in a ViewModel

class NotesViewModel(dao: NoteDao) : ViewModel() {

val notes = dao.observeNotes()

.stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), emptyList())

}