Flow

class BusScheduleViewModel(private val scheduleDao: ScheduleDao): ViewModel() {
    fun fullSchedule(): Flow<List<Schedule>> = scheduleDao.getAll()
}

lifecycleScope.launch(Dispatchers.IO) {
    viewModel.fullSchedule().collect {
        busStopAdapter.submitList(it)
    }
}

ViewModel define

class MyViewModel(val dao: MyDao): ViewModel()

// use viewmodel
private val viewModel: MyViewModel by viewModels()

Room Database

@Database(entities = [Schedule::class], version = 1)
abstract class AppDatabase: RoomDatabase() {

    abstract fun scheduleDao(): ScheduleDao

    companion object {

        @Volatile
        private var INSTANCE: AppDatabase? = null

        fun getDatabase(context: Context): AppDatabase {
            return INSTANCE ?: synchronized(this) {
                val instance =
                    Room.databaseBuilder(context, AppDatabase::class.java, "app_database")
                        .createFromAsset("database/bus_schedule.db")
                        .build()

                INSTANCE = instance
                instance
            }
        }
    }
}

// use database
val database: AppDatabase by lazy { AppDatabase.getDatabase(this) }

Room DAO

Flow를 사용하기 위해 return 타입을 Flow로 포장

@Dao
interface ScheduleDao {
    @Query("SELECT * FROM schedule ORDER BY arrival_time ASC")
    fun getAll(): Flow<List<Schedule>> 

    @Query("SELECT * FROM schedule WHERE stop_name = :stopName ORDER BY arrival_time ASC")
    fun getByStopName(stopName: String): Flow<List<Schedule>>
}

Room entry

@Entity
data class Schedule(
   @PrimaryKey val id: Int,
   @NonNull @ColumnInfo(name = "stop_name") val stopName: String,
   @NonNull @ColumnInfo(name = "arrival_time") val arrivalTime: Int
)

Bus Scheduler App

This folder contains the source code for the Bus Scheduler app codelab.
https://developer.android.com/codelabs/basic-android-kotlin-training-intro-room-flow

Introduction

The Bus Scheduler app displays a list of bus stops and arrival times. Tapping a bus stop on the first screen will display a list of all arrival times for that particular stop.

The bus stops are stored in a Room database. Schedule items are represented by the Schedule class and queries on the data table are made by the ScheduleDao class. The app includes a view model to access the ScheduleDao and format data to be display in a list, using Flow to send data to a recycler view adapter.

Pre-requisites

• Experience with Kotlin syntax.
• Familiarity with activities, fragments, and recycler views.
• Basic knowledge of SQL databases and performing basic queries.

Getting Started

1. Install Android Studio, if you don't already have it.
2. Download the sample.
3. Import the sample into Android Studio.
4. Build and run the sample.