Android - Kotlin - Room

data class ListItem (
    val title: String,
    val subtitle: String
)

class MainListAdapter : RecyclerView.Adapter<MainListAdapter.ViewHolder>() {
    private var list: List<ListItem> = emptyList()

    class ViewHolder(view: View): RecyclerView.ViewHolder(view)

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MainListAdapter.ViewHolder {
        return ViewHolder(
            LayoutInflater
                .from(parent.context)
                .inflate(R.layout.list_main, parent, false)
        )
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        holder.itemView.title.text = list[position].title
        holder.itemView.subtitle.text = list[position].subtitle
        holder.itemView.setOnClickListener {
            println("Clicked on position $position")
        }
        println("Recycling view - $position")
    }

    override fun getItemCount(): Int {
        return list.size
    }

    internal fun setListItems(items: List<ListItem>) {
        this.list = items
        notifyDataSetChanged()
    }
}

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

@Entity(tableName = "names_table")
class Name(
    @PrimaryKey(autoGenerate = true) val id: Int? = null,
    @ColumnInfo(name = "name") val name: String
)

import androidx.lifecycle.LiveData
import androidx.room.Dao
import androidx.room.Insert
import androidx.room.OnConflictStrategy
import androidx.room.Query

@Dao
interface NameDao {
    @Query("SELECT * from names_table ORDER BY name ASC")
    fun getAlphabetizedNames(): LiveData<List<Name>>

    @Insert(onConflict = OnConflictStrategy.IGNORE)
    suspend fun insert(name: Name)

    @Query("DELETE FROM names_table")
    suspend fun deleteAll()
}

import androidx.lifecycle.LiveData

class NameRepository(private val nameDao: NameDao) {

    val allNames: LiveData<List<Name>> = nameDao.getAlphabetizedNames()

    suspend fun insert(name: Name) {
        nameDao.insert(name)
    }
}

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

@Database(entities = [Name::class], version = 1, exportSchema = false)
public abstract class NameRoomDatabase : RoomDatabase() {

    abstract fun nameDao(): NameDao

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

        fun getDatabase(context: Context): NameRoomDatabase {
            val tempInstance = INSTANCE
            if (tempInstance != null) {
                return tempInstance
            }
            synchronized(this) {
                val instance = Room.databaseBuilder(
                    context.applicationContext,
                    NameRoomDatabase::class.java,
                    "name_database"
                ).build()
                INSTANCE = instance
                return instance
            }
        }
    }
}

import android.app.Application
import androidx.lifecycle.AndroidViewModel
import androidx.lifecycle.LiveData
import androidx.lifecycle.viewModelScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.launch

class NameViewModel(application: Application) : AndroidViewModel(application) {

    private val repository: NameRepository
    val allNames: LiveData<List<Name>>

    init {
        val namesDao = NameRoomDatabase.getDatabase(application).nameDao()
        repository = NameRepository(namesDao)
        allNames = repository.allNames
    }

    fun insert(name: Name) = viewModelScope.launch(Dispatchers.IO) {
        repository.insert(name)
    }
}

class MainActivity : AppCompatActivity() {
    private lateinit var nameViewModel: NameViewModel
    private val newNameActivityRequestCode = 1

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val listAdapter = MainListAdapter()
        listView.adapter = listAdapter

        nameViewModel = ViewModelProvider(this).get(NameViewModel::class.java)
        nameViewModel.allNames.observe(this, Observer { names ->
            names?.let { listAdapter.setListItems(it.map { ListItem(it.name, "ID: ${it.id}") }) }
        })

        fab.setOnClickListener {
            startActivityForResult(
                Intent(this@MainActivity, AddItemActivity::class.java),
                newNameActivityRequestCode)
        }
    }

    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        val text = data?.getStringExtra(AddItemActivity.EXTRA_REPLY)
        if (newNameActivityRequestCode == requestCode && resultCode == Activity.RESULT_OK && text != null) {
            nameViewModel.insert(Name(null, text!!))
        } else {
            Toast.makeText(applicationContext, R.string.save_error, Toast.LENGTH_LONG)
        }
    }
}

class AddItemActivity: AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_additem)
        saveButton.setOnClickListener {
            val replyIntent = Intent()
            if (TextUtils.isEmpty(editTextWord.text)) {
                setResult(Activity.RESULT_CANCELED, replyIntent)
            } else {
                replyIntent.putExtra(EXTRA_REPLY, editTextWord.text.toString())
                setResult(Activity.RESULT_OK, replyIntent)
            }
            finish()
        }
    }

    companion object {
        const val EXTRA_REPLY = "com.sagar_r_kothari.mycardsholder.AdditemActivity.Reply"
    }
}