【Kotlin/Android Studio】RoomDataBaseの使い方！Daoとは？

build.gradle(Module)
plugins {
    // 〜〜〜〜〜〜〜
    id 'kotlin-kapt'
}

android {
    
    // 〜〜〜〜〜〜〜

    tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile) {
        kotlinOptions {
            jvmTarget = "1.8"
        }
    }
}

dependencies {

    // 〜〜〜〜〜〜〜
    def room_version = "2.5.0"

    implementation "androidx.room:room-runtime:$room_version"
    implementation "androidx.room:room-ktx:$room_version"
    kapt "androidx.room:room-compiler:$room_version"

    implementation 'androidx.room:room-rxjava2:2.3.0'
    implementation 'io.reactivex.rxjava2:rxjava:2.2.4'
    implementation 'io.reactivex.rxjava2:rxkotlin:2.3.0'
    implementation 'io.reactivex.rxjava2:rxandroid:2.1.0'

}

User.kt
@Entity(tableName = "user_table")
data class User(
    @PrimaryKey(autoGenerate = true) val id: Int,
    val name: String,
    val age: Int,
    val hobby: String
)

@ColumnInfo(name = "first_name") val firstName: String?,
@ColumnInfo(name = "last_name") val lastName: String?

UserDao.kt
@Dao
interface UserDao {
    @Insert
    fun insert(user: User)

    @Query("SELECT * FROM user_table")
    fun getAll(): Flowable<List<User>>

    @Query("SELECT * FROM user_table WHERE id = :id")
    fun getLiveDataUser(id: Int): LiveData<User>

    @Query("DELETE FROM user_table")
    fun deleteAll()

}

UserDatabase.kt
@Database(entities = [User::class], version = 1, exportSchema = false)
abstract class UserDatabase : RoomDatabase() {
    abstract fun userDao(): UserDao

    companion object {
        @Volatile
        private var INSTANCE: UserDatabase? = null
        fun getDatabase(context: Context): UserDatabase {
            return INSTANCE ?: synchronized(this) {
                val instance = Room.databaseBuilder(
                    context.applicationContext,
                    UserDatabase::class.java,
                    "user_database"
                )
                    .fallbackToDestructiveMigration()
                    .build()
                INSTANCE = instance
                return instance
            }
        }
    }
}

@Database(entities = [User::class], version = 1, exportSchema = false)
abstract class UserDatabase : RoomDatabase() { ... }

abstract fun userDao(): UserDao

companion object { ... }

@Volatile
private var INSTANCE: UserDatabase? = null

fun getDatabase(context: Context): UserDatabase { 
  return INSTANCE ?: synchronized(this) { ... } 
}

val instance = Room.databaseBuilder(
    context.applicationContext,
    UserDatabase::class.java,
    "user_database"
)
    .fallbackToDestructiveMigration()
    .build()

MainActivity.kt
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Button
import android.widget.EditText
import android.widget.TextView
import androidx.recyclerview.widget.DividerItemDecoration
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView
import com.example.room.room.User
import com.example.room.room.UserDao
import com.example.room.room.UserDatabase
import io.reactivex.Completable
import io.reactivex.android.schedulers.AndroidSchedulers
import io.reactivex.disposables.CompositeDisposable
import io.reactivex.schedulers.Schedulers

class MainActivity : AppCompatActivity() {

    lateinit var db : UserDatabase
    lateinit var dao : UserDao

    private val compositeDisposable = CompositeDisposable()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val display:TextView = findViewById(R.id.display_text)
        val addButton:Button = findViewById(R.id.add_button)
        val getButton:Button = findViewById(R.id.get_button)
        val allDeleteButton:Button = findViewById(R.id.all_delete_button)
        val input:EditText = findViewById(R.id.input_text)

        db = UserDatabase.getDatabase(this)
        dao = db.userDao()

        addButton.setOnClickListener{
            if (input.text.toString() != "") {
                display.text = "入力したよ"
                insUser(input.text.toString())
            }else{
                display.text = "未入力です"
            }
        }

        getButton.setOnClickListener {
            display.text = "取得したよ"
            getUser(0) // 取得したいIDを渡す
        }

        allDeleteButton.setOnClickListener {
            display.text = "リセットしました"
            deleteAll()
        }

        subscribeAllUser()
    }

    override fun onDestroy() {
        super.onDestroy()
        compositeDisposable.clear()
    }

    private fun insUser(name: String) {
        val user = User(0, name, 20, "サーフィン")

        Completable.fromAction {
            dao.insert(user)
        }.subscribeOn(Schedulers.io())
            .subscribe()
            .addTo(compositeDisposable)
    }

    private fun getUser(id : Int){
        val display: TextView = findViewById(R.id.display_text)

        val userLiveData = dao.getLiveDataUser(id)
        userLiveData.observe(this, { user ->
            if (user != null) {
                display.text = user.name
            } else {
                display.text = "User not found"
            }
        })
    }

    private fun deleteAll(){
        Completable.fromAction { dao.deleteAll() }
            .subscribeOn(Schedulers.io())
            .subscribe()
            .addTo(compositeDisposable)
    }

    private fun subscribeAllUser(){
        val recyclerView: RecyclerView = findViewById(R.id.main_list)
        recyclerView.layoutManager = LinearLayoutManager(this)
        recyclerView.addItemDecoration(
            DividerItemDecoration(this, DividerItemDecoration.VERTICAL)
        )
        dao.getAll()
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(
                {
                    //データ取得完了時の処理
                    val data = ArrayList<User>()
                    it.forEach {
                            user -> data.add(user)
                    }
                    val adapter = MainAdapter(data)
                    recyclerView.adapter = adapter
                }
            ).addTo(compositeDisposable)
    }
}