Room

简单使用介绍

1. 导包 ```kotlin plugins { … id ‘kotlin-kapt’ }

def room_version = “2.3.0” implementation “androidx.room:room-runtime:$room_version” kapt “androidx.room:room-compiler:$room_version” implementation “androidx.room:room-ktx:$room_version” testImplementation “androidx.room:room-testing:$room_version”

2. 实体类
```kotlin
@Entity(tableName = "user")
class User2(
    @PrimaryKey(autoGenerate = true) var id: Long = 0,
    @ColumnInfo(name = "user_id")var userId: Long = 0,
    @ColumnInfo(name = "user_name") var userName: String,
    @ColumnInfo(defaultValue = "china") var address: String,
    @Ignore var sex: Boolean
) {
    //必须要有构造函数
    constructor(): this(0,0,"alice","china",false)
//    @PrimaryKey(autoGenerate = true)
//    var id: Long = 0
//    @ColumnInfo(name = "user_name")
//    var userName: String = ""
//    @ColumnInfo(defaultValue = "china")
//    var address: String = ""
//    @Ignore
//    var sex: Boolean = false
    override fun toString(): String {
        return "id = $id ,userid = $userId, userName = $userName , address = $address, sex = $sex"
    }
}

1. Dao 类 ```kotlin @Dao interface UserDao { @Query(“select * from user”) fun queryAll(): List

   @Query(“select * from user where id = :id”) fun queryById(id: Long): User2

   @Query(“select * from user”) fun queryAll3(): LiveData>

   @Insert(onConflict = OnConflictStrategy.REPLACE) fun addUser(user2: User2)

   @Delete fun delete(user2: User2)

   @Query(“delete from user where id = :id”) fun deleteById(id: Long)

   @Query(“delete from user”) fun deleteAll()

   @Update fun update(user2: User2)

   @Query(“update user set user_name = :updateName where id = :id”) fun updateOne(id: Long, updateName: String)

}

4. DataBase 类
```kotlin
@Database(entities = [User2::class], version = 2)
abstract class UserDatabase : RoomDatabase() {
    abstract fun userDao(): UserDao
    companion object {
        private var instance: UserDatabase? = null
        fun getInstance(context: Context): UserDatabase {
            if (instance == null) {
                instance = Room.databaseBuilder(context, UserDatabase::class.java, "user.db")
                    .addMigrations(object : Migration(1,2){//数据库升级
                        override fun migrate(database: SupportSQLiteDatabase) {
                        database.execSQL("ALTER TABLE user "
                                + " ADD COLUMN user_id Long NOT NULL DEFAULT 0")
                        }
                    })
                    .allowMainThreadQueries().build()
            }
            return instance!!
        }
    }
}

1. 使用示例

   class RoomActivity: AppCompatActivity(R.layout.activity_room) {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        //使用
        val userDao = UserDatabase.getInstance(this).userDao()
        btnRoomCreate.setOnClickListener {
            for (i in 0..9){
                val user = User2(userId = i.toLong(),userName = "name$i",address = "address$i",sex = true)
                userDao.addUser(user)
            }
        }
        btnRoomQuery.setOnClickListener {
            //简单查询所有
            val list = userDao.queryAll()
            list.forEach {
                Log.e(TAG, "onCreate: $it")
            }
            //使用 livedata 查询和使用 flow 效果一样
             //返回livedata,新增的瞬间立马就会监听到，不适合用在查询，适合用在监听上
            userDao.queryAll3().observe(this){
                Log.e(TAG, "live-onCreate: $it")
            }
        }
        btnRoomQuery2.setOnClickListener {
            val user = userDao.queryById(8)
            Log.e(TAG, "onCreate: $user")
        }
        btnRoomAdd.setOnClickListener {
            val user = User2(userName = "add",address = "add address",sex = true)
            userDao.addUser(user)
        }
        btnRoomUpdate.setOnClickListener {
            userDao.updateOne(8,"hello")
        }
        btnRoomUpdate2.setOnClickListener {
            val user = User2(id = 7,userName = "7777",address = "7777 address",sex = true)
            userDao.update(user)
        }
        btnRoomDelete.setOnClickListener {
            userDao.deleteAll()
        }
        btnRoomDelete2.setOnClickListener {
            userDao.deleteById(8)
        }
    }
   }

   参考

   SQL基本语句
   Android Jetpack架构组件 — Room入坑详解