目录

1.导入依赖库

2.数据实体类

3.数据访问对象 (DAO)

4.数据库类

5.调用DAO里面的“增、删、改、查”方法

6.数据库升降级处理

升级（保存数据库历史数据）：

升级（不保存数据库历史数据）：

降级（不保存数据库历史数据）：

---

1.导入依赖库

kapt "androidx.room:room-compiler:2.2.5"
api("androidx.room:room-ktx:2.2.5"

2.数据实体类

@Entity(tableName = "areas", indices = [Index(value = ["id"], unique = true)])
data class Area(@PrimaryKey(autoGenerate = true)val id: Int,/*国家*/val country: String? = "",/*省、州*/val state: String? = "",/*城市*/val city: String? = "",/*省、州代码*/val stateCode: String? = "",/*城市代码*/val cityCode: String? = "",/*国家代码*/val countryCode: String? = "",/*新增时间*/val addTime: Long? = 0L,/*更新时间*/val updateTime: Long? = 0L,/*新增时间字符串*/val addTimeStr: String? = "",/*更新时间字符串*/val updateTimeStr: String? = "",val isDelete: Int,/*语言*/val language: String
)

3.数据访问对象 (DAO)

@Dao
interface AreaDao {/*** 获取所有国家列表（以国家ID分组）*/@Query("SELECT * FROM areas WHERE language=:language GROUP BY countryCode")fun getCountryList(language: String): List<Area>/*** 获取所有州/省列表*/@Query("SELECT * FROM areas WHERE countryCode=:countryCode AND language=:language GROUP BY stateCode")fun getStateList(countryCode: String, language: String): List<Area>/*** 获取所有城市列表*/@Query("SELECT * FROM areas WHERE countryCode=:countryCode AND stateCode=:stateCode AND language=:language GROUP BY cityCode")fun getCityList(countryCode: String, stateCode: String, language: String): List<Area>/*** 获取所有城市列表*/@Query("SELECT * FROM areas WHERE countryCode=:countryCode AND language=:language GROUP BY cityCode")fun getCityList(countryCode: String, language: String): List<Area>/*** 批量插入地区记录*/@Insert(onConflict = OnConflictStrategy.REPLACE)fun insertAreas(tunnels: List<Area>): List<Long>/*** 删除所有地区记录*/@Query("DELETE FROM areas")fun deleteAllArea()
}

4.数据库类

@Database(entities = [Area::class, AlarmClock::class, DeviceInfo::class],version = 4, exportSchema = false
)
abstract class FlowFitDatabase : RoomDatabase() {abstract fun areaDao(): AreaDaoabstract fun alarmClockDao(): AlarmClockDaoabstract fun deviceInfoDao(): DeviceInfoDaocompanion object {/*单例模式*/@Volatileprivate var instance: FlowFitDatabase? = nullfun getInstance(context: Context): FlowFitDatabase {return instance ?: synchronized(this) {instance ?: buildDatabase(context).also { instance = it }}}/*创建并填充数据库*/private fun buildDatabase(context: Context): FlowFitDatabase {return Room.databaseBuilder(context,FlowFitDatabase::class.java,FLOW_FIT_DATABASE_NAME).addMigrations(object : Migration(1, 2) {override fun migrate(database: SupportSQLiteDatabase) {database.execSQL("drop table IF EXISTS  StepTimeSharing ")database.execSQL("CREATE TABLE IF NOT EXISTS \"StepTimeSharing\" (   " +"   \"date\" INTEGER NOT NULL,   " +"   \"calories\" INTEGER NOT NULL,   " +"   \"distances\" INTEGER NOT NULL,   " +"   \"steps\" INTEGER NOT NULL,   " +"   PRIMARY KEY (\"date\") " +");")}}).addCallback(object : RoomDatabase.Callback() {override fun onCreate(db: SupportSQLiteDatabase) {super.onCreate(db)GlobalScope.launch(Dispatchers.IO) {val database: FlowFitDatabase = getInstance(context)/*初始化闹钟提醒记录*/database.alarmClockDao().insertAll(listOf(AlarmClock(id = 0,time = 0L,repeat = 0,isOpen = 0,hour = 0,minute = 0,label = 0,isShow = 0)))}}override fun onOpen(db: SupportSQLiteDatabase) {super.onOpen(db)}}).fallbackToDestructiveMigrationOnDowngrade()//降级的时候，当未匹配到版本的时候就会直接删除表然后重新创建
//                .fallbackToDestructiveMigration() //升级,清空数据库.build()}}
}

5.调用DAO里面的“增、删、改、查”方法

//获取所有国家列表
FlowFitDatabase.getInstance(CONTEXT).areaDao().getCountryList(language)

6.数据库升降级处理

处理代码已包含在步骤4中：

升级（保存数据库历史数据）：

1.修改version版本号

2.添加升级适配代码

升级（不保存数据库历史数据）：

.fallbackToDestructiveMigration() //升级,清空数据库

降级（不保存数据库历史数据）：

.fallbackToDestructiveMigrationOnDowngrade()//降级的时候，当未匹配到版本的时候就会直接删除表然后重新创建