Kotlin使用Room数据库

1.导入依赖

  //room database
    implementation 'androidx.room:room-runtime:2.2.3'
    annotationProcessor 'androidx.room:room-compiler:2.2.3'
    kapt 'androidx.room:room-compiler:2.2.3'
    androidTestImplementation 'androidx.room:room-testing:2.2.3'

2.gradle设置，在在顶部添加

apply plugin: 'kotlin-kapt'

defaultConfig括号内添加

           javaCompileOptions {
                annotationProcessorOptions {
                    arguments = [
                            "room.schemaLocation"  : "$projectDir/schemas".toString(),
                            "room.incremental"     : "true",
                            "room.expandProjection": "true"]
                }
            }

3.新建实体类，就是建数据库表

@Entity(tableName = "worker" ,primaryKeys = arrayOf("id"))
data class Worker(
    @ColumnInfo(name = "id")
    var id: Long?,
    @ColumnInfo(name = "name")
    var name:String?
    )

4.新建Dao类，就是映射数据库操作到方法中（个人理解）

@Dao
interface WorkerDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun insertWorker(worker: Worker)

    @Query("DELETE FROM worker WHERE name=:name")
    fun deleteWorker(name: String)

    @Query("SELECT * FROM worker")
    fun queryAllWorkers(): List<Worker>

    @Query("SELECT * FROM worker WHERE id = :id")
    fun getWorkerById(id: Long): Worker
}

5.新建KotDatabase类，生成数据库

@Database(entities = [Worker::class], version = 1)
abstract class KotDatabase : RoomDatabase() {
    abstract fun getWorkerDao(): WorkerDao

    companion object {
        @Volatile
        private var INSTANCE: KotDatabase? = null
        fun getInstance(context: Context): KotDatabase =
            INSTANCE ?: synchronized(this) {
                INSTANCE ?: buildDatabase(context).also { INSTANCE = it }
            }
        private fun buildDatabase(context: Context) =
            Room.databaseBuilder(context.applicationContext, KotDatabase::class.java, "kot.db").allowMainThreadQueries()
                .build()
    }
}

这里啰嗦一句，如果不加上.allowMainThreadQueries()，你只能在子线程中操作，不能在UI主线程里操作，不然报错闪退
6.insert举例操作

  Thread{
               val worker = Worker(0,"李华")
               KotDatabase.getInstance(applicationContext).getWorkerDao().insertWorker(worker)
               val worker2 = Worker(11,"luoyanda")
               KotDatabase.getInstance(applicationContext).getWorkerDao().insertWorker(worker2)
               var workerM = KotDatabase.getInstance(applicationContext).getWorkerDao().getWorkerById(11);
               var message:Message = Message.obtain()
               message.what = 100
               message.obj = workerM.name
               myHandler.sendMessage(message);
          }.start()

7.读写内存权限加上。

   if(ContextCompat.checkSelfPermission(applicationContext,android.Manifest.permission.WRITE_EXTERNAL_STORAGE)!=PackageManager.PERMISSION_GRANTED||
            ContextCompat.checkSelfPermission(applicationContext,android.Manifest.permission.READ_EXTERNAL_STORAGE)!=PackageManager.PERMISSION_GRANTED
                ){
             ActivityCompat.requestPermissions(this@MainActivity, arrayOf(android.Manifest.permission.WRITE_EXTERNAL_STORAGE,
                 android.Manifest.permission.READ_EXTERNAL_STORAGE),REQ_CODE_PERMISSION)

        }else{
   //todo
}
        。。。。
            override fun onRequestPermissionsResult(
        requestCode: Int,
        permissions: Array<out String>,
        grantResults: IntArray
    ) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults)
        when(requestCode){
            REQ_CODE_PERMISSION ->{
                if(grantResults.size>0&&
                    grantResults[0]==PackageManager.PERMISSION_GRANTED&&
                    grantResults[1] ==PackageManager.PERMISSION_GRANTED){
                    //todo
                }
            }
        }
    }

8.不用data class也行，不然有自增id时无法使用，这里使用普通的class也许，构造器里不添加自增id就行，kotlin会自动曾家get/set方法的。

@Entity(tableName = "student")
class Student{
    @PrimaryKey(autoGenerate = true)
    var id: Long = 0   //不影响第一个元素为1，后面自增
    var firstName: String? = null
    var lastName: String? = null
    var test:String? = null

    constructor(firstName: String?, lastName: String?, test: String?) {
        this.firstName = firstName
        this.lastName = lastName
        this.test = test
    }

}

8.迁移，由于要改数据库表中的字段，比如之前没有test变量，在这里增加，保证之前的数据不会丢失。

//这里的version由1改为2，狠！狠！狠！重要，表明现在是2代数据库，如果后面再改以此为依据
@Database(entities = [Worker::class,Student::class], version = 2)
abstract class KotDatabase : RoomDatabase() {
    abstract fun getWorkerDao(): WorkerDao
   abstract  fun getStudentDao():StudentDao
    companion object {
        @Volatile
        private var INSTANCE: KotDatabase? = null
        fun getInstance(context: Context): KotDatabase =
            INSTANCE ?: synchronized(this) {
                INSTANCE ?: buildDatabase(context).also { INSTANCE = it }//不空就执行方括号里的代码
            }
        var dd: Migration = object : Migration(1,2){                 //由一代往二代迁移要做的操作
            override fun migrate(database: SupportSQLiteDatabase) {
                database.execSQL("ALTER TABLE student ADD COLUMN test TEXT default 'test' ")
            }
        }
        //调用迁移
        private fun buildDatabase(context: Context) =
            Room.databaseBuilder(context.applicationContext, KotDatabase::class.java, "kot.db").addMigrations(
                dd).allowMainThreadQueries()
                .build()
    }
}