[android] jetpack Room 持久性库，数据库操作如此简单

三个类搞定数据操作 @Database、@Entity、@Dao
添加依赖

plugins {
    id 'com.android.application'
    id 'kotlin-android'
    id 'kotlin-android-extensions'
    id 'kotlin-kapt'
}

  def roomVersion = "2.3.0"

    implementation("androidx.room:room-runtime:$roomVersion")
    annotationProcessor("androidx.room:room-compiler:$roomVersion")
    kapt("androidx.room:room-compiler:$roomVersion")

创建实体类 用@Entity注解

@Entity
class Student() {

    //主键，自动增长
    @PrimaryKey(autoGenerate = true)
    var id: Int = 0

    @ColumnInfo(name = "name")//可以加别名，优先级更高，如果加了就是用别名的名称
    lateinit var name: String

    @ColumnInfo(name = "age")
    var age: Int = 0

    constructor(name: String,age: Int): this(){
        this.name = name
        this.age = age
    }

}

创建访问数据库的接口类 使用@Dao注解

@Dao
interface StudentDao {

    //vararg 可变参数
    //void a(Student... args) vs vararg students: Student
    @Insert
    fun insertStudents(vararg students: Student)

    @Update
    fun updateStudents(vararg students: Student)

    //删除全部
    @Query("DELETE FROM student")
    fun deleteAllStudents()

    //删除
    @Delete
    fun deleteStudents(vararg students: Student)

    @Query("SELECT * FROM student ORDER BY ID DESC")
    fun queryAllStudents(): List<Student>
}

数据库 增删改查操作，使用@Database注解

@Database(entities = [Student::class], version = 1)
abstract class StudentDatabase: RoomDatabase() {

    abstract fun getStudentDao() : StudentDao


    companion object{

        private var INSTANCE : StudentDatabase? = null
        // Application 这个函数给Application 用
        fun getDatabase(context: Context): StudentDatabase?{
            if(INSTANCE == null){
                INSTANCE = Room.databaseBuilder(context,StudentDatabase::class.java,"student_database.db")
                    .allowMainThreadQueries()//允许在主线程运行
                    .build()
            }
            return INSTANCE
        }
        //项目中使用这个方法
        fun getDatabase(): StudentDatabase? = INSTANCE

    }

}