Room学习笔记

Room是android Architecture Components中简化数据库读写的组件

集成方法：

implementation "android.arch.persistence.room:runtime:1.0.0-rc1"
annotationProcessor "android.arch.persistence.room:compiler:1.0.0-rc1"
testImplementation "android.arch.persistence.room:testing:1.0.0-rc1"
implementation "android.arch.persistence.room:rxjava2:1.0.0-rc1"

Room分为三部分

• DataBase: 代表一个数据库，提供抽象方法访问Dao对象。通过Room.databaseBuilder或者Room.inMemoryDatabaseBuilder创建。
• Entity：每个entity类代表一个table，每个实例代表一行
• Dao: 数据访问接口，增删查Entity的接口

Database

@Database(version = 1, entities = {Entity.class})

mark一个类是Database类，提供dao的获取方法，entities指定有哪些表

Entity

 @Entity
 public class User {
   @PrimaryKey
   private int uid;
   private String name;
   @ColumnInfo(name = "last_name")
   private String lastName;
   // getters and setters are ignored for brevity but they are required for Room to work.
 }

@Entity(tableName="", indices={@Index("name")}, foreignKeys={}, primaryKeys="", inheritSuperIndices="")

Entity的构造函数要么或者构造函数的参数对应到field

tranisent或者@Ignore修饰的field不会持久化

至少要有一个@PrimaryKey的field 或者Entity.primaryKeys()定义主键

以下注解可以修饰field

@PrimaryKey(autoGenerate=true|false)

@ColumnInfo(index=true|false, collate= UNSPECIFIED|BINARY|NOCASE|RTRIM, name="", typeAffinity= UNDEFINED|TEXT|INTEGER|REAL|BLOB)

构造表示时列的一些属性

@Embedded(prefix="")

如果field是另外一个class，那么这个class的属性会放到主类中

   public class Coordinates {
       double latitude;
       double longitude;
   }
   public class Address {
       String street;
       @Embedded
       Coordinates coordinates;
   }

@Relation(entity = class, entityColumn = "", parentColumn = "", projection = "")

定义一到多的关系，修饰的field需要是List或者set。用于自动获取关联的relation

 @Entity
 public class Pet {
     @ PrimaryKey
     int id;
     int userId;
     String name;
     // other fields
 }
 public class UserNameAndAllPets {
   public int id;
   public String name;
   @Relation(parentColumn = "id", entityColumn = "userId")
   public List<Pet> pets;
 }

DAO

DAO可以是接口或是抽象类，提供一系列insert，udpate或者delete的函数

@Insert(onConflict=ABORT|FAIL|IGNORE|REPLACE|ROLLBACK)

将传入的对象在一个transaction中插入数据库

@Dao
public interface MyDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    public void insertUsers(User... users);

    @Insert
    public void insertBothUsers(User user1, User user2);

    @Insert
    public void insertUsersAndFriends(User user, List<User> friends);
}

@Update(onConflict = ABORT|FAIL|IGNORE|REPLACE|ROLLBACK)

根据传入对象的primaryKey寻找数据库中的行，修改对象所在行

@Dao
public interface MyDao {
    @Update
    public void updateUsers(User... users);
}

@Delete

删除传入对象所在行

@Dao
public interface MyDao {
    @Delete
    public void deleteUsers(User... users);
}

@Query

运行一段query，可以是SELECT，UPDATE，DELETE

通过:name来在query中引用方法参数

用返回的column来构建返回对象

@Dao
public interface MyDao {
    @Query("SELECT * FROM user")
    public User[] loadAllUsers();
}

TypeConverter

当有自定义类文件要转化成Room可以持久化的数据类型时，需要定义TypeConverter

public class Converters {
    @TypeConverter
    public static Date fromTimestamp(Long value) {
        return value == null ? null : new Date(value);
    }

    @TypeConverter
    public static Long dateToTimestamp(Date date) {
        return date == null ? null : date.getTime();
    }
}

然后在Database类上指定TypeConverter定义的类

@Database(entities = {User.class}, version = 1)
@TypeConverters({Converters.class})
public abstract class AppDatabase extends RoomDatabase {
    public abstract UserDao userDao();
}

Migration

用于指定从低version升级到高version时，进行的数据库操作

Room.databaseBuilder(getApplicationContext(), MyDb.class, "database-name")
        .addMigrations(MIGRATION_1_2, MIGRATION_2_3).build();

static final Migration MIGRATION_1_2 = new Migration(1, 2) {
    @Override
    public void migrate(SupportSQLiteDatabase database) {
        database.execSQL("CREATE TABLE `Fruit` (`id` INTEGER, "
                + "`name` TEXT, PRIMARY KEY(`id`))");
    }
};

static final Migration MIGRATION_2_3 = new Migration(2, 3) {
    @Override
    public void migrate(SupportSQLiteDatabase database) {
        database.execSQL("ALTER TABLE Book "
                + " ADD COLUMN pub_year INTEGER");
    }
};