[移动开发]Android Jetpack架构组件之Room

Room

https://developer.android.google.cn/topic/libraries/architecture/room

Room重要概念:
Entity:实体类，对应的是数据库的一张表结构，使用注解@Entity标记>
Dao:包含访问一系列访问数据库的方法，使用注解@Dao标记。//Data Access Objects
Database:数据库持有者，作为与应用持久化相关数据的底层连接的主要接入点。使用注解@Database标记，另外需满足以下条件:定义的类必须是一个继承于RoomDatabase的抽象类，在注解中需要定义与数据库相关联的实体类列表。包含一个没有参数的抽象方法并且返回一个Dao对象。

app从Database得到get DAO,从DAO得到Entity，从Entity得到和设置对象值

implementation 'androidx.room: room-runtime: 2.2.5'
//注解器：抽象类自动生成实现类
annotationProcessor 'androidx .room:room-compiler:2.2.5'

Entity

	@Entity (tableName - "student")
    public class Student {

        @PrimaryKey (autoGenerate = true)
        @ColumnInfo(name = "id",typeAffinity = ColumnInfo.INTEGER)
        public int id;
        

        @ColumnInfo(name = "name", typeAffinity = ColumnInfo.TEXT)
        public string name;

        @columnInfo(name = "name",typeAffinity = ColumnInfo.TEXT)
        public int age;
        
        public student (int id,String name, int age){
			this.id = id;
			this.name = name;
            this.age = age;
        }
       
    // Ignore注解 room会自动忽略此构造函数      
	@Ignores
	public student (string name, int age) {
		this.name = name;
		this.age = age;
	}

Dao：增删查改接口类

@Dao
public interface StudentDao {

    @Insert
	void insertStudent (student. .- students) ;

    @Delete
	void deletestudent (student. . . students);

    @Update
	void updateStudent (Student . . . students) ;

    @Query ( "SELECT *FROMstudent")
	List<Student> getAllstudent ();

    @Query ("SELECT * FROM student WHERE id = :id")
    List<Student> getStudentById(int id);
}

Database

@Database (entities = {Student.class}, version = 1, exportSchema = false)
public abstract class MyDatabase extends RoomDatabase {

    private static final string DATABASE_NAME= "my_db. db"
    private static MyDatabase mInstance;

    public static synchronized MyDatabase getInstance (Context context) {
		if (mInstance == nul1) {
			mInstance = Room. databaseBuilder(context.getApplicationContext(),
			MyDatabase.class,
			DATABASE_NAME').build() ;
		}
		return minstance;
	}
	public abstract StudentDao getStudentDao ();
}

使用

MyDatabase database = MyDatabase.getInstance (this);
studentDao = database.getStudentDao ( ) ;


public void mlnsert (View view){
	student s1 = new Student ( name: "Jack" , age: 20) ;
    Student s2 = new Student( name: "Rose",age: 1 ;new 		
    InsertStudentTask (studentDao).execute (s1);
}

//room不允许在主线程执行数据库操作
class InsertStudentTask extends AsyncTask<Student, Void,void> {
	private studentDao studentDao;
	
    public InsertStudentTask (StudentDao studentDao){
		this . studentDao - studentDao;
	}

    @override
	protected Void doInBackground (Student. . . students) {
		studentDao.insertStudent ( students) ;
        rerurn null;
    }
}

进一步优化

问题:每当数据库数据发生变化时，都需要开启一个工作线程去重新获取数据库中的数据。
解决:当数据发生变化时，通过LiveData通知View层，实现数据自动更新。

Room+ViewModel+LiveData

//Repository:通过Repository操作数据库
public class studentRepository {
	private StudentDao studentDao;
    
	public StudentRepository (Context context){
		MyDatabase database = MyDatabase.getInstance ( context) ;
        this.studentDao = database.getStudentDao ( ) ;
	}
    
    public void insertStudent (Student. . . students){
		new InsertStudentTask(studentDao) .execute (students);
    }

    class InsertStudentTask extends AsyncTask<Student, Void,Void> {
		private studentDao studentDao;
		public InsertStudentTask(StudentDao studentDao) { 
            this.studentDa
			
            @override
			protected Void doInBackground (Student. . . students) {
				studentDao.insertstudent (students) ;
				return nul1;
			}
        }
}

//ViewModel:ViewModel传递上下文对象需要用AndroidViewModel
//ViewModel保存数据
public class studentViewModel extends AndroidViewModel {
	private studentRepository repository ;
    
	public StudentViewModel ( @NonNull Application application) {
		super(application) ;
		this.repository = new StudentRepository(application) ;
	}
    
	public void insertStudent (Student. . . students) {
		repository.insertStudent(students) ;
    }
}

//Activity只需要和ViewModel交互即可
studentViewModel = new ViewModelProvider ( owner: this, new
ViewModelProvider.AndroidviewModelFactory(getApplication()).get(StudentViewModel.class)
//Livedata自动更新
studentViewModel.getAllStudentsLive () .observe ( owner: this，new Observer<List<Student>>()
	@override
	public void onChanged (List<Student> students){
		adapter.setstudents ( students ) ;
		adapter.notifyDataSetChanged ( );
	});
 }
                                          

public void mInsert (View view) {
	Student s1 = new student ( name: "Jack", age: 20) ;
    Student s2 = new Student ( name: "Rose", age: 18);
    studentviewModel .insertStudent (s1,s2) ;
}

room版本升级

1.构造Migration
static final Migration MIGRATION_1_2 = new Migration (1,2){
	@override
	public void migrate (@NonNull SupportSQLiteDatabase database){
		database.execSQL ( "ALTERTABLE student ADD COLUMN sex INTEGER NOT NULLDEFAULT 1" );
};

2.添加addMigrations
public static synchronized MyDatabase getInstance (Context context){
	if ( mInstance -= nul1){
		mInstance = Room. databaseBuilder(context.getapplicationContext(),	
                                          MyDatabase.class,
                                          DATABASE_NAME)
	.addMigrations (MIGRATION_1_2,MIGRATION_2_3)
            .build () ;
}
return mlnstance;
}
    
3.修改注解
@Database (entities = {Student.class}, version = 2, exportSchema = false)

使用Migration升级数据库

问题:如果用户设备上数据库版本为1，而当前要安装的App数据库版本为3，怎么办?
Room会先判断当前有没有直接从1到3的升级方案，如果有，就直接执行从1到3的升级方案，如果没有，那么Room会按照顺序先后执行Migration(1,2)、Migration(2,3)以完成升级。

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