Room是Android Jetpack中的一个组件，它提供了一个抽象层，帮助开发者在本地数据库（如SQLite）上进行持久化数据存储。Room通过简化数据库操作，使得数据管理变得更加容易和高效。

Room重要的概念

• 实体（Entity）：实体是用于表示数据库表的类。通过使用@Entity注解，可以定义一个实体类，并将其映射到数据库表。实体类中的属性对应于数据库表中的列。
• 数据库（Database）：数据库类是一个抽象类，用于定义数据库版本和包含的实体。通过使用@Database注解，可以创建一个继承自RoomDatabase的抽象类，并指定实体类和数据库版本。
• 数据访问对象（DAO）：DAO接口用于定义对数据库的访问操作。通过使用@Dao注解，可以定义一个接口，并在其中声明各种CRUD方法。这些方法会被转换为对应的SQL语句。
  

在Android开发中，使用DAO（Data Access Object）、Repository和ViewModel的架构模式是为了实现数据与界面的清晰分离，提高应用的可维护性和扩展性。

• DAO（Data Access Object）：DAO是负责直接与数据库交互的组件。它定义了操作数据库的方法，如查询、插入、更新和删除。通过使用DAO，开发者可以将数据库操作逻辑从业务逻辑中分离出来，使得代码更加模块化和易于管理。
• Repository：Repository层作为数据源和ViewModel之间的中介，负责处理数据请求和响应。它可以封装多个数据源，例如本地数据库和远程API，并提供一个统一的接口供ViewModel调用。这样，当数据源发生变化时，只需要修改Repository层，而不必触及ViewModel或View层，从而提高了代码的复用性和可维护性。
• ViewModel：ViewModel是一个用于存储和管理UI相关数据的架构组件。它的主要目的是将UI控制器（如Activity和Fragment）与数据相关的业务逻辑分开，使得UI控制器能够专注于展示数据和响应用户交互，而数据的获取和处理则交由ViewModel来管理。这种分离能够使代码更加清晰、易于测试和维护。

案例：Room + ViewModel + LiveData 实现操作

使用步骤：
1.引入依赖

    def room_version = "2.6.1"implementation "androidx.room:room-runtime:$room_version"annotationProcessor "androidx.room:room-compiler:$room_version"

2.定义实体

@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 = "age",typeAffinity = ColumnInfo.INTEGER)public int age;public Student(int id, int age, String name) {this.id = id;this.age = age;this.name = name;}@Ignorepublic Student(int age, String name) {this.age = age;this.name = name;}@Ignorepublic Student(int id) {this.id = id;}
}

3.定义Dao

@Dao
public interface StudentDao {@Insertvoid insert(Student... students);@Deletevoid delete(Student... students);@Updatevoid update(Student... students);@Query("select * from student")List<Student> getAllStudent();@Query("select * from student where id = :id")Student getStudentById(int id);
}

3.定义MyDatabase

@Database(entities = {Student.class}, version = 1,exportSchema = false)
public abstract class MyDatabase extends RoomDatabase {private static final String DB_NAME = "my_db.db";private static  MyDatabase mInstance;public static synchronized MyDatabase getInstance(Context context){if(mInstance == null){mInstance = Room.databaseBuilder(context.getApplicationContext(),MyDatabase.class,DB_NAME).build();}return mInstance;}public abstract StudentDao studentDao();
}

4.定义StudentRepository

public class StudentRepository {private StudentDao studentDao;public StudentRepository(Context context){MyDatabase db = MyDatabase.getInstance(context);studentDao = db.studentDao();}public void insert(Student... students){new InsertStudentTask(studentDao).execute(students);}class InsertStudentTask extends AsyncTask<Student, Void,Void>{private StudentDao studentDao;public InsertStudentTask(StudentDao studentDao) {this.studentDao = studentDao;}@Overrideprotected Void doInBackground(Student... students) {studentDao.insert(students);return null;}}public void update(Student... students){new UpdateStudentTask(studentDao).execute(students);}class UpdateStudentTask extends AsyncTask<Student, Void,Void>{private StudentDao studentDao;public UpdateStudentTask(StudentDao studentDao) {this.studentDao = studentDao;}@Overrideprotected Void doInBackground(Student... students) {studentDao.update(students);return null;}}public void delete(Student... students){new DeleteStudentTask(studentDao).execute(students);}class DeleteStudentTask extends AsyncTask<Student, Void,Void>{private StudentDao studentDao;public DeleteStudentTask(StudentDao studentDao) {this.studentDao = studentDao;}@Overrideprotected Void doInBackground(Student... students) {studentDao.delete(students);return null;}}public void deleteAllStudent(){new DeleteAllStudentTask(studentDao).execute();}class DeleteAllStudentTask extends AsyncTask<Void, Void,Void>{private StudentDao studentDao;public DeleteAllStudentTask(StudentDao studentDao) {this.studentDao = studentDao;}@Overrideprotected Void doInBackground(Void... voids) {studentDao.deleteAll();return null;}}public LiveData<List<Student>> getAllStudent(){return studentDao.getAllStudent();}}

5.定义StudentViewModel

public class StudentViewModel extends AndroidViewModel {private StudentRepository studentRepository;public StudentViewModel(@NonNull Application application) {super(application);studentRepository = new StudentRepository(application);}public void insert(Student... students){studentRepository.insert(students);}public void delete(Student... students){studentRepository.delete(students);}public void deleteAllStudent(){studentRepository.deleteAllStudent();}public void update(Student... students){studentRepository.update(students);}public LiveData<List<Student>> getAllStudnet(){return studentRepository.getAllStudent();}}

6.在MainActivity操作实现数据新增删除等操作

public class MainActivity extends AppCompatActivity {private StudentRecyclerViewAdapter adapter;private StudentViewModel studentViewModel;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);RecyclerView recyclerView = findViewById(R.id.recyclerView);recyclerView.setLayoutManager(new LinearLayoutManager(this));List<Student> studentList = new ArrayList<>();adapter = new StudentRecyclerViewAdapter(studentList);recyclerView.setAdapter(adapter);studentViewModel = new ViewModelProvider(this,new ViewModelProvider.AndroidViewModelFactory()).get(StudentViewModel.class);studentViewModel.getAllStudnet().observe(this, new Observer<List<Student>>() {@Overridepublic void onChanged(List<Student> students) {adapter.setStudentList(students);adapter.notifyDataSetChanged();}});}public void mInsert(View view) {Student s1 = new Student(18,"张三");Student s2 = new Student(19,"李四");studentViewModel.insert(s2);studentViewModel.insert(s1);}public void mUpdate(View view) {Student s1 = new Student(4,99,"新张三");studentViewModel.update(s1);}public void mDelete(View view) {Student s1 = new Student(2);studentViewModel.delete(s1);}public void mClear(View view) {studentViewModel.deleteAllStudent();}
}