Android，LiveData+ViewModel+Room的案例

MainActivity.java

package com.example.app1;

import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {
    private RecyclerView recyclerView;
    private UserRecyclerViewAdapter adapter;
    private UserDao userDao;
    private UserViewModel userViewModel;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        recyclerView = findViewById(R.id.recyclerView);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        List<User> arrayList = new ArrayList<>();
        adapter = new UserRecyclerViewAdapter(arrayList);
        recyclerView.setAdapter(adapter);

        userViewModel = new ViewModelProvider(this,new ViewModelProvider.AndroidViewModelFactory(getApplication())).get(UserViewModel.class);
        userViewModel.queryUserAllLiveData().observe(this, new Observer<List<User>>() {
            @Override
            public void onChanged(List<User> users) {
                adapter.setList(users);
                adapter.notifyDataSetChanged();
            }
        });
    }

    public void insertData(View view) {
        User user1 = new User("小明",21);
        User user2 = new User("小红",20);
        userViewModel.insertUser(user1,user2);

    }

    public void deleteData(View view) {
        User user1 = new User(11);
        User user2 = new User(12);
        userViewModel.deleteUser(user1,user2);

    }

    public void updateData(View view) {
        User user = new User(13,"Einstein",144);
        userViewModel.updateUser(user);
        Toast.makeText(this, "修改成了'小明'的名字和年龄", Toast.LENGTH_SHORT).show();
    }

    public void mClear(View view) {
        userViewModel.deleteAllUsers();
    }
}

User.java:

package com.example.app1;

import androidx.room.ColumnInfo;
import androidx.room.Entity;
import androidx.room.Ignore;
import androidx.room.PrimaryKey;

@Entity(tableName = "user")
public class User {
    @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 User(int id, String name, int age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }

    @Ignore
    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Ignore
    public User(int id) {
        this.id = id;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

UserDao.java:

package com.example.app1;

import androidx.lifecycle.LiveData;
import androidx.room.Dao;
import androidx.room.Delete;
import androidx.room.Insert;
import androidx.room.Query;
import androidx.room.Update;

import java.util.List;

@Dao
public interface UserDao {
    @Insert
    void insert(User... users);

    @Delete
    void delete(User... users);

    @Update
    void update(User... users);

    @Query("SELECT * FROM user")
    LiveData<List<User>> queryAll();

    @Query("DELETE FROM user")
    void deleteAll();

    @Query("SELECT * FROM user WHERE id = :id")
    List<User> query(int id);
}

UserDatabase.java

package com.example.app1;

import android.content.Context;

import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;

@Database(entities = {User.class},version = 1,exportSchema = false)
public abstract class UserDatabase extends RoomDatabase {
    private static final String DATABASE_NAME = "user_db.db";
    private static UserDatabase mInstance;
    public static synchronized UserDatabase getInstance(Context context){
        if (mInstance == null){
            mInstance = Room.databaseBuilder(context.getApplicationContext(),
                    UserDatabase.class,
                    DATABASE_NAME)
                    .build();
        }
        return mInstance;
    }

    public abstract UserDao getUserDao();
}

UserRecyclerViewAdapter.java:

package com.example.app1;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import java.util.List;

public class UserRecyclerViewAdapter extends RecyclerView.Adapter<UserRecyclerViewAdapter.MyViewHolder> {
    private List<User> list;

    public UserRecyclerViewAdapter(List<User> arrayList) {
        this.list = arrayList;
    }

    public void setList(List<User> list) {
        this.list = list;
    }

    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item,parent,false);
        return new MyViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        User user = list.get(position);
        TextView text_id = holder.itemView.findViewById(R.id.text_id);
        text_id.setText(String.valueOf(user.id));
        TextView text_name = holder.itemView.findViewById(R.id.text_name);
        text_name.setText(user.name);
        TextView text_age = holder.itemView.findViewById(R.id.text_age);
        text_age.setText(String.valueOf(user.age));
    }

    @Override
    public int getItemCount() {
        return list.size();
    }

    public class MyViewHolder extends RecyclerView.ViewHolder {
        public MyViewHolder(@NonNull View itemView) {
            super(itemView);
        }
    }
}

UserRepository.java

package com.example.app1;

import android.content.Context;
import android.os.AsyncTask;

import androidx.lifecycle.LiveData;

import java.util.List;

public class UserRepository {
    private UserDao userDao;

    public UserRepository(Context context) {
        UserDatabase userDatabase = UserDatabase.getInstance(context);
        this.userDao = userDatabase.getUserDao();
    }

    //添加user
    public void insertUser(User... users){
        new InsertUserTask(userDao).execute(users);
    }

    class InsertUserTask extends AsyncTask<User,Void,Void> {
        private UserDao userDao;

        public InsertUserTask(UserDao userDao) {
            this.userDao = userDao;
        }

        @Override
        protected Void doInBackground(User... users) {
            userDao.insert(users);
            return null;
        }
    }

    //删除user
    public void deleteUser(User... users){
        new DeleteUserTask(userDao).execute(users);
    }

    class DeleteUserTask extends AsyncTask<User,Void,Void>{
        private UserDao userDao;

        public DeleteUserTask(UserDao userDao) {
            this.userDao = userDao;
        }

        @Override
        protected Void doInBackground(User... users) {
            userDao.delete(users);
            return null;
        }
    }

    //删除所有数据
    public void deleteAllUsers(){
        new DeleteUserAll(userDao).execute();
    }

    class DeleteUserAll extends AsyncTask<Void,Void,Void>{
        private UserDao userDao;

        public DeleteUserAll(UserDao userDao) {
            this.userDao = userDao;
        }

        @Override
        protected Void doInBackground(Void... voids) {
            userDao.deleteAll();
            return null;
        }
    }

    //修改user
    public void updateUser(User... users){
        new UpdatetUserTask(userDao).execute(users);
    }

    class UpdatetUserTask extends AsyncTask<User,Void,Void>{
        private UserDao userDao;

        public UpdatetUserTask(UserDao userDao) {
            this.userDao = userDao;
        }

        @Override
        protected Void doInBackground(User... users) {
            userDao.update(users);
            return null;
        }
    }

    //查询所有数据
    public LiveData<List<User>> queryUserAllLiveData(){
        return userDao.queryAll();

    }
}

UserViewModel.java:

package com.example.app1;

import android.app.Application;

import androidx.annotation.NonNull;
import androidx.lifecycle.AndroidViewModel;
import androidx.lifecycle.LiveData;

import java.util.List;

public class UserViewModel extends AndroidViewModel {
    private UserRepository userRepository;
    public UserViewModel(@NonNull Application application) {
        super(application);
        this.userRepository = new UserRepository(application);
    }
    //添加user
    public void insertUser(User... users){
        userRepository.insertUser(users);
    }

    //删除user
    public void deleteUser(User... users){
        userRepository.deleteUser(users);
    }

    //修改user
    public void updateUser(User... users){
        userRepository.updateUser(users);
    }

    //删除所有数据
    public void deleteAllUsers(){
        userRepository.deleteAllUsers();
    }

    //查询所有数据
    public LiveData<List<User>> queryUserAllLiveData(){
        return userRepository.queryUserAllLiveData();
    }
}

activity_main.xml:

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <androidx.constraintlayout.widget.Guideline
        android:id="@+id/guideline5"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        app:layout_constraintGuide_percent="0.41" />

    <Button
        android:id="@+id/insertBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="76dp"
        android:layout_marginBottom="92dp"
        android:text="添加数据"
        app:layout_constraintBottom_toTopOf="@+id/guideline5"
        app:layout_constraintStart_toStartOf="parent"
        android:onClick="insertData"/>

    <Button
        android:id="@+id/deleteBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="76dp"
        android:layout_marginBottom="32dp"
        android:text="删除数据"
        app:layout_constraintBottom_toTopOf="@+id/guideline5"
        app:layout_constraintStart_toStartOf="parent"
        android:onClick="deleteData"/>

    <Button
        android:id="@+id/updateBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="68dp"
        android:layout_marginBottom="92dp"
        android:text="修改数据"
        app:layout_constraintBottom_toTopOf="@+id/guideline5"
        app:layout_constraintEnd_toEndOf="parent"
        android:onClick="updateData"/>

    <Button
        android:id="@+id/mClear"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="68dp"
        android:layout_marginBottom="32dp"
        android:text="清空数据"
        app:layout_constraintBottom_toTopOf="@+id/guideline5"
        app:layout_constraintEnd_toEndOf="parent"
        android:onClick="mClear"/>


    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:layout_marginBottom="184dp"
        android:background="#03A9F4"
        android:text="数据库的相关操作"
        android:textSize="34sp"
        app:layout_constraintBottom_toTopOf="@+id/guideline5"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.527" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerView"
        android:layout_width="match_parent"
        android:layout_height="370dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:layout_marginBottom="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="@+id/guideline5"
        app:layout_constraintVertical_bias="0.38" />


androidx.constraintlayout.widget.ConstraintLayout>

item.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/text_id"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:background="#03A9F4"
        android:text="id"
        android:gravity="center"
        android:textSize="34sp"/>

    <TextView
        android:id="@+id/text_name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:background="#FFC107"
        android:text="名字"
        android:gravity="center"
        android:textSize="34sp"/>

    <TextView
        android:id="@+id/text_age"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:background="#FF5722"
        android:text="年龄"
        android:gravity="center"
        android:textSize="34sp"/>
LinearLayout>