Andriod — JetPack ：初识 JetPack
Andriod — JetPack ：LifeCycle 的诞生
Andriod — JetPack ：ViewModel 的诞生
Andriod — JetPack ：BaseObservable 与 ObservableField 双向绑定
Andriod — JetPack ：DataBinding + LiveData +ViewModel 简单实例
Andriod — JetPack ：Room 增删改查
Andriod — JetPack ：Room + ViewModel + LiveData 增删改查实例
Andriod — JetPack ：LiveData setValue 和 postValue 的区别
一、前言1.为什么使用 LiveData + ViewModel ? 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。所以，当数据变化时，我们可以通过 LiveData 通知 View 层，实现数据的自动更新。 2.图解 
二、代码实例
build.gradle
// Room
implementation "androidx.room:room-runtime:2.2.5"
annotationProcessor "androidx.room:room-compiler:2.2.5"
implementation 'androidx.recyclerview:recyclerview:1.0.0'
activity_main.xml
item.xml
Student.java
package com.example.room2;
import androidx.room.ColumnInfo;
import androidx.room.Entity;
import androidx.room.Ignore;
import androidx.room.PrimaryKey;
@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, String name, int age) {
this.id = id;
this.name = name;
this.age = age;
}
@Ignore
public Student(String name, int age) {
this.name = name;
this.age = age;
}
@Ignore
public Student(int id) {
this.id = id;
}
}
StudentDao.java
package com.example.room2;
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 StudentDao {
@Insert
void insertStudent(Student... students); // 可以传入多个 Student
@Delete
void deleteStudent(Student... students);
@Query("DELETE FROM student")
void deleteAllStudent();
@Update
void updateStudent(Student... students);
@Query("SELECT * FROM student")
LiveData getAllStudentsLive();
@Query("SELECT * FROM student WHERE id = :id")
List getStudentById(Integer id);
}
MyDataBase.java
package com.example.room2;
import android.content.Context;
import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
// 一定是抽象类
@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;
// 构建 database 单例
public static synchronized MyDataBase getInstance(Context context) {
if(mInstance == null) {
mInstance = Room.databaseBuilder(context.getApplicationContext(), MyDataBase.class, DATABASE_NAME)
//.allowMainThreadQueries() // 允许在主线程操作数据库
.build();
}
return mInstance;
}
public abstract StudentDao getStudentDao();
}
StudentRepository.java
package com.example.room2;
import android.content.Context;
import android.os.AsyncTask;
import androidx.lifecycle.LiveData;
import java.util.List;
public class StudentRepository {
private StudentDao studentDao;
public StudentRepository(Context context) {
MyDataBase dataBase = MyDataBase.getInstance(context);
this.studentDao = dataBase.getStudentDao();
this.studentDao = studentDao;
}
public void insertStudent(Student... students) {
new InsertStudentTask(studentDao).execute(students);
}
public void deleteStudent(Student... students) {
new DeleteStudentTask(studentDao).execute(students);
}
public void deleteAllStudents() {
new DeleteAllStudentsTask(studentDao).execute();
}
public void updateStudent(Student... students) {
new UpdateStudentTask(studentDao).execute(students);
}
public LiveData getAllStudentsLive() {
return studentDao.getAllStudentsLive();
}
// 异步线程添加学生
class InsertStudentTask extends AsyncTask {
private StudentDao studentDao;
public InsertStudentTask(StudentDao studentDao) {
this.studentDao = studentDao;
}
@Override
protected Void doInBackground(Student... students) {
studentDao.insertStudent(students);
return null;
}
}
// 异步修改学生
class UpdateStudentTask extends AsyncTask {
private StudentDao studentDao;
public UpdateStudentTask(StudentDao studentDao) {
this.studentDao = studentDao;
}
@Override
protected Void doInBackground(Student... students) {
studentDao.updateStudent(students);
return null;
}
}
// 异步删除学生
class DeleteStudentTask extends AsyncTask {
private StudentDao studentDao;
public DeleteStudentTask(StudentDao studentDao) {
this.studentDao = studentDao;
}
@Override
protected Void doInBackground(Student... students) {
studentDao.deleteStudent(students);
return null;
}
}
// 异步删除所有学生
class DeleteAllStudentsTask extends AsyncTask {
private StudentDao studentDao;
public DeleteAllStudentsTask(StudentDao studentDao) {
this.studentDao = studentDao;
}
@Override
protected Void doInBackground(Void... Void) {
studentDao.deleteAllStudent();
return null;
}
}
}
StudentViewModel.java
package com.example.room2;
import android.app.Application;
import androidx.annotation.NonNull;
import androidx.lifecycle.AndroidViewModel;
import androidx.lifecycle.LiveData;
import java.util.List;
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);
}
public void deleteAllStudents() {
repository.deleteAllStudents();
}
public void deleteStudent(Student... students) {
repository.deleteStudent(students);
}
public void updateStudent(Student... students) {
repository.updateStudent(students);
}
public LiveData getAllStudentsLive() {
return repository.getAllStudentsLive();
}
}
MainActivity.java
package com.example.room2;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModel;
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 StudentRecycleViewAdapter adapter;
private StudentDao studentDao;
private StudentViewModel studentViewModel;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
List students = new ArrayList();
RecyclerView recyclerView = findViewById(R.id.rv_01);
adapter = new StudentRecycleViewAdapter(students);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
recyclerView.setAdapter(adapter);
// 得到 ViewModel
studentViewModel = new ViewModelProvider(this, new ViewModelProvider.AndroidViewModelFactory(getApplication())).get(StudentViewModel.class);
// 自动更新操作，给 LiveData 监听
studentViewModel.getAllStudentsLive().observe(this, new Observer() {
@Override
public void onChanged(List students) {
adapter.setStudents(students);
adapter.notifyDataSetChanged();
}
});
}
// Room 不允许在主线程操作数据库，如果非在主线程操作数据库，请修改 MyDataBase 文件
// 增加
public void mInsert(View view) {
Student s1 = new Student("Jack", 20);
Student s2 = new Student("Rose", 22);
studentViewModel.insertStudent(s1, s2);
}
// 删除
public void mDelete(View view) {
Student s1 = new Student(4);
studentViewModel.deleteStudent(s1);
}
// 修改
public void mUpdate(View view) {
Student s1 = new Student(3, "Jason", 21);
studentViewModel.updateStudent(s1);
}
public void mClear(View view) {
studentViewModel.deleteAllStudents();
}
}
StudentRecycleViewAdapter.java
package com.example.room2;
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 StudentRecycleViewAdapter extends RecyclerView.Adapter {
List students;
public void setStudents(List students) {
this.students = students;
}
public StudentRecycleViewAdapter(List students) {
this.students = students;
}
@NonNull
@Override
public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
View root = LayoutInflater.from(parent.getContext()).inflate(R.layout.item,parent, false);
return new MyViewHolder(root);
}
@Override
public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
Student student = students.get(position);
holder.tvId.setText(String.valueOf(student.id));
holder.name.setText(student.name);
holder.age.setText(String.valueOf(student.age));
}
@Override
public int getItemCount() {
return students == null ? 0 :students.size();
}
public class MyViewHolder extends RecyclerView.ViewHolder {
private TextView tvId, name, age;
public MyViewHolder(View view) {
super(view);
tvId = view.findViewById(R.id.tvId);
name = view.findViewById(R.id.name);
age = view.findViewById(R.id.age);
}
}
}
