android----Room数据库的操作(2)

 这篇文章是对上篇文章的优化。增加两个类,一个ViewModel管理界面数据,一个仓库类存放类。

  1、Room本身支持LiveData,使用LiveData,我们就可以不用每次对页面进行刷新操作(自己写的页面刷新函数 )。我们可以用observe来观察它是否发生改变,如果改变则进行页面改变。

  2、在Database改成Singleton,因为在database构造实例是非常消耗资源的,所有我们只让它构造一个实例,节省资源

  3、改变线程。上一篇文章数据库的操作是在主线程,这是不对的,应该把它放到工作线程。我们可以用AsyncTask。要用AsyncTask对数据库进行操作,就需要定义多个类来继承AsyncTask这个抽象类。

  

  实体类:

android----Room数据库的操作(2)
package com.example.roomtest;

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

@Entity // 实体类
public class Word {
    @PrimaryKey(autoGenerate = true) // 将id设置为主键,并且自增
    private int id;
    @ColumnInfo(name = "English")   // 设置每列的名称
    private String English_word;
    @ColumnInfo(name = "Chinese")
    private String Chinese_meaning;
    // 主键不用构造函数
    public  Word(){}
    public Word(String english_word, String chinese_meaning) {
        English_word = english_word;
        Chinese_meaning = chinese_meaning;
    }
    public void setId(int id){
        this.id = id;
    }
    public int getId() {
        return id;
    }
    public String getEnglish_word() {
        return English_word;
    }

    public void setEnglish_word(String english_word) {
        English_word = english_word;
    }

    public String getChinese_meaning() {
        return Chinese_meaning;
    }

    public void setChinese_meaning(String chinese_meaning) {
        Chinese_meaning = chinese_meaning;
    }
}
android----Room数据库的操作(2)

  Dao:

android----Room数据库的操作(2)
package com.example.roomtest;

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    // 访问数据库操作的接口 Database access object
public interface WordDao {
    //我们所有对数据库的操作都在Dao里声明
    @Insert   // 这些修饰词会在编译时候生成代码
    void insertWords(Word... words);   // 如果是一个参数就写 Word word,多个参数就这样写
    @Update
    void updataWords(Word... words);
    @Delete
    void deleteWords(Word... words);
    @Query("DELETE  FROM WORD")
    void deleteAllWords();
    @Query("SELECT * FROM WORD ORDER BY ID DESC")  // 获取所有的WORD,并且按照id降序排序
    LiveData<List<Word>> getAllWordsLive();
}
android----Room数据库的操作(2)

  Database:

  

android----Room数据库的操作(2)
package com.example.roomtest;

import android.content.Context;

import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
// singleton
/*
    这里有个参数,第一个参数entities是一个集合,因为只有一个实体类,如果有多个可以逗号隔开
    第二个参数是version(版本):每一次数据库的结构发生改变,版本都得改变
    第三个参数是生成数据库表:这里先选择false
 */
@Database(entities = {Word.class},version = 1,exportSchema = false)
public abstract class WordDatabase extends RoomDatabase {
    private static WordDatabase INSTANCE;
    // 因为需要一个Context,所有我们得传来一个Context
    static synchronized WordDatabase getDatabase(Context context){
        if(INSTANCE==null){
            INSTANCE = Room.databaseBuilder(context.getApplicationContext(), WordDatabase.class, "word_database")
            .allowMainThreadQueries().build();
        }
        return INSTANCE;
    }
    public abstract  WordDao  getWordDao();
}
android----Room数据库的操作(2)

  ViewModel:

  

android----Room数据库的操作(2)
package com.example.roomtest;

import android.app.Application;
import android.os.AsyncTask;

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

import java.util.List;

public class WordViewModel extends AndroidViewModel {
    private WordRepository wordRepository;
    public WordViewModel(@NonNull Application application) {
        super(application);
        wordRepository = new WordRepository(application);
    }

    public LiveData<List<Word>> getAllWordLive() {
        return wordRepository.getAllWordLive();
    }

    // 定义类的接口-- 插入
    public void insertWords(Word...words){
        wordRepository.insertWords(words);
    }
    // 定义类的接口-- 删除
    public void deleteWords(Word...words){
        wordRepository.deleteWords(words);
    }
    // 定义类的接口-- 清空
    public void deleteAllWords(){
        wordRepository.deleteAllWords();
    }
    // 定义类的接口-- 修改
    public void updataWords(Word...words){
        wordRepository.updataWords(words);
    }
}
android----Room数据库的操作(2)

  Repository:

  

android----Room数据库的操作(2)
package com.example.roomtest;

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

import androidx.lifecycle.LiveData;

import java.util.List;

public class WordRepository {
    private LiveData<List<Word>> allWordLive;
    private  WordDao wordDao;
    public WordRepository(Context context) {
        WordDatabase wordDatabase = WordDatabase.getDatabase(context.getApplicationContext());
        wordDao = wordDatabase.getWordDao();
        allWordLive = wordDao.getAllWordsLive();
    }

    public LiveData<List<Word>> getAllWordLive() {
        return allWordLive;
    }
    // 定义类的接口-- 插入
    public void insertWords(Word...words){
        new InsertAsyncTask(wordDao).execute(words);
    }
    // 定义类的接口-- 删除
    public void deleteWords(Word...words){
        new DeleteAsyncTask(wordDao).execute(words);
    }
    // 定义类的接口-- 清空
    public void deleteAllWords(){
        new DeleteAllAsyncTask(wordDao).execute();
    }
    // 定义类的接口-- 修改
    public void updataWords(Word...words){
        new UpdataAsyncTask(wordDao).execute(words);
    }
    // 插入类
    static class InsertAsyncTask extends AsyncTask<Word,Void,Void> {
        private WordDao wordDao;
        public InsertAsyncTask(WordDao wordDao) {
            this.wordDao = wordDao;
        }
        // 将在主线程的工作放到后台来执行
        @Override
        protected Void doInBackground(Word... words) {
            wordDao.insertWords(words);
            return null;
        }
    }
    // 删除类
    static class DeleteAsyncTask extends AsyncTask<Word,Void,Void> {
        private WordDao wordDao;
        public DeleteAsyncTask(WordDao wordDao) {
            this.wordDao = wordDao;
        }
        // 将在主线程的工作放到后台来执行
        @Override
        protected Void doInBackground(Word... words) {
            wordDao.deleteWords(words);
            return null;
        }
    }
    // 清空类
    static class DeleteAllAsyncTask extends AsyncTask<Void,Void,Void> {
        private WordDao wordDao;
        public DeleteAllAsyncTask(WordDao wordDao) {
            this.wordDao = wordDao;
        }
        // 将在主线程的工作放到后台来执行
        @Override
        protected Void doInBackground(Void... voids) {
            wordDao.deleteAllWords();
            return null;
        }
    }
    // 修改类
    static class UpdataAsyncTask extends AsyncTask<Word,Void,Void> {
        private WordDao wordDao;
        public UpdataAsyncTask(WordDao wordDao) {
            this.wordDao = wordDao;
        }
        // 将在主线程的工作放到后台来执行
        @Override
        protected Void doInBackground(Word... words) {
            wordDao.updataWords(words);
            return null;
        }
    }
}
android----Room数据库的操作(2)

  运行结果:

  android----Room数据库的操作(2)

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