数据库框架Room2 进阶

1. 对WordDatabase使用单例模式（双锁懒汉式）

    private static volatile WordDatabase instance = null;

    static WordDatabase getInstance(Context context){
        if (instance == null) {
            synchronized (WordDatabase.class) {
                if (instance == null) {
                    instance = Room.databaseBuilder(context.getApplicationContext(), WordDatabase.class, "word_database")
                            .build();
                }
            }
        }
        return instance;
    }

 

2.创建仓库WordRepository来连接dao层

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

import androidx.lifecycle.LiveData;

import java.util.List;

/**
 * 仓库，直接连接dao层
 * 静态内部类继承自AsyncTask使用异步来操作数据库
 * 然后使用方法对其进行调用
 */
class WordRepository {

    private WordDao wordDao;
    private LiveData<List<Word>> allWordsLive;

    WordRepository(Context context) {
        WordDatabase wordDatabase = WordDatabase.getInstance(context);
        wordDao = wordDatabase.getWordDao();
        allWordsLive = wordDao.getAllWordsLive();
    }

    LiveData<List<Word>> getAllWordsLive() {
        return allWordsLive;
    }

    void insertWords(Word... words) {
        new InsertAsyncTask(wordDao).execute(words);
    }

    void updateWords(Word... words) {
        new UpdateAsyncTask(wordDao).execute(words);
    }

    void deleteWords(Word... words) {
        new DeleteAsyncTask(wordDao).execute(words);
    }

    void deleteAllWords() {
        new DeleteAllAsyncTask(wordDao).execute();
    }

    static class InsertAsyncTask extends AsyncTask<Word, Void, Void> {
        private WordDao wordDao;

        InsertAsyncTask(WordDao wordDao) {
            this.wordDao = wordDao;
        }

        @Override
        protected Void doInBackground(Word... words) {
            wordDao.insertWords(words);
            return null;
        }
    }

    static class UpdateAsyncTask extends AsyncTask<Word, Void, Void> {
        private WordDao wordDao;

        UpdateAsyncTask(WordDao wordDao) {
            this.wordDao = wordDao;
        }

        @Override
        protected Void doInBackground(Word... words) {
            wordDao.updateWords(words);
            return null;
        }
    }

    static class DeleteAsyncTask extends AsyncTask<Word, Void, Void> {
        private WordDao wordDao;

        DeleteAsyncTask(WordDao wordDao) {
            this.wordDao = wordDao;
        }

        @Override
        protected Void doInBackground(Word... words) {
            wordDao.deleteWords(words);
            return null;
        }
    }

    static class DeleteAllAsyncTask extends AsyncTask<Void, Void, Void> {
        private WordDao wordDao;

        DeleteAllAsyncTask(WordDao wordDao) {
            this.wordDao = wordDao;
        }

        @Override
        protected Void doInBackground(Void... voids) {
            wordDao.deleteAllWords();
            return null;
        }
    }
}

 3. 使用ViewModel来连接repository

public class WordViewModel extends AndroidViewModel {

    private WordRepository wordRepository;

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

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

    void insertWords(Word... words){
        wordRepository.insertWords(words);
    }

    void updateWords(Word... words){
        wordRepository.updateWords(words);
    }

    void deleteWords(Word... words){
        wordRepository.deleteWords(words);
    }

    void deleteAllWords(){
        wordRepository.deleteAllWords();
    }
}

 

4. activity中注册viewmodel和监听livedata

        wordViewModel = ViewModelProviders.of(this).get(WordViewModel.class);

        wordViewModel.getAllWordsLive().observe(this, words -> {
            StringBuilder text = new StringBuilder();
            for (Word word : words) {
                text.append(word.getId()).append(": ").append(word.getWord()).append(" = ").append(word.getChineseMeaning()).append("\n");
            }
            textView.setText(text);
        });