如何使用 Rxjava 预填充 android 房间数据库

Question

如何使用Rxjava 正确预填充数据库

这是我的道

@Query("SELECT * from questions WHERE difficulty  = :difficulty")
Flowable<List<Question>> getQuestions(String difficulty);
// Emits the number of users added to the database.
@Insert
public Maybe<long[]> insertQuestions(List<Question> questions);

这是我的数据库类

public static synchronized QuestionDatabase getInstance(Context context) {
if (instance == null) {
instance = Room.databaseBuilder(context.getApplicationContext(), QuestionDatabase.class, "questions_database")
.fallbackToDestructiveMigration()
//.addMigrations(MIGRATION_1_2)
.addCallback(roomCallback)
.build();
}
return instance;
}
private static RoomDatabase.Callback roomCallback = new RoomDatabase.Callback() {
@Override
public void onCreate(@NonNull SupportSQLiteDatabase db) {
super.onCreate(db);
QuestionDao questionDao = instance.questionDao();
List<Question> questions = new ArrayList<>();
questions.add(new Question("A is correct", "A", "B", "C", 1, GameConstants.DIFFICULTY_EASY));
questions.add(new Question("C is correct", "A", "B", "C", 3, GameConstants.DIFFICULTY_HARD));
questions.add(new Question("B is correct", "A", "B", "C", 2, GameConstants.DIFFICULTY_EASY));
//inserting records
questionDao.insertQuestions(questions)
.subscribeOn(Schedulers.io())
//.observeOn(AndroidSchedulers.mainThread())
.subscribe(
new Consumer<long[]>() {
@Override
public void accept(long[] longs) throws Exception {
}
},
new Consumer<Throwable>() {
@Override
public void accept(Throwable throwable) throws Exception {
}
});

}
};

我遇到的问题如下：

仅当我在活动 p.Ex 上调用方法时，才会填充数据库：

//getting flowable to subscribe consumer that will access the data from Room database.
questionDao.getQuestions(GameConstants.DIFFICULTY_HARD)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(
new Consumer<List<Question>>() {
@Override
public void accept(List<Question> questions) throws Exception {
questionList = (ArrayList<Question>)questions;
questionCountTotal = questionList.size();
Collections.shuffle(questionList);
showNextQuestion();
}
},
new Consumer<Throwable>() {
@Override
public void accept(Throwable throwable) throws Exception {
}
}
);

当我调用这种方法时，我得到了两次问题。我通过在 questionDao.getQuestions 的接受方法中放置一个断点来检查这一点。 我相信两个线程正在运行一个用于填充，另一个用于获取问题。

为什么会发生这种情况，我如何以正确的方式实现它？

提前致谢

Answer 1

我相信以下更改将起作用

1. 更改getInstance方法以强制打开数据库，从而导致onCreate在您获取实例时运行，而不是在您尝试访问数据库之前不运行。

根据(见评论(：-

public static synchronized QuestionDatabase getInstance(Context context) {
if (instance == null) {
instance = Room.databaseBuilder(context.getApplicationContext(), QuestionDatabase.class, "questions_database")
.fallbackToDestructiveMigration()
//.addMigrations(MIGRATION_1_2)
.addCallback(roomCallback)
.build();
}
instance.getOpenHelper().getWritableDatabase(); //<<<<<<<<<< Forces an Open thus creation of the Database
return instance;
}

2. 添加初始问题而不订阅

即代替

//inserting records
questionDao.insertQuestions(questions)
.subscribeOn(Schedulers.io())
//.observeOn(AndroidSchedulers.mainThread())
.subscribe(
new Consumer<long[]>() {
@Override
public void accept(long[] longs) throws Exception {
}
},
new Consumer<Throwable>() {
@Override
public void accept(Throwable throwable) throws Exception {
}
});

只需使用 ：-

questionDao.insertQuestions(questions);

当您在活动中恢复实例时，将填充数据库。