如何强制 Room 的 OnCreate 回调工作？

Question

我非常坚持这一点。我正在关注 Google 的“ArchitectureComponentsBasic1”，您可以通过转到 File -> New -> Import Sample 将其导入 Android Studio。我也遵循了 Maxim 在this SO 文章中提供的建议。什么都没有工作。如何让 OnCreate 回调运行，以便预填充我的数据库？

数据库：请注意，我正在我的 buildDatabase 方法中运行执行程序，以尝试运行一个虚拟查询以根据上面链接的 SO 运行 getWritableDatabase()。这不起作用。

        version = 1,
        exportSchema = false)
public abstract class ProjectDatabase extends RoomDatabase {
    private final MutableLiveData<Boolean> isDatabaseCreated = new MutableLiveData<>();
    private static ProjectDatabase instance;
    private static final String DATABASE_NAME = "project.db";
    private static final String TAG = ProjectDatabase.class.getSimpleName();

    public abstract ProjectDao projectDao();

    public static ProjectDatabase getInstance(final Context context, final AppExecutors appExecutors) {
        if (instance == null) {
            synchronized (ProjectDatabase.class) {
                if (instance == null) {
                    instance = buildDatabase(context.getApplicationContext(), appExecutors);
                    instance.updateDatabaseCreated(context.getApplicationContext());
                }
            }
        }
        return instance;
    }

    private static ProjectDatabase buildDatabase(final Context appContext, final AppExecutors appExecutors) {
        Log.d(TAG, "building database.");
        ProjectDatabase db = Room.databaseBuilder(appContext, ProjectDatabase.class, DATABASE_NAME)
                .addCallback(new Callback() {
                    @Override
                    public void onCreate(@NonNull SupportSQLiteDatabase db) {
                        super.onCreate(db);
                        Log.d(TAG, "callback was called.");
                        appExecutors.diskIO().execute(() -> {
                            ProjectDatabase database = ProjectDatabase.getInstance(appContext, appExecutors);
                            List<Project> projects = DataGenerator.generateProjects();

                            Log.d(TAG, "project data: " + projects.get(0).getTitle());

                            insertData(database, projects);
                            database.setDatabaseCreated();
                        });
                    }
                })
                .build();

        appExecutors.diskIO().execute(new Runnable() {
            @Override
            public void run() {
                Log.d(TAG, "populate database");
                db.projectDao().populateDatabase();
            }
        });

        return db;
    }

    /**
     * Check whether the database already exists and expose it via {@link #getDatabaseCreated()}
     */
    private void updateDatabaseCreated(final Context context) {
        if (context.getDatabasePath(DATABASE_NAME).exists()) {
            setDatabaseCreated();
        }
    }

    private void setDatabaseCreated(){
        Log.d(TAG, "database created");
        isDatabaseCreated.postValue(true);
    }

    private static void insertData(final ProjectDatabase database, final List<Project> projects) {
        database.runInTransaction(() -> {
            database.projectDao().insertAll(projects);
        });
    }

DAO 项目

@Dao
public interface ProjectDao {
    @Insert(onConflict = OnConflictStrategy.IGNORE)
    void insertAll(List<Project> projects);

    @Insert(onConflict = OnConflictStrategy.IGNORE)
    void insert(Project project);

    @Query("SELECT * FROM project")
    LiveData<List<Project>> loadAllProjects();

    @Query("SELECT * FROM project WHERE is_favorite = 1")
    LiveData<Project[]> getAllFavorites();

    @Query("SELECT is_favorite FROM project WHERE id = :projectId")
    int isFavorite(int projectId);

    @Query("UPDATE project SET is_favorite = 1 WHERE id = :projectId")
    int addToFavorites(int projectId);

    @Query("UPDATE project SET is_favorite = 0 WHERE id = :projectId")
    int removeFromFavorites(int projectId);


    // Dummy query needed to activate the Room callback method to prepopulate the database
    @Query("SELECT * from project")
    LiveData<Project> populateDatabase();
}

Answer 1

您需要为任何返回 LiveData 的 dao 方法注册至少一个观察者。当观察者注册时，您的 roomCallback onCreate 将被调用。

要删除那些嵌套的意大利面条代码，您可以使用 AsyncTask 预填充 db，所以它看起来像这样：

public static synchronized ProjectDatabase getInstance(Context context) {
    if (instance == null) {
        instance = Room.databaseBuilder(context.getApplicationContext(),
                ProjectDatabase.class, DATABASE_NAME)
                .addCallback(roomCallback)
                .build();
    }
    return instance;
    }

 private static RoomDatabase.Callback roomCallback = new RoomDatabase.Callback() {
    @Override
    public void onCreate(@NonNull SupportSQLiteDatabase db) {
        super.onCreate(db);
        Log.d(TAG, "callback was called.");
        new PopulateDbAsyncTask(instance).execute();
    }
};

private static class PopulateDbAsyncTask extends AsyncTask<Void, Void, Void>
{
    private ProjectDao projectDao;
    private PopulateDbAsyncTask(ProjectDatabase db){
        projectDao=db.projectDao();
    }
    @Override
    protected Void doInBackground(Void... voids) {
        List<Project> projects = DataGenerator.generateProjects();
        projectDao.insertAll(projects )
        return null;
    }
}

}