【发布时间】：2021-04-04 19:05:37
【问题描述】：
我正在使用 SQLiteOpenHelper 类编写一个应用程序，并决定重新构建它，使用 Room 实现 MVVM 模式以获得额外的可用性。我的第一个问题是 Room 需要使用 .createFromAsset("database/data_table.db") 从 assets 文件夹加载数据库文件。它应该将它复制到设备上，然后所有进程都从那里开始，但它没有按要求复制它，开发者网站除了如何调用它之外没有指定更多内容。这里是 数据库类，每个方法的解释用于学习目的，因为我正在学习过程中。
@Database(entities = MarkerObject.class, version = 1, exportSchema = false)
public abstract class MarkerDatabase extends RoomDatabase {
/**We create the instance so we wouldn't create multiple instances
of the database and use the same instance throughout the application
which is accessed through the static variable **/
private static MarkerDatabase instance;
/**
*This is used to access the Dao,
* no body is provided because Room takes care of that,
* abstract classes don't have methods but the room generates it because of the @Dao annotation.
*/
public abstract MarkerDao markerDao();
/**
* This method creates a single instance of the database,
* then we can call this method from the outside so we have a handle to it.
* Synchronized means that only one thread at a time can access the database
* eliminating creating multiple instances.
*/
public static synchronized MarkerDatabase getInstance(Context context){
/** we want to instantiate the database
* only if we don't have an instance, thus the if condition */
if (instance == null) {
instance = Room.databaseBuilder(context.getApplicationContext(),
MarkerDatabase.class, "locations_table")
.createFromAsset("database/locations_table.db")
.build();
}
return instance;
}
}
DAO
@Dao
public interface MarkerDao {
@Query("SELECT * FROM locations_table")
LiveData<List<MarkerObject>> getAllMarkers();
}
存储库
public class MarkerRepository {
private MarkerDao markerDao;
private LiveData<List<MarkerObject>> allMarkers;
public MarkerRepository(Application application){
MarkerDatabase markerDatabase = MarkerDatabase.getInstance(application);
markerDao = markerDatabase.markerDao();
allMarkers = markerDao.getAllMarkers();
}
public LiveData<List<MarkerObject>> getAllMarkers(){return allMarkers; }
}
视图模型
public class MarkerViewModel extends AndroidViewModel {
private MarkerRepository repository;
private LiveData<ArrayList<MarkerObject>> allMarkers;
/**
* We use the application as context in the constructor
* because the ViewModel outlives the activities lifecycle.
* To avoid memory leaks we use application as context instead of activities or views.
*/
public MarkerViewModel(@NonNull Application application) {
super(application);
repository = new MarkerRepository(application);
allMarkers = repository.getAllMarkers();
}
public LiveData<ArrayList<MarkerObject>> getAllMarkers(){
return allMarkers;
}
}
主活动
public class MainActivity extends AppCompatActivity {
private MarkerViewModel markerViewModel;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
TextView textView = findViewById(R.id.textView);
markerViewModel = new ViewModelProvider(this).get(MarkerViewModel.class);
markerViewModel.getAllMarkers().observe(this, new Observer<List<MarkerObject>>() {
@Override
public void onChanged(List<MarkerObject> markerObjects) {
textView.setText(markerObjects.toString());
}
});
}
}
现在我明白这对某些人来说可能是一个愚蠢的问题，我可能会因此受到评判，但除了互联网之外，我没有其他来源可以学习。非常感谢所有帮助。
【问题讨论】：
-
“但它没有按要求复制它”——这意味着您的数据库已经存在。如果您之前运行过应用，然后添加了此行，则需要清除应用数据或卸载应用。
-
从另一个答案中，我得到必须首先执行查询才能创建数据库，但是正如我在提供查询后在那里指定的那样，然后我在调用 ViewModel 时遇到错误。但无论哪种方式都谢谢你。
标签： java android sqlite android-room