CursorLoader的onLoadFinished回调中的RxJava2

Question

要从数据库中获取数据,我CursorLoader在应用程序中使用.一旦onLoadFinished()回调方法调用app的逻辑,就将Cursor对象转换为List业务模型需求中的对象.如果有大量数据,那么转换(繁重操作)需要一些时间.这会降低UI线程的速度.我尝试Thread使用RxJava2传递Cursor对象在非UI中启动转换,但得到Exception:

Caused by: android.database.StaleDataException: Attempting to access a closed CursorWindow.Most probable cause: cursor is deactivated prior to calling this method.

这是Fragment代码的一部分:

@Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        QueryBuilder builder;
        switch (id) {
            case Constants.FIELDS_QUERY_TOKEN:
                builder = QueryBuilderFacade.getFieldsQB(activity);
                return new QueryCursorLoader(activity, builder);
            default:
                return null;
        }
    }

    @Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) {
        if (cursor.getCount() > 0) {
            getFieldsObservable(cursor)
                    .subscribeOn(Schedulers.io())
                    .observeOn(AndroidSchedulers.mainThread())
                    .subscribe(this::showFields);
        } else {
            showNoData();
        }
    }

private static Observable<List<Field>> getFieldsObservable(Cursor cursor) {
            return Observable.defer(() -> Observable.just(getFields(cursor))); <-- Exception raised at this line

        }

private static List<Field> getFields(Cursor cursor) {
            List<Field> farmList = CursorUtil.cursorToList(cursor, Field.class);
            CursorUtil.closeSafely(cursor);
            return farmList;
        }

CursorLoader这里使用的目的是在更新数据存储时从DB获取通知.

更新 如Tin Tran建议,我删除了CursorUtil.closeSafely(cursor);,现在我得到另一个例外:

Caused by: java.lang.IllegalStateException: attempt to re-open an already-closed object: /data/user/0/com.my.project/databases/db_file
                                                          at android.database.sqlite.SQLiteClosable.acquireReference(SQLiteClosable.java:55)
                                                          at android.database.CursorWindow.getNumRows(CursorWindow.java:225)
                                                          at android.database.sqlite.SQLiteCursor.onMove(SQLiteCursor.java:121)
                                                          at android.database.AbstractCursor.moveToPosition(AbstractCursor.java:236)
                                                          at android.database.AbstractCursor.moveToNext(AbstractCursor.java:274)
                                                          at android.database.CursorWrapper.moveToNext(CursorWrapper.java:202)
                                                          at com.db.util.CursorUtil.cursorToList(CursorUtil.java:44)
                                                          at com.my.project.MyFragment.getFields(MyFragment.java:230)

cursorToList() 的方法 CursorUtil

public static <T> ArrayList<T> cursorToList(Cursor cursor, Class<T> modelClass) {
        ArrayList<T> items = new ArrayList<T>();
        if (!isCursorEmpty(cursor)) {
            while (cursor.moveToNext()) { <-- at this line (44) of the method raised that issue
                final T model = buildModel(modelClass, cursor);
                items.add(model);
            }
        }
        return items;
    }

Answer 1

从我对您的问题的评论中可以看出,我对getFieldsObservable()尚未返回的数据是否正在更新感兴趣.我收到了您对评论感兴趣的信息.

我可以判断,这是你的情况:

• onLoadFinished() 用Cursor-1调用
• RxJava的方法正在另一个使用Cursor-1的线程上执行(尚未完成,这里使用Cursor-1)
• onLoadFinished()使用Cursor-2调用,LoaderManager API负责关闭Cursor-1,RxJava仍然在另一个线程上查询

因此,会产生异常.

因此,您最好坚持创建自定义AsyncTaskLoader(从中CursorLoader扩展).这AsyncTaskLoader将包含所有逻辑CursorLoader(基本上是一对一的副本),但将返回已排序/过滤器对象onLoadFinished(YourCustomObject).因此,您希望使用RxJava执行的操作实际上是由您的加载器在其loadInBackground()方法中完成的.

这有一个变化的快照MyCustomLoader将有loadInBackground()方法:

public class MyCustomLoader extends AsyncTaskLoader<PojoWrapper> {
  ...
  /* Runs on a worker thread */
  @Override
  public PojoWrapper loadInBackground() {
    ...
    try {
      Cursor cursor = getContext().getContentResolver().query(mUri, mProjection, mSelection,
          mSelectionArgs, mSortOrder, mCancellationSignal);
      ...

      // `CursorLoader` performs following:
      // return cursor;

      // We perform some operation here with `cursor`
      // and return PojoWrapper, that consists of `cursor` and `List<Pojo>`
      List<Pojo> list = CursorUtil.cursorToList(cursor, Field.class);
      return new PojoWrapper(cursor, list);
    } finally {
      ...
    }
  }
  ...
}

在哪里PojoWrapper:

public class PojoWrapper {
  Cursor cursor;
  List<Pojo> list;

  public PojoWrapper(Cursor cursor, List<Pojo> list) {
    this.cursor = cursor;
    this.list = list;
  }
}

因此,onLoadFinished()您不必将工作委托给另一个线程,因为您已经在Loader实现中完成了:

@Override public void onLoadFinished(Loader<PojoWrapper> loader, PojoWrapper data) {
      List<Pojo> alreadySortedList = data.list;
}

这是整个代码MyCustomLoader.

Answer 2

一旦加载程序知道应用程序不再使用数据，它就会释放数据。例如，如果数据是来自 CursorLoader 的游标，则不应自行对其调用 close()。来自： https: //developer.android.com/guide/components/loaders.html

您不应该自己关闭光标，我认为CursorUtil.closeSafely(cursor)确实如此。

您可以使用switchMap运算符来实现它。它正是我们想要的

private PublishSubject<Cursor> cursorSubject = PublishSubject.create()

public void onCreate(Bundle savedInstanceState) {
    cursorSubject
        .switchMap(new Func1<Cursor, Observable<List<Field>>>() {
             @Override public Observable<List<Field>> call(Cursor cursor) {
                  return getFieldsObservable(cursor);
             }
        })
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe(this::showFields);
}

@Override
public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) {
    cursorSubject.onNext(cursor)
}

您现在需要修改showFields为 并getFieldsObservable考虑空Cursor