ios - 带有 sortDescriptors 的 NSFetchResultsController 错误

Question

我有一个表格视图控制器，其中包含一个由 fetchResultsController 处理的 Formations 列表。

这是我的核心数据实体的样子：

我尝试按dateRange对我的 fetchResultsController 进行排序，如下所示：

// |fetchedResultsController| custom setter
- (NSFetchedResultsController *)fetchedResultsController {
    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:self.fetchRequest managedObjectContext:self.mainManagedObjectContext sectionNameKeyPath:@"dateRange" cacheName:kFormationsFetchedCacheName];
    _fetchedResultsController.delegate = self;

    return _fetchedResultsController;
}

// |fetchRequest| custom setter
- (NSFetchRequest *)fetchRequest {
    if (_fetchRequest != nil) {
        return _fetchRequest;
    }

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"student == %@", self.currentStudent];
    NSSortDescriptor *dateDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"dateRange" ascending:NO];
    NSSortDescriptor *nameDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"name" ascending:NO];

    _fetchRequest = [[NSFetchRequest alloc] initWithEntityName:kBSFormation];
    _fetchRequest.predicate = predicate;
    _fetchRequest.sortDescriptors = [NSArray arrayWithObjects: dateDescriptor, nameDescriptor, nil];

    return _fetchRequest;
}

当我尝试添加第一个 Formation 时，一切都很好，但对于接下来的，我有这些错误：

2013-01-30 22:43:08.370 [7202:c07] -[BSDateRange compare:]: unrecognized selector sent to instance 0x81781a0

2013-01-30 22:43:08.371 [7202:c07] CoreData: error: Serious application error.  Exception was caught during Core Data change processing.  This is usually a bug within an observer of NSManagedObjectContextObjectsDidChangeNotification.  -[BSDateRange compare:]: unrecognized selector sent to instance 0x81781a0 with userInfo (null)

2013-01-30 22:43:08.372 [7202:c07] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[BSDateRange compare:]: unrecognized selector sent to instance 0x81781a0'

如果我评论这一行：NSSortDescriptor *dateDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"dateRange" ascending:NO];，它可以工作，但我的表格视图很乱，因为sectionNameKeyPath 设置为dateRange

有人正在弄清楚这里的问题是什么？ ：/

Answer 1

您告诉它根据dateRange 关系进行排序。但是dateRange 是与BSDateRange 的关系，Core Data 如何比较它们？它应该使用from，还是to，或者它们的某种组合？您不能只告诉它对这样的对象进行排序，因为排序应该如何工作并不明显。

相反，首先弄清楚排序的含义。然后适当地修改您的排序描述符。例如，如果您决定排序取决于 from 值，请将排序描述符更改为使用 from 的键路径：

NSSortDescriptor *dateDescriptor = [NSSortDescriptor
    sortDescriptorWithKey:@"dateRange.from"
    ascending:NO];

如果您需要同时基于from 和to 进行排序，请使用多个排序描述符。