ios - 数据源方法 collectionView : numberOfItemsInSection: Crashes App "index 0 beyond bounds for empty array"

Question

我整天都在处理这段代码，所以它可能只是我忽略的一些愚蠢的东西，但我似乎无法找出问题所在，而且我无计可施。

-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    if(collectionView == self.titleBarCollectionView) {
        return 1;
    }
    NSInteger photoCount = [[self.fetchedResultsController fetchedObjects] count];
    NSInteger sectionCount = (photoCount + 3) / 3;
    return sectionCount;
}

-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    if(collectionView != self.titleBarCollectionView) {
        id <NSFetchedResultsSectionInfo> sectionInfo = [self.fetchedResultsController sections][section];
        NSInteger numberInSection = [sectionInfo numberOfObjects];
        if(numberInSection < 3) numberInSection++;
        return numberInSection;
    }
    return 3;
}

我有一个显示用户照片的 UICollectionView。照片取自核心数据，但是 Collection View 中的第一个索引(项目 0，部分 0)被替换为“添加照片”图像，并且所有照片的 indexPaths 都进行了调整以适应这一点。它工作了几个小时都很好，但出于某种原因在删除应用程序(重置核心数据)后，我似乎无法加载 View 。它立即崩溃并显示错误 <strong>* Terminating app due to uncaught exception 'NSRangeException', reason: '*</strong> -[__NSArrayM objectAtIndex:]: index 0 beyond bounds for empty array'当它到达 id sectionInfo = [self.fetchedResultsController sections][section]; 行时.我这辈子都弄不明白。这是我的其余数据源方法。

-(void)configureCell:(UICollectionViewCell *)cell atIndexPath:(NSIndexPath *)indexPath
{
    if([cell.reuseIdentifier isEqualToString:@"Profile Title Cell"]) { 
        ... Irrelevant code ...
    } else if(indexPath.row == 0 && indexPath.section == 0) {
        UIImageView *imageView = (UIImageView *)[cell viewWithTag:300];
        imageView.image = [UIImage imageNamed:@"plusIcon.png"];
    } else {
        NSIndexPath *newIndexPath = [self newIndexPathFromIndexPath:indexPath];
        Photo *photo = [self.fetchedResultsController objectAtIndexPath:newIndexPath];
        UIImageView *imageView = (UIImageView *)[cell viewWithTag:300];

        NSData *imageData = [[NSData alloc] initWithBase64EncodedString:photo.thumbnailData options:0];
        imageView.image = [UIImage imageWithData:imageData];

        if(photo.imageData) {
            imageView.image = [UIImage imageWithData:photo.imageData];
        } else if(photo.imageURL && ![photo.imageURL isEqualToString:@""]) {
            NSString *imageURL = [NSString stringWithFormat:@"%@%@", PHOTOS_BASE_URL, photo.imageURL];
            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];
                self.activeThreads += 1;
                NSData *imageURLData = [NSData dataWithContentsOfURL:[NSURL URLWithString:imageURL]];
                dispatch_async(dispatch_get_main_queue(), ^{
                    photo.imageData = imageURLData;
                    [self.imageCollectionView reloadItemsAtIndexPaths:@[indexPath]];
                    self.activeThreads -= 1;
                    if(self.activeThreads < 1)
                        [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];
                });
            });
        }
    }
}

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewCell *cell = nil;
    if(collectionView == self.titleBarCollectionView) {
        cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Profile Title Cell" forIndexPath:indexPath];
    } else {
        cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Profile Image Cell" forIndexPath:indexPath];
    }
    [self configureCell:cell atIndexPath:indexPath];
    return cell;
}

- (NSIndexPath *)newIndexPathFromIndexPath:(NSIndexPath *)indexPath
{
    NSInteger row = 0;
    NSInteger section = 0;
    if(indexPath.row > 0) {
        row = indexPath.row - 1;
        section = indexPath.section;
    } else {
        row = 2;
        section = indexPath.section - 1;
    }
    return [NSIndexPath indexPathForItem:row inSection:section];
}

编辑:这是我的 fetchedResultsController setter

- (NSFetchedResultsController *)fetchedResultsController
{
    if(!_fetchedResultsController) {
        NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Photo"];

        fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"uploadDate" ascending:YES]];

        fetchRequest.predicate = [NSPredicate predicateWithFormat:@"ownerID == %@", self.currentUser.userID];

        _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                                                        managedObjectContext:[RKManagedObjectStore defaultStore].mainQueueManagedObjectContext
                                                                          sectionNameKeyPath:@"sectionIdentifier"
                                                                                   cacheName:nil];

        _fetchedResultsController.delegate = self;

        [_fetchedResultsController performFetch:nil];
    }
    return _fetchedResultsController;
}

Answer 1

您在评论中说 fetchedObjects 返回 8 个对象。在 numberOfSectionsInCollectionView 中，您使用它来计算 Collection View 中的部分数:

NSInteger photoCount = [[self.fetchedResultsController fetchedObjects] count];
NSInteger sectionCount = (photoCount + 3) / 3;
return sectionCount;

假设计数真的是 8，这意味着 sectionCount 是 3(11/3，整数除法)。请注意，您并不是在询问获取的结果 Controller 它有多少个部分，而是在告诉 Collection View 它应该包含三个部分。

由于您在上面返回 3，collectionView:numberOfItemsInSection: 将被调用 3 次，section 值为 0、1 和 2。您将 section 值传递给获取结果 Controller :

id <NSFetchedResultsSectionInfo> sectionInfo = [self.fetchedResultsController sections][section];

这仅在 fetched results controller 实际上有 3 个部分时有效。 但你从来没有问过它有多少节，所以不能保证它有那么多节。在这种情况下，极有可能发生崩溃。

看起来发生的事情是:

• 创建抓取结果 Controller 时没有使用sectionNameKeyPath
• 因此 [self.fetchedResultsController sections] 返回一个空数组
• 但是您尝试在那个空数组中查找一个对象，因为您假设会有 3 个部分而没有检查这是否属实。

因此你崩溃了。您不能只告诉 fetched results controller 它应该有多少个部分。部分的数量基于获取结果，除非包含 sectionNameKeyPath，否则没有任何部分。