Ich habe einen ViewController mit einem CollectionView im Inneren. Wenn die Ansicht geladen wird, werden die sichtbaren Zellen (9 Zellen) korrekt angezeigt. Wenn ich nach unten scrolle, möchte ich die sichtbaren Elemente in der Sammlungsansicht mit loadImagesForOnscreenRows laden, indem ich den IndexPathsForVisibleItems für partnerCollectionView aufrufe. Bei loadImagesForOnscreenRows enthält der IndexPathsForVisibleItems jedoch immer die ersten 9 Zellen, auch wenn die Zellen 10 bis 18 auf dem Bildschirm sichtbar sein sollten. Der Code, den ich benutze, ist:

#import "PartnerListViewController.h"
#import "AppDelegate.h"
#import "Partner.h"
#import "ImageLoader.h"
#import "PartnerDetailViewController.h"

@interface PartnerListViewController ()

@end

@implementation PartnerListViewController

@synthesize lblTitle;
@synthesize partnerCollectionView;

@synthesize imageDownloadsInProgress;

@synthesize fetchedResultsController;
@synthesize managedObjectContext;

- (void)viewDidLoad
{
   [super viewDidLoad];
   // Do any additional setup after loading the view.

   AppDelegate * appDelegate = (AppDelegate *) [[UIApplication sharedApplication] delegate];
   managedObjectContext = [appDelegate managedObjectContext];
   imageDownloadsInProgress = [NSMutableDictionary dictionary];
   appDelegate = nil;

   [self setupFetchedResultsController];
   [partnerCollectionView reloadData];
}

- (void)didReceiveMemoryWarning
{
   [super didReceiveMemoryWarning];
   // Dispose of any resources that can be recreated.
}

#pragma mark - Collection view data source

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView {
    return [[fetchedResultsController sections] count];
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
   id <NSFetchedResultsSectionInfo> sectionInfo = [[fetchedResultsController sections] objectAtIndex:section];
   return [sectionInfo numberOfObjects];
}

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
   static NSString *CellIdentifier = @"PartnerCell";
   UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:CellIdentifier forIndexPath:indexPath];

   Partner *partner = [self.fetchedResultsController objectAtIndexPath:indexPath];

   UIImageView *imageView = (UIImageView *)[cell viewWithTag:100];
   if (!partner.image)
   {
       imageView.image = [UIImage imageNamed:@"annotationMap.png"];
       if (self.partnerCollectionView.dragging == NO && self.partnerCollectionView.decelerating == NO)
       {
           [self startDownload:partner.imageUrl forIndexPath:indexPath];
       }
   } else {
       imageView.image = [UIImage imageWithData:partner.image];
   }

   UILabel *lblTitlePartner = (UILabel *)[cell viewWithTag:101];
   lblTitlePartner.text = partner.title;
   lblTitlePartner.font = [UIFont fontWithName:fontName size:10];

   return cell;
}

#pragma mark - Table cell image support
- (void)startDownload:(NSString *)urlString forIndexPath:(NSIndexPath *)indexPath
{
   NSLog(@"startDownload:%ld", (long)indexPath.row);

   ImageLoader *imageLoader = [imageDownloadsInProgress objectForKey:indexPath];
   imageLoader = [[ImageLoader alloc] init];
   imageLoader.urlString = urlString;
   imageLoader.indexPathTableView = indexPath;
   imageLoader.delegate = (id)self;
   [imageDownloadsInProgress setObject:imageLoader forKey:indexPath];
   [imageLoader startDownload];
}

// this method is used in case the user scrolled into a set of cells that don't have their app icons yet
- (void)loadImagesForOnscreenRows
{
   NSArray *visiblePaths = [self.partnerCollectionView indexPathsForVisibleItems];
   NSMutableArray *rowsArray = [NSMutableArray arrayWithCapacity:[visiblePaths count]];
   [visiblePaths enumerateObjectsUsingBlock:^(NSIndexPath *indexPath, NSUInteger idx, BOOL *stop) {
       NSLog(@"loadImagesForOnscreenRows1%@", @(indexPath.item));
       [rowsArray addObject:@(indexPath.item)];
   }];
   for (NSIndexPath *indexPath in visiblePaths)
   {
       NSLog(@"loadImagesForOnscreenRows2:%ld", (long)indexPath.row);

       Partner *item = [self.fetchedResultsController objectAtIndexPath:indexPath];

       if (!item.image) // avoid the app icon download if the app already has an icon
       {
           [self startDownload:item.imageUrl forIndexPath:indexPath];
       }
   }
}

// called by our ImageDownloader when an icon is ready to be displayed
- (void)imageLoaderDidFinishDownloading:(NSIndexPath *)indexPath
{
   NSLog(@"imageLoaderDidFinishDownloading:%ld", (long)indexPath.row);

   ImageLoader *imageLoader = [imageDownloadsInProgress objectForKey:indexPath];
   if (imageLoader != nil)
   {
       // Save the newly loaded image
       Partner *item = [self.fetchedResultsController objectAtIndexPath:indexPath];
       item.image = UIImageJPEGRepresentation(imageLoader.image, 1.0);

       [self performSelectorOnMainThread:@selector(saveItem) withObject:nil waitUntilDone:YES];
       [self performSelectorOnMainThread:@selector(reloadData) withObject:nil waitUntilDone:YES];
   }
}

- (void)saveItem
{
   [self.managedObjectContext save:nil];
}

- (void)reloadData
{
   [self.partnerCollectionView reloadData];
}

#pragma mark deferred image loading (UIScrollViewDelegate)

// Load images for all onscreen rows when scrolling is finished
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
   if (!decelerate)
{
       [self loadImagesForOnscreenRows];
   }
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
   [self loadImagesForOnscreenRows];
}

- (void)setupFetchedResultsController
{
   // 1 - Decide what Entity you want
   NSString *entityName = @"Partner"; // Put your entity name here
   NSLog(@"Setting up a Fetched Results Controller for the Entity named %@", entityName);

   // 2 - Request that Entity
   NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:entityName];

   // 4 - Sort it if you want
   request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"title" ascending:NO selector:@selector(localizedCaseInsensitiveCompare:)]];
   // 5 - Fetch it
   self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:nil];
   NSError *error = nil;
  if (![[self fetchedResultsController] performFetch:&error]) {
      NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
   }
}

@end

Und das Ergebnis in der Ausgabe:

Erstausstellung der sichtbaren Gegenstände

2013-09-02 06:45:21.940 [2564:c07] startDownload:0
2013-09-02 06:45:21.943 [2564:c07] imageLoaderDidFinishDownloading:0
2013-09-02 06:45:21.950 [2564:c07] startDownload:1
2013-09-02 06:45:21.951 [2564:c07] imageLoaderDidFinishDownloading:1
2013-09-02 06:45:21.958 [2564:c07] startDownload:2
2013-09-02 06:45:21.959 [2564:c07] imageLoaderDidFinishDownloading:2
2013-09-02 06:45:21.965 [2564:c07] startDownload:3
2013-09-02 06:45:22.063 [2564:c07] imageLoaderDidFinishDownloading:3
2013-09-02 06:45:22.072 [2564:c07] startDownload:4
2013-09-02 06:45:22.073 [2564:c07] imageLoaderDidFinishDownloading:4
2013-09-02 06:45:22.081 [2564:c07] startDownload:5
2013-09-02 06:45:22.082 [2564:c07] imageLoaderDidFinishDownloading:5
2013-09-02 06:45:22.089 [2564:c07] startDownload:6
2013-09-02 06:45:22.090 [2564:c07] imageLoaderDidFinishDownloading:6
2013-09-02 06:45:22.098 [2564:c07] startDownload:7
2013-09-02 06:45:22.099 [2564:c07] imageLoaderDidFinishDownloading:7
2013-09-02 06:45:22.104 [2564:c07] startDownload:8
2013-09-02 06:45:22.163 [2564:c07] imageLoaderDidFinishDownloading:8

Nach dem Scrollen zu Punkt 10 bis 19:

2013-09-02 06:45:26.212 [2564:c07] loadImagesForOnscreenRows1:8
2013-09-02 06:45:26.212 [2564:c07] loadImagesForOnscreenRows1:0
2013-09-02 06:45:26.212 [2564:c07] loadImagesForOnscreenRows1:1
2013-09-02 06:45:26.212 [2564:c07] loadImagesForOnscreenRows1:6
2013-09-02 06:45:26.213 [2564:c07] loadImagesForOnscreenRows1:2
2013-09-02 06:45:26.213 [2564:c07] loadImagesForOnscreenRows1:3
2013-09-02 06:45:26.213 [2564:c07] loadImagesForOnscreenRows1:4
2013-09-02 06:45:26.213 [2564:c07] loadImagesForOnscreenRows1:5
2013-09-02 06:45:26.213 [2564:c07] loadImagesForOnscreenRows1:7
2013-09-02 06:45:26.214 [2564:c07] loadImagesForOnscreenRows2:8
2013-09-02 06:45:26.214 [2564:c07] loadImagesForOnscreenRows2:0
2013-09-02 06:45:26.214 [2564:c07] loadImagesForOnscreenRows2:1
2013-09-02 06:45:26.214 [2564:c07] loadImagesForOnscreenRows2:6
2013-09-02 06:45:26.214 [2564:c07] loadImagesForOnscreenRows2:2
2013-09-02 06:45:26.215 [2564:c07] loadImagesForOnscreenRows2:3
2013-09-02 06:45:26.215 [2564:c07] loadImagesForOnscreenRows2:4
2013-09-02 06:45:26.215 [2564:c07] loadImagesForOnscreenRows2:5
2013-09-02 06:45:26.215 [2564:c07] loadImagesForOnscreenRows2:7

Wie Sie nach dem Scrollen sehen können, bleiben die sichtbaren Indexpfade der Elemente gleich. Hat jemand anderes dies erlebt oder eine Idee für eine Lösung? Oder verstehe ich ein Prinzip der Sammlungsbetrachtung falsch?

Vielen Dank im Voraus! Herzliche Grüße, Jan