Problems on UICollectionView with Parse database

Question

I'm currently building an UICollectionView which display photos from my parse database. I've make some researches on this topic and find if there are any tutorial. I've followed the instruction give by a tutorial on YouTube: https://www.youtube.com/watch?v=OTXDi0XHsnU but the app crashed when I connected the UICollectionView with dataSource and delegate. I would like to know if there are any tutorial on this topic and how can I fix this problem. My app is written in object-c and I've checked that there are no repeat connection in the storyboard.

AlbumView.h

#import <UIKit/UIKit.h>
#import "AlbumViewCell.h"
#import <Parse/Parse.h>

@interface AlbumView : UIViewController {

    NSArray *imageFilesArray;
    NSMutableArray *imagesArray;
}

@property (assign, nonatomic) IBOutlet UICollectionView *imagesCollection;
@end

AlbumView.m

#import "AlbumView.h"

@interface AlbumView ()

@end

@implementation AlbumView

@synthesize imagesCollection;


- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    [self queryParseMethod];

}

- (IBAction)cancel:(id)sender {
    [self dismissViewControllerAnimated:YES completion:nil];
}

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

- (void)queryParseMethod {
    NSLog(@"Start query");
    PFQuery *query = [PFQuery queryWithClassName:@"ati8"];

    [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
        if (!error) {
            imageFilesArray = [[NSArray alloc] initWithArray:objects];
            NSLog(@"%@", imageFilesArray);

            [imagesCollection reloadData];
        }
    }];
}

-(NSInteger)numberOfSelectionsInCollectionView:(UICollectionView *)
collectionView {
    return 1;
}

-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSelection: (NSInteger)selection {
    return [imageFilesArray count];
}

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {

    static NSString *cellIdentifer = @"imageCell";
    AlbumViewCell *cell = (AlbumViewCell *)[collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifer forIndexPath:indexPath];

    PFObject *imageObject = [imageFilesArray objectAtIndex:indexPath.row];
    PFFile *imageFile = [imageObject objectForKey:@"photos"];

    [imageFile getDataInBackgroundWithBlock:^(NSData *data, NSError *error) {
        if (!error) {
            cell.parseImage.image = [UIImage imageWithData:data];
            [imagesCollection reloadData];
        }
    }];

    return cell;
}

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {

}

@end

AlbumViewCell.h

#import <UIKit/UIKit.h>

@interface AlbumViewCell : UICollectionViewCell

@property (assign, nonatomic) IBOutlet UIImageView *parseImage;
@end

AlbumViewCell.m

#import "AlbumViewCell.h"

@implementation AlbumViewCell

@synthesize parseImage;

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        // Custom initialization
    }
    return self;
}

@end

---

UPDATE: added crash report

2015-08-31 16:31:01.118 HikingHK Online[14345:444090] Start query
2015-08-31 16:31:01.140 HikingHK Online[14345:444090] -[AlbumView collectionView:numberOfItemsInSection:]: unrecognized selector sent to instance 0x7fce7276d1f0
2015-08-31 16:31:01.147 HikingHK Online[14345:444090] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[AlbumView collectionView:numberOfItemsInSection:]: unrecognized selector sent to instance 0x7fce7276d1f0'
*** First throw call stack:
(
    0   CoreFoundation                      0x0000000108937c65 __exceptionPreprocess + 165
    1   libobjc.A.dylib                     0x00000001085d0bb7 objc_exception_throw + 45
    2   CoreFoundation                      0x000000010893f0ad -[NSObject(NSObject) doesNotRecognizeSelector:] + 205
    3   CoreFoundation                      0x000000010889513c ___forwarding___ + 988
    4   CoreFoundation                      0x0000000108894cd8 _CF_forwarding_prep_0 + 120
    5   UIKit                               0x000000010938f5fd -[UICollectionViewData _updateItemCounts] + 335
    6   UIKit                               0x0000000109391c09 -[UICollectionViewData numberOfSections] + 22
    7   UIKit                               0x000000010937b771 -[UICollectionViewFlowLayout _getSizingInfos] + 347
    8   UIKit                               0x000000010937c8a9 -[UICollectionViewFlowLayout _fetchItemsInfoForRect:] + 526
    9   UIKit                               0x00000001093780ed -[UICollectionViewFlowLayout prepareLayout] + 251
    10  UIKit                               0x000000010938fae6 -[UICollectionViewData _prepareToLoadData] + 67
    11  UIKit                               0x00000001093901bf -[UICollectionViewData validateLayoutInRect:] + 54
    12  UIKit                               0x0000000109354850 -[UICollectionView layoutSubviews] + 194
    13  UIKit                               0x0000000108db09eb -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 536
    14  QuartzCore                          0x00000001073b1ed2 -[CALayer layoutSublayers] + 146
    15  QuartzCore                          0x00000001073a66e6 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 380
    16  UIKit                               0x0000000108da4635 -[UIView(Hierarchy) layoutBelowIfNeeded] + 607
    17  UIKit                               0x0000000108e9141e -[UINavigationController _layoutViewController:] + 1135
    18  UIKit                               0x0000000108e8e468 -[UINavigationController navigationTransitionView:didEndTransition:fromView:toView:] + 522
    19  UIKit                               0x0000000108d900f6 -[UIViewAnimationState sendDelegateAnimationDidStop:finished:] + 209
    20  UIKit                               0x0000000108d8e430 +[UIViewAnimationState popAnimationState] + 289
    21  UIKit                               0x0000000109095036 -[UINavigationTransitionView transition:fromView:toView:] + 2569
    22  UIKit                               0x0000000108e92170 -[UINavigationController _startTransition:fromViewController:toViewController:] + 2984
    23  UIKit                               0x0000000108e92408 -[UINavigationController _startDeferredTransitionIfNeeded:] + 523
    24  UIKit                               0x0000000108e92ece -[UINavigationController __viewWillLayoutSubviews] + 43
    25  UIKit                               0x0000000108fdd6d5 -[UILayoutContainerView layoutSubviews] + 202
    26  UIKit                               0x0000000108db09eb -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 536
    27  QuartzCore                          0x00000001073b1ed2 -[CALayer layoutSublayers] + 146
    28  QuartzCore                          0x00000001073a66e6 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 380
    29  QuartzCore                          0x00000001073a6556 _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 24
    30  QuartzCore                          0x000000010731286e _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 242
    31  QuartzCore                          0x0000000107313a22 _ZN2CA11Transaction6commitEv + 462
    32  UIKit                               0x0000000108d2e9ed -[UIApplication _reportMainSceneUpdateFinished:] + 44
    33  UIKit                               0x0000000108d2f6b1 -[UIApplication _runWithMainScene:transitionContext:completion:] + 2648
    34  UIKit                               0x0000000108d2e095 -[UIApplication workspaceDidEndTransaction:] + 179
    35  FrontBoardServices                  0x000000010e09b5e5 __31-[FBSSerialQueue performAsync:]_block_invoke_2 + 21
    36  CoreFoundation                      0x000000010886b41c __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    37  CoreFoundation                      0x0000000108861165 __CFRunLoopDoBlocks + 341
    38  CoreFoundation                      0x0000000108860947 __CFRunLoopRun + 887
    39  CoreFoundation                      0x0000000108860366 CFRunLoopRunSpecific + 470
    40  UIKit                               0x0000000108d2db02 -[UIApplication _run] + 413
    41  UIKit                               0x0000000108d308c0 UIApplicationMain + 1282
    42  HikingHK Online                     0x0000000106c273e3 main + 99
    43  libdyld.dylib                       0x000000010dc6f145 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb) 

Code that crash the app:

return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));

Answer 1

You have this method in your code:

-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSelection: (NSInteger)selection {
    return [imageFilesArray count];
}

But you dont mean numberOfItemsInSelection, you mean numberOfItemsInSection !!!

-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection: (NSInteger)selection {
    return [imageFilesArray count];
}

Then it will work

Answer 2

You have to, in short, use DLImageLoader.

General example

[DLImageLoader loadImageFromURL:imageURL
           completed:^(NSError *error, NSData *imgData) {
           imageView.image = [UIImage imageWithData:imgData];
        [imageView setContentMode:UIViewContentModeCenter];
    }];

(i) Parse's demo "PFImage" class which supposedly caches images, doesn't really work.

(ii) You can see many posts about this around the net

(iii) If you make an app that handles images, you must deal with caching. It's actually "all about" the caching.

(iv) If you don't prefer DLImageLoader, use an alternative (SDWebImage or whatever), or perhaps write your own, but DLImageLoader is the simplest and most reliable.