#import <Foundation/Foundation.h>

@interface LoadGalleryThumbOp : NSObject

@property (nonatomic, strong) NSString *documentPath;
@property (nonatomic, assign) NSInteger indexPathInTableView;
@property (nonatomic, weak) id<LoadGalleryThumbDelegate> delegate;

- (instancetype)init;
- (void)startDownload;
- (void)setImageFromDisk:(NSString *)filePath;

@end

#import "LoadGalleryThumbOp.h"

@implementation LoadGalleryThumbOp

- (instancetype)init {
    self = [super init];
    if (self) {
        _documentPath = @"";
        _indexPathInTableView = 0;
        _delegate = nil;
    }
    return self;
}

- (void)startDownload {
    // Implement download logic here
}

- (void)setImageFromDisk:(NSString *)filePath {
    // Implement image loading logic here
}

@end

#import <UIKit/UIKit.h>
#import "LoadGalleryThumbOp.h"

@interface PhotoGalleryVC : UIViewController <LoadGalleryThumbDelegate>

@property (nonatomic, strong) UITableView *albumListTableView;
@property (nonatomic, strong) NSMutableDictionary *imageDownloadsInProgress;

- (void)viewDidLoad {
    // Implement view did load logic here
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    // Implement cell configuration logic here
}

- (void)appImageDidLoad:(NSIndexPath *)indexPath {
    // Implement image loaded delegate method here
}

@end

#import "PhotoGalleryVC.h"

@implementation PhotoGalleryVC

- (void)viewDidLoad {
    [super viewDidLoad];
    self.imageDownloadsInProgress = [NSMutableDictionary dictionary];
    // ... implement view did load logic ...
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"Cell"];
    if (!cell) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"Cell"];
    }
    // ... implement cell configuration logic ...
    
    LoadGalleryThumbOp *galleryThumbDownloader = [imageDownloadsInProgress objectForKey:indexPath];
    if (galleryThumbDownloader == nil) {
        // ... create new downloader and start download ...
    } else if (galleryThumbDownloader.imageDownload == NO) {
        // ... restart download if image is not yet loaded ...
    }
    
    return cell;
}

- (void)appImageDidLoad:(NSIndexPath *)indexPath {
    CustomCellPhotoGalary *cell = (CustomCellPhotoGalary*)[albumListTableView cellForRowAtIndexPath:indexPath];
    LoadGalleryThumbOp *galleryThumbDownloader = [imageDownloadsInProgress objectForKey:indexPath];
    if (galleryThumbDownloader != nil && galleryThumbDownloader.imageDownload) {
        // ... load image from disk and configure cell ...
    } else {
        // ... display no thumb image ...
    }
    
    [albumListTableView reloadData];
}

@end

@protocol LoadGalleryThumbDelegate <NSObject>

- (void)appImageDidLoad:(NSIndexPath *)indexPath;

@end

#import <UIKit/UIKit.h>

@interface CustomCellPhotoGallery : UITableViewCell

@property (nonatomic, strong) UIImageView *albumCoverImageView;
@property (nonatomic, strong) UIActivityIndicatorView *activityIndicator;

@end

- (void)viewDidLoad {
    [super viewDidLoad];
    self.albumListTableView = [[UITableView alloc] initWithFrame:self.view.bounds style:UITableViewStyleGrouped];
    
    CustomCellPhotoGallery *cell = [[CustomCellPhotoGallery alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"Cell"];
    cell.albumCoverImageView.image = [UIImage imageNamed:@"no_thumb.png"];
    self.albumListTableView.registerClass:[CustomCellPhotoGallery class], forCellReuseIdentifier:@"Cell"];
    
    self.imageDownloadsInProgress = [NSMutableDictionary dictionary];
    // ... implement rest of the view controller ...
}

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
    LoadGalleryThumbOp *galleryThumbDownloader = [self.imageDownloadsInProgress objectForKey:indexPath];
    if (galleryThumbDownloader == nil) {
        galleryThumbDownloader = [[LoadGalleryThumbOp alloc] init];
        galleryThumbDownloader.documentPath = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0] stringByAppendingPathComponent:[NSString stringWithFormat:@"Gallery/%@/images/thumb.jpg", [[self albumListTableView cellForRowAtIndexPath:indexPath].albumCoverImageView.image]];
        gallerythumbDownloader.indexPathInTableView = indexPath;
        galleryThumbDownloader.delegate = self;
        [self.imageDownloadsInProgress setObject:galleryThumbDownloader forKey:indexPath];
    }
    
    [galleryThumbDownloader startDownload];
}

- (void)appImageDidLoad:(NSIndexPath *)indexPath {
    CustomCellPhotoGallery *cell = [self.albumListTableView cellForRowAtIndexPath:indexPath];
    LoadGalleryThumbOp *gallerythumbDownloader = [self.imageDownloadsInProgress objectForKey:indexPath];
    if (gallerythumbDownloader != nil && gallerythumbDownloader.imageDownload) {
        cell.albumCoverImageView.image = [[UIImage alloc] initWithData:[NSData dataWithContentsOfFile:gallerythumbDownloader.documentPath]];
    }
}

@end