Simple UICollectionView example Objective-C: http://swiftiostutorials.com/tutorial-using-uicollectionview-uicollectionviewflowlayout/

GalleryItem.h

#import <Foundation/Foundation.h>

@interface GalleryItem : NSObject

@property (nonatomic, strong) NSString *itemImage;

+ (instancetype)galleryItemWithDictionary:(NSDictionary *)dictionary;

@end

GalleryItem.m

#import "GalleryItem.h"

@implementation GalleryItem

+ (instancetype)galleryItemWithDictionary:(NSDictionary *)dictionary
{
    GalleryItem *item = [[GalleryItem alloc] init];
    
    item.itemImage = dictionary[@"itemImage"];
    
    return item;
}

@end

GalleryItemCollectionViewCell.h

#import <UIKit/UIKit.h>

@class GalleryItem;

@interface GalleryItemCollectionViewCell : UICollectionViewCell

@property (nonatomic, weak) IBOutlet UIImageView *itemImageView;

- (void)setGalleryItem:(GalleryItem *)item;

@end

GalleryItemCollectionViewCell.m

#import "GalleryItemCollectionViewCell.h"
#import "GalleryItem.h"

@implementation GalleryItemCollectionViewCell

- (void)setGalleryItem:(GalleryItem *)item
{
    _itemImageView.image = [UIImage imageNamed:item.itemImage];
}

@end

GalleryItemCommentView.h

#import <UIKit/UIKit.h>

@interface GalleryItemCommentView : UICollectionReusableView

@property (nonatomic, weak) IBOutlet UILabel *commentLabel;

@end

items.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<array>
	<dict>
		<key>itemImage</key>
		<string>pic_1</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_2</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_3</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_4</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_5</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_6</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_7</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_8</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_9</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_10</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_11</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_12</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_1</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_2</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_3</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_4</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_5</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_6</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_7</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_8</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_9</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_10</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_11</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_12</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_1</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_2</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_3</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_4</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_5</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_6</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_7</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_8</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_9</string>
	</dict>
	<dict>
		<key>itemImage</key>
		<string>pic_10</string>
	</dict>
</array>
</plist>

ViewController.h

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController

@property (nonatomic, weak) IBOutlet UICollectionView *collectionView;

@end

ViewController1.m

#import "ViewController.h"

@interface ViewController ()
{
    NSArray *_galleryItems; // NEW PROPERTY
}

@end

@implementation ViewController
@end

ViewController10.m

#import "ViewController.h"

@implementation ViewController

...

#pragma mark - UICollectionViewDataSource

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

@end

ViewController11.m

#import "ViewController.h"

@implementation ViewController

#pragma mark - UICollectionViewDataSource

...

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

@end

ViewController12.m

#import "ViewController.h"

@implementation ViewController

#pragma mark - UICollectionViewDataSource

...

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    GalleryItemCollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"GalleryItemCollectionViewCell" forIndexPath:indexPath];
    [cell setGalleryItem:_galleryItems[indexPath.row]];
    return cell;
}

@end

ViewController13.m

#import "ViewController.h"

@implementation ViewController

#pragma mark - UICollectionViewDataSource

...

- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
{
    GalleryItemCommentView *commentView = [collectionView dequeueReusableSupplementaryViewOfKind:kind withReuseIdentifier:@"GalleryItemCommentView" forIndexPath:indexPath];
    
    commentView.commentLabel.text = [NSString stringWithFormat:@"Supplementary view of kind %@", kind];
    return commentView;
}

@end

ViewController2.h

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController <UICollectionViewDataSource, UICollectionViewDelegate>

@property (nonatomic, weak) IBOutlet UICollectionView *collectionView;

@end

ViewController2.m

#import "ViewController.h"

...

@implementation ViewController

- (void)initGalleryItems
{
    NSMutableArray *items = [NSMutableArray array];
    
    NSString *inputFile = [[NSBundle mainBundle] pathForResource:@"items" ofType:@"plist"];
    NSArray *inputDataArray = [NSArray arrayWithContentsOfFile:inputFile];
    
    for (NSDictionary *inputItem in inputDataArray)
    {
        [items addObject:[GalleryItem galleryItemWithDictionary:inputItem]];
    }
    
    _galleryItems = items;
}

@end

ViewController20.m

#import "ViewController.h"

@implementation ViewController

...

#pragma mark - UICollectionViewDelegate

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
    UIAlertController *controller = [UIAlertController alertControllerWithTitle: @"didSelectItemAtIndexPath:"
                                                                        message: [NSString stringWithFormat: @"Indexpath = %@", indexPath]
                                                                 preferredStyle: UIAlertControllerStyleAlert];
    
    UIAlertAction *alertAction = [UIAlertAction actionWithTitle: @"Dismiss"
                                                          style: UIAlertActionStyleDestructive
                                                        handler: nil];
    
    [controller addAction: alertAction];
    
    [self presentViewController: controller animated: YES completion: nil];
}

@end

ViewController3.m

#import "ViewController.h"

...

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    [self initGalleryItems];
    [_collectionView reloadData];
}

- (void)initGalleryItems
{
    ....
}

@end

ViewController30.m

#import "ViewController.h"

@implementation ViewController

...

#pragma mark - UICollectionViewFlowLayout

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    CGFloat picDimension = self.view.frame.size.width / 4.0f;
    return CGSizeMake(picDimension, picDimension);
}

- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout insetForSectionAtIndex:(NSInteger)section
{
    CGFloat leftRightInset = self.view.frame.size.width / 14.0f;
    return UIEdgeInsetsMake(0, leftRightInset, 0, leftRightInset);
}

@end