Layout:

.h

#import <UIKit/UIKit.h>

@interface FlowLayout : UICollectionViewFlowLayout

@end

.m

#import "FlowLayout.h"
@implementation FlowLayout

- (void)prepareLayout{
    
    [super prepareLayout];
    
    self.itemSize = self.collectionView.bounds.size;
    self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    self.sectionInset = UIEdgeInsetsMake(0, 0, 0, 0);
    self.minimumLineSpacing = 0;
    self.minimumInteritemSpacing = 0;
    self.collectionView.showsVerticalScrollIndicator = NO;
    self.collectionView.showsHorizontalScrollIndicator = NO;
    self.collectionView.bounces = NO;
    self.collectionView.pagingEnabled = YES;
    
}

@end

CollectionView

.h

#import <UIKit/UIKit.h>

@interface CollectionView : UICollectionView

- (instancetype)initWithUrls:(NSArray <NSURL *>*)urls;

@end

.m

#import "CollectionView.h"
#import "CollectionViewCell.h"
#import "FlowLayout.h"

static NSString * const reuseIdentifier = @"reuseIdentifier";

@interface CollectionView () <UICollectionViewDataSource,UICollectionViewDelegate>

@end

@implementation CollectionView{
    
    NSArray <NSURL *>*_urlArray;
}

- (instancetype)initWithUrls:(NSArray <NSURL *>*)urls{
    self = [super initWithFrame:CGRectZero collectionViewLayout:[[FlowLayout alloc]init]];
    if (self) {
        _urlArray = urls;
        
        [self registerClass:[CollectionViewCell class] forCellWithReuseIdentifier:reuseIdentifier];
        self.dataSource = self;
        self.delegate = self;
        
        dispatch_async(dispatch_get_main_queue(), ^{
            
            NSIndexPath *indexPath = [NSIndexPath indexPathForItem:_urlArray.count inSection:0];
            [self scrollToItemAtIndexPath:indexPath atScrollPosition:0 animated:NO];
        });
        
        
    }
    return self;
}


#pragma mark --UICollectionViewDataSource

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
    
    return _urlArray.count * 2;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
    
    CollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifier forIndexPath:indexPath];

    cell.url = _urlArray[indexPath.item % _urlArray.count];
    
    return cell;
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView{
    
    CGFloat contentOffset = scrollView.contentOffset.x;
    
    NSInteger currentImg = contentOffset / scrollView.bounds.size.width;
    
    
    if (currentImg == 0) {
        
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:_urlArray.count inSection:0];
        [self scrollToItemAtIndexPath:indexPath atScrollPosition:0 animated:NO];
    }
    
    if (currentImg == [self numberOfItemsInSection:0] - 1) {
        
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:_urlArray.count - 1 inSection:0];
        [self scrollToItemAtIndexPath:indexPath atScrollPosition:0 animated:NO];
    }
    
}

@end

在自定义的构造函数中,手动滚动CollectionView时,之所以使用了主队列异步,是由于构造函数在数据源方法前执行,使用主队列异步,保证数据源方法执行完毕后,再滚动CollectionView

CollectionCell

.h

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

@interface CollectionViewCell : UICollectionViewCell

@property (nonatomic,strong) NSURL *url;

@end

.m

#import "CollectionViewCell.h"
#import "Masonry.h"

@interface CollectionViewCell()

@property (nonatomic,strong) NSOperationQueue *queue;

@end

@implementation CollectionViewCell{
    
    UIImageView *_imageView;
}


- (instancetype)initWithFrame:(CGRect)frame{
    self = [super initWithFrame:frame];
    if (self) {
        NSLog(@"%s",__FUNCTION__);
        _imageView = [[UIImageView alloc]initWithFrame:self.bounds];
        [self.contentView addSubview:_imageView];
        
    }
    return self;
}


- (void)setUrl:(NSURL *)url{
    
    _url = url;
    
    NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
        
        NSData *data = [NSData dataWithContentsOfURL:url];
        
        UIImage *image = [UIImage imageWithData:data];
        
        dispatch_sync(dispatch_get_main_queue(), ^{
            
            _imageView.image = image;
            
        });
        
    }];
    
    [self.queue addOperation:operation];
    
    
}


- (NSOperationQueue *)queue{
    if (_queue == nil) {
        _queue = [[NSOperationQueue alloc] init];
    }
    return  _queue;
}

@end

将CollectionView分别抽取出CollectionView,Layout和Cell三个类,让代码的可读性更好,当外界使用使用:

#import "ViewController.h"
#import "CollectionView.h"
#import "Masonry/Masonry.h"

@interface ViewController ()

@end

@implementation ViewController{
    NSArray *_urlArr;
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [self loadData];
    
    self.view.backgroundColor = [UIColor whiteColor];
    
    CollectionView *collectionView = [[CollectionView alloc]initWithUrls:_urlArr];
    
    [self.view addSubview:collectionView];
    [collectionView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.left.mas_equalTo(self.view).mas_offset(20);
        make.top.mas_equalTo(self.view).mas_offset(100);
        make.right.mas_equalTo(self.view).mas_offset(-20);
        make.height.mas_equalTo(300);
    }];
    
}

- (void)loadData{
    
    NSMutableArray *mArr = [NSMutableArray array];
    
    for (int i = 0; i<3; i ++) {
        NSString *urlString = @"http://img2.duitang.com/uploads/item/201208/18/20120818150713_zarnG.jpeg";
        [mArr addObject:[NSURL URLWithString:urlString]];
    }
    
    _urlArr = mArr.copy;
}

@end