UICollectionView实现无限轮播

利用UICollectionView封装一个图片轮播器
.h

#import 

NS_ASSUME_NONNULL_BEGIN

@interface CarouselView : UICollectionView

@property (nonatomic, copy) NSArray  *imageNames;

- (instancetype)initWithFrame:(CGRect)frame imageNames:(NSArray *)imageNames;

@end

NS_ASSUME_NONNULL_END

.m

#import "CarouselView.h"
#import "CarouselFlowLayout.h"
#import "CarouselViewCell.h"

@interface CarouselView ()

@property (nonatomic, assign) NSInteger index;

@end

@implementation CarouselView

NSString * const reuseIdentifier = @"CarouselViewCell";

- (instancetype)initWithFrame:(CGRect)frame imageNames:(NSArray *)imageNames
{
    CarouselFlowLayout *layout = [[CarouselFlowLayout alloc] init];
    self = [super initWithFrame:frame collectionViewLayout:layout];
    if (self) {
        self.backgroundColor = [UIColor orangeColor];
        self.dataSource = self;
        self.delegate = self;
        self.pagingEnabled = YES;
        self.bounces = NO;
        self.showsHorizontalScrollIndicator = NO;
        self.imageNames = imageNames;
        [self registerClass:[CarouselViewCell class] forCellWithReuseIdentifier:reuseIdentifier];
    }
    return self;
}

- (instancetype)initWithFrame:(CGRect)frame collectionViewLayout:(UICollectionViewLayout *)layout
{
    
    self = [super initWithFrame:frame collectionViewLayout:layout];
    if (self) {
        self.backgroundColor = [UIColor orangeColor];
        self.dataSource = self;
        self.delegate = self;
        self.pagingEnabled = YES;
        self.bounces = NO;
        self.showsHorizontalScrollIndicator = NO;
        [self registerClass:[CarouselViewCell class] forCellWithReuseIdentifier:reuseIdentifier];
    }
    return self;
}

- (void)setImageNames:(NSArray *)imageNames{
    _imageNames = imageNames;
    
    NSAssert(_imageNames.count > 2, @"轮播图必须大于2张");
    
    [self scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:1 inSection:0] atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:NO];
}


#pragma mark -- UICollectionViewDataSource
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
    return self.imageNames.count;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
    CarouselViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifier forIndexPath:indexPath];
    
    NSInteger next = indexPath.item - 1;
    next = (self.index + next + self.imageNames.count) % self.imageNames.count;
    
    cell.imageName = self.imageNames[next];
    
    return cell;
}

#pragma mark -- UICollectionViewDelegate
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath{
    
}

#pragma mark -- UISCrollViewDelegate
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView{
    
    NSInteger offsetX = scrollView.contentOffset.x / scrollView.frame.size.width - 1;
    
    self.index =  (self.index + offsetX + self.imageNames.count) % self.imageNames.count;
    
    dispatch_async(dispatch_get_main_queue(), ^{
        [self scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:1 inSection:0] atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:NO];
        [self reloadData];
    });
}

@end

还有layout,这里只设置一个外边距，也可以自定义一个layout，使用UICollectionView原生的初始化方法就可以了
.h

#import 
@class CarouselFlowLayout;
NS_ASSUME_NONNULL_BEGIN

@protocol CarouselFlowLayoutDelegate 

@optional

/**
 CollectionView外边距
 */
- (UIEdgeInsets)carouselFlowLayoutEdgeInsets:(CarouselFlowLayout *)layout;

@end

@interface CarouselFlowLayout : UICollectionViewFlowLayout

@property (nonatomic, weak) id delegate;

@end

NS_ASSUME_NONNULL_END

.m

#import "CarouselFlowLayout.h"

static UIEdgeInsets const DefaultEdgeInsets = {10, 10, 10, 10};//外边距

@interface CarouselFlowLayout ()

@property (nonatomic, strong) NSMutableArray  *attributeArray;

@end

@implementation CarouselFlowLayout

- (instancetype)init{
    self = [super init];
    if (self) {
        self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    }
    return self;
}

- (void)prepareLayout{
    [super prepareLayout];
    [self.attributeArray removeAllObjects];
    
    NSInteger itemCount = [self.collectionView numberOfItemsInSection:0];
    for (int i = 0; i < itemCount; i ++) {
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i inSection:0];
        [self.attributeArray addObject:[self layoutAttributesForItemAtIndexPath:indexPath]];
    }
}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect{
    return self.attributeArray;
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath{
    
    UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];

    CGFloat collectionViewWidth = CGRectGetWidth(self.collectionView.frame);
    CGFloat collectionViewHeight = CGRectGetHeight(self.collectionView.frame);
    
    CGFloat width = collectionViewWidth - [self edgeInsets].left - [self edgeInsets].right;
    CGFloat height = collectionViewHeight - [self edgeInsets].top - [self edgeInsets].bottom - [self safeAreaInsets].bottom - [self safeAreaInsets].top;
    
    CGFloat orginX = [self edgeInsets].left + (width + [self edgeInsets].right + [self edgeInsets].left) * indexPath.item;
    CGFloat orginY = [self safeAreaInsets].top + [self edgeInsets].top;
   
    attributes.frame = CGRectMake(orginX, orginY, width, height);
    
    return attributes;
}

- (CGSize)collectionViewContentSize{
    return CGSizeMake(CGRectGetWidth(self.collectionView.frame) * self.attributeArray.count, 0);
}

- (UIEdgeInsets)edgeInsets{
    if ([self.delegate respondsToSelector:@selector(carouselFlowLayoutEdgeInsets:)]) {
        return [self.delegate carouselFlowLayoutEdgeInsets:self];
    }
    return DefaultEdgeInsets;
}

#pragma mark -- iPhoneX 的安全区域
- (UIEdgeInsets)safeAreaInsets{
    UIEdgeInsets safeAreaInsets = UIEdgeInsetsMake(0, 0, 0, 0);
    if (@available(iOS 11.0, *)) {
        safeAreaInsets = [[UIApplication sharedApplication] delegate].window.safeAreaInsets;
    }
    return safeAreaInsets;
}

#pragma mark -- 懒加载
- (NSMutableArray *)attributeArray{
    if (!_attributeArray) {
        _attributeArray = [NSMutableArray array];
    }
    return _attributeArray;
}

@end