参考链接

• 创建一个UICollectionView

- (void)viewDidLoad {
    [super viewDidLoad];
    
    CGFloat x = 0.0f;
    CGFloat y = 60.0f;
    CGFloat width = self.view.frame.size.width;
    CGFloat height = 200.0f;
    
    _flowLayout = [[UICollectionViewFlowLayout alloc] init];
    _flowLayout.minimumLineSpacing = 10.0f;
    _flowLayout.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    _flowLayout.itemSize = CGSizeMake(width - 50.0f, height - 20.0f);
//    _flowLayout.sectionInset = UIEdgeInsetsMake(10.0f, 10.0f, 10.0f, 10.0f);
    
    _collectionView = [[UICollectionView alloc] initWithFrame:CGRectMake(x, y, width, height) collectionViewLayout:_flowLayout];
    _collectionView.backgroundColor = [UIColor orangeColor];
    _collectionView.delegate = self;
    _collectionView.dataSource = self;
    _collectionView.pagingEnabled = NO;
    [_collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:CollectionCellIdentifier];
    [self.view addSubview:_collectionView];
    
}

• 数据源代理

#pragma mark - UICollectionViewDataSource
- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    return 1;
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return 20;
}

- (__kindof UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:CollectionCellIdentifier forIndexPath:indexPath];
    
    cell.backgroundColor = [UIColor colorWithRed:arc4random_uniform(255)/255.0 green:arc4random_uniform(255)/255.0 blue:arc4random_uniform(255)/255.0 alpha:1.0f];
    
    return cell;
}

• 关键代码 UIScrollViewDelegate的一个方法

- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset
{
    if (![scrollView isKindOfClass:[UICollectionView class]]) {
        return;
    }
    UICollectionView *collectionView = (UICollectionView *)scrollView;
    
    CGPoint originalTargetContentOffset = CGPointMake(targetContentOffset->x, targetContentOffset->y);
    CGPoint targetCenter = CGPointMake(originalTargetContentOffset.x + CGRectGetWidth(collectionView.bounds)/2, CGRectGetHeight(collectionView.bounds) / 2);
    NSIndexPath *indexPath = nil;
    while (indexPath == nil) {
        targetCenter = CGPointMake(originalTargetContentOffset.x + CGRectGetWidth(collectionView.bounds)/2, CGRectGetHeight(collectionView.bounds) / 2);
        indexPath = [collectionView indexPathForItemAtPoint:targetCenter];
    }
    
    // 这里用attributes比用cell要好很多，因为cell可能因为不在屏幕范围内导致cellForItemAtIndexPath返回nil
    UICollectionViewLayoutAttributes *attributes = [collectionView.collectionViewLayout layoutAttributesForItemAtIndexPath:indexPath];
    if (attributes) {
        
        *targetContentOffset = CGPointMake(attributes.center.x - CGRectGetWidth(collectionView.bounds)/2, originalTargetContentOffset.y);
    } else {
        NSLog(@"center is %@; indexPath is {%@, %@}; cell is %@",NSStringFromCGPoint(targetCenter), @(indexPath.section), @(indexPath.item), attributes);
    }
    NSLog(@"indexPath.row - %ld",(long)indexPath.row);
    
}