UICollectionViewFlowLayout 流水布局

Model cell

@interface ImageCell : UICollectionViewCell
@property (nonatomic, copy) NSString *image;
@end

@interface ImageCell()
@property (weak, nonatomic) IBOutlet UIImageView *imageView;
@end

@implementation ImageCell

- (void)awakeFromNib {
    self.imageView.layer.borderWidth = 3;
    self.imageView.layer.borderColor = [UIColor whiteColor].CGColor;
    self.imageView.layer.cornerRadius = 3;
    self.imageView.clipsToBounds = YES;
}

- (void)setImage:(NSString *)image
{
    _image = [image copy];

    self.imageView.image = [UIImage imageNamed:image];
}

@end

LineLayout

#import "LineLayout.h"

static const CGFloat ItemWH = 100;

@implementation LineLayout

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

/**
 *  只要显示的边界发生改变就重新布局:
 内部会重新调用prepareLayout和layoutAttributesForElementsInRect方法获得所有cell的布局属性
 */
- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
{
    return YES;
}

/**
 *  用来设置collectionView停止滚动那一刻的位置
 *
 *  @param proposedContentOffset 原本collectionView停止滚动那一刻的位置
 *  @param velocity              滚动速度
 */
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
    // 1.计算出scrollView最后会停留的范围
    CGRect lastRect;
    lastRect.origin = proposedContentOffset;
    lastRect.size = self.collectionView.frame.size;

    // 计算屏幕最中间的x
    CGFloat centerX = proposedContentOffset.x + self.collectionView.frame.size.width * 0.5;

    // 2.取出这个范围内的所有属性
    NSArray *array = [self layoutAttributesForElementsInRect:lastRect];

    // 3.遍历所有属性
    CGFloat adjustOffsetX = MAXFLOAT;
    for (UICollectionViewLayoutAttributes *attrs in array) {
        if (ABS(attrs.center.x - centerX) < ABS(adjustOffsetX)) {
            adjustOffsetX = attrs.center.x - centerX;
        }
    }

    return CGPointMake(proposedContentOffset.x + adjustOffsetX, proposedContentOffset.y);
}

/**
 *  一些初始化工作最好在这里实现
 */
- (void)prepareLayout
{
    [super prepareLayout]; 

    // 每个cell的尺寸
    self.itemSize = CGSizeMake(ItemWH, ItemWH);
    CGFloat inset = (self.collectionView.frame.size.width - ItemWH) * 0.5;
    self.sectionInset = UIEdgeInsetsMake(0, inset, 0, inset);
    // 设置水平滚动
    self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    self.minimumLineSpacing = ItemWH * 0.7;

    // 每一个cell(item)都有自己的UICollectionViewLayoutAttributes
    // 每一个indexPath都有自己的UICollectionViewLayoutAttributes
}

/** 有效距离:当item的中间x距离屏幕的中间x在ActiveDistance以内,才会开始放大, 其它情况都是缩小 */
static CGFloat const ActiveDistance = 150;
/** 缩放因素: 值越大, item就会越大 */
static CGFloat const ScaleFactor = 0.6;

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    // 0.计算可见的矩形框
    CGRect visiableRect;
    visiableRect.size = self.collectionView.frame.size;
    visiableRect.origin = self.collectionView.contentOffset;

    // 1.取得默认的cell的UICollectionViewLayoutAttributes
    NSArray *array = [super layoutAttributesForElementsInRect:rect];
    // 计算屏幕最中间的x
    CGFloat centerX = self.collectionView.contentOffset.x + self.collectionView.frame.size.width * 0.5;

    // 2.遍历所有的布局属性
    for (UICollectionViewLayoutAttributes *attrs in array) {
        // 如果不在屏幕上,直接跳过
        if (!CGRectIntersectsRect(visiableRect, attrs.frame)) continue;

        // 每一个item的中点x
        CGFloat itemCenterX = attrs.center.x;

        // 差距越小, 缩放比例越大
        // 根据跟屏幕最中间的距离计算缩放比例
        CGFloat scale = 1 + ScaleFactor * (1 - (ABS(itemCenterX - centerX) / ActiveDistance));
        attrs.transform = CGAffineTransformMakeScale(scale, scale);
    }

    return array;
}

@end

ViewController

#import "ViewController.h"
#import "ImageCell.h"
#import "LineLayout.h"

@interface ViewController () <UICollectionViewDataSource, UICollectionViewDelegate>
@property (nonatomic, strong) NSMutableArray *images;
@property (nonatomic, weak) UICollectionView *collectionView;
@end

@implementation ViewController

static NSString *const ID = @"image";

- (NSMutableArray *)images
{
    if (!_images) {
        self.images = [[NSMutableArray alloc] init];

        for (int i = 1; i<=20; i++) {
            [self.images addObject:[NSString stringWithFormat:@"%d", i]];
        }
    }
    return _images;
}

- (void)viewDidLoad {
    [super viewDidLoad];

    CGFloat w = self.view.frame.size.height;
    CGRect rect = CGRectMake(0, 0, w, 200);
    UICollectionView *collectionView = [[UICollectionView alloc] initWithFrame:rect collectionViewLayout:[[LineLayout alloc] init]];
    collectionView.dataSource = self;
    collectionView.delegate = self;
    [collectionView registerNib:[UINib nibWithNibName:@"ImageCell" bundle:nil] forCellWithReuseIdentifier:ID];
    [self.view addSubview:collectionView];
    self.collectionView = collectionView;
    // collectionViewLayout :
    // UICollectionViewLayout
    // UICollectionViewFlowLayout
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    if ([self.collectionView.collectionViewLayout isKindOfClass:[LineLayout class]]) {
        [self.collectionView setCollectionViewLayout:[[UICollectionViewFlowLayout alloc] init] animated:YES];
    } else {
        [self.collectionView setCollectionViewLayout:[[LineLayout alloc] init] animated:YES];
    }
}

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

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    ImageCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:ID forIndexPath:indexPath];
    cell.image = self.images[indexPath.item];
    return cell;
}

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
    // 删除模型数据
    [self.images removeObjectAtIndex:indexPath.item];

    // 删UI(刷新UI)
    [collectionView deleteItemsAtIndexPaths:@[indexPath]];
}

@end

效果图