瀑布流现在是一些APP常用的场景，如电子商城、旅游、社区等。实现起来很简单。
项目github下载地址

WaterfallCollectionViewCell

先定义一个带XIB的WaterfallCollectionViewCell

#import <UIKit/UIKit.h>

@interface WaterfallCollectionViewCell : UICollectionViewCell

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

@end

XIB部分推荐运用AutoLayout进行布局，可以节省好多时间。

XIB

WaterfallCollectionViewLayout

定义WaterfallCollectionViewLayout集成自UICollectionViewLayout，这里的思路是：

• 实现瀑布流
• 需要根据要现实的内容计算每个Item的高度？
• 每个Item的上下左右边距是多少？
• 瀑布流要现实多少列？
  那么问题来了要怎么去实现呢？这里需要运用到代理，一直有一些初学者对代理这个东西用起来感觉有点绕，其实可以这么理解。
• 我是一个CollectionViewController
• 我想要计算每个Item的宽高？
• 我想要计算下一个Item要进行布局时，放在哪个列的下面？
• 但这个事情我做不合适，为什么呢？
• 因为我是一个大领导有好多事情要管，你们每个Item自己管好了，我给你们提供一个沟通渠道（代理实现的方法）告诉你们我想要什么就行，那么剩下的事情就委托给你们自己了。

#import <UIKit/UIKit.h>

@class WaterfallCollectionViewLayout;

@protocol WaterfallCollectionViewDelegate <NSObject>

@required
- (CGFloat)waterflowLayout:(WaterfallCollectionViewLayout *)waterflowLayout heightForItemAtIndex:(NSUInteger)index itemWidth:(CGFloat)itemWidth;

@optional
- (NSInteger)columnCountInWaterflowLayout:(WaterfallCollectionViewLayout *)waterflowLayout;
- (CGFloat)columnMarginInWaterflowLayout:(WaterfallCollectionViewLayout *)waterflowLayout;
- (CGFloat)rowMarginInWaterflowLayout:(WaterfallCollectionViewLayout *)waterflowLayout;
- (UIEdgeInsets)edgeInsetsInWaterflowLayout:(WaterfallCollectionViewLayout *)waterflowLayout;

@end


@interface WaterfallCollectionViewLayout : UICollectionViewLayout

@property (nonatomic, weak) id<WaterfallCollectionViewDelegate> delegate;

@end

//
//  WaterfallCollectionViewLayout.m
//  UICollectionVIewDemo
//
//  Created by DaLei on 2017/6/8.
//  Copyright © 2017年 DaLei. All rights reserved.
//

#import "WaterfallCollectionViewLayout.h"

/** 默认的列数 */
static const NSInteger DefaultColumnCount = 3;
/** 每一列之间的间距 */
static const CGFloat DefaultColumnMargin = 10;
/** 每一行之间的间距 */
static const CGFloat DefaultRowMargin = 10;
/** 边缘间距 */
static const UIEdgeInsets DefaultEdgeInsets = {10, 10, 10, 10};




@interface WaterfallCollectionViewLayout()

/** 存放所有cell的布局属性 */
@property (nonatomic, strong) NSMutableArray *attrsArray;
/** 存放所有列的当前高度 */
@property (nonatomic, strong) NSMutableArray *columnHeights;
/** 内容的高度 */
@property (nonatomic, assign) CGFloat contentHeight;

- (CGFloat)rowMargin;
- (CGFloat)columnMargin;
- (NSInteger)columnCount;
- (UIEdgeInsets)edgeInsets;

@end



@implementation WaterfallCollectionViewLayout

#pragma mark - 常见数据处理

- (CGFloat)rowMargin {
    if ([self.delegate respondsToSelector:@selector(rowMarginInWaterflowLayout:)]) {
        return [self.delegate rowMarginInWaterflowLayout:self];
    } else {
        return DefaultRowMargin;
    }
}

- (CGFloat)columnMargin {
    if ([self.delegate respondsToSelector:@selector(columnMarginInWaterflowLayout:)]) {
        return [self.delegate columnMarginInWaterflowLayout:self];
    } else {
        return DefaultColumnMargin;
    }
}

- (NSInteger)columnCount {
    if ([self.delegate respondsToSelector:@selector(columnCountInWaterflowLayout:)]) {
        return [self.delegate columnCountInWaterflowLayout:self];
    } else {
        return DefaultColumnCount;
    }
}

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

#pragma mark - 懒加载

- (NSMutableArray *)columnHeights {
    if (!_columnHeights) {
        _columnHeights = [NSMutableArray array];
    }
    return _columnHeights;
}

- (NSMutableArray *)attrsArray {
    if (!_attrsArray) {
        _attrsArray = [NSMutableArray array];
    }
    return _attrsArray;
}

- (void)prepareLayout {
    [super prepareLayout];
    
    self.contentHeight = 0;
    
    // 清除以前计算的所有高度
    [self.columnHeights removeAllObjects];
    for (NSInteger i = 0; i < self.columnCount; i++) {
        [self.columnHeights addObject:@(self.edgeInsets.top)];
    }
    
    // 清除之前所有的布局属性
    [self.attrsArray removeAllObjects];
    // 开始创建每一个cell对应的布局属性
    NSInteger count = [self.collectionView numberOfItemsInSection:0];
    for (NSInteger i = 0; i < count; i++) {
        // 创建位置
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i inSection:0];
        // 获取indexPath位置cell对应的布局属性
        UICollectionViewLayoutAttributes *attrs = [self layoutAttributesForItemAtIndexPath:indexPath];
        [self.attrsArray addObject:attrs];
    }
}

/**
 * 决定cell的排布
 */
- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    return self.attrsArray;
}

/**
 * 返回indexPath位置cell对应的布局属性
 */
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    // 创建布局属性
    UICollectionViewLayoutAttributes *attrs = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
    
    // collectionView的宽度
    CGFloat collectionViewW = self.collectionView.frame.size.width;
    
    // 设置布局属性的frame
    CGFloat w = (collectionViewW - self.edgeInsets.left - self.edgeInsets.right - (self.columnCount - 1) * self.columnMargin) / self.columnCount;
    CGFloat h = [self.delegate waterflowLayout:self heightForItemAtIndex:indexPath.item itemWidth:w];
    
    // 找出高度最短的那一列
    NSInteger destColumn = 0;
    CGFloat minColumnHeight = [self.columnHeights[0] doubleValue];
    for (NSInteger i = 1; i < self.columnCount; i++) {
        // 取得第i列的高度
        CGFloat columnHeight = [self.columnHeights[i] doubleValue];
        
        if (minColumnHeight > columnHeight) {
            minColumnHeight = columnHeight;
            destColumn = i;
        }
    }
    
    CGFloat x = self.edgeInsets.left + destColumn * (w + self.columnMargin);
    CGFloat y = minColumnHeight;
    if (y != self.edgeInsets.top) {
        y += self.rowMargin;
    }
    attrs.frame = CGRectMake(x, y, w, h);
    
    // 更新最短那列的高度
    self.columnHeights[destColumn] = @(CGRectGetMaxY(attrs.frame));
    
    // 记录内容的高度
    CGFloat columnHeight = [self.columnHeights[destColumn] doubleValue];
    if (self.contentHeight < columnHeight) {
        self.contentHeight = columnHeight;
    }
    return attrs;
}

- (CGSize)collectionViewContentSize {
    return CGSizeMake(0, self.contentHeight + self.edgeInsets.bottom);
}


@end

WaterfallCollectionViewController

新建WaterfallCollectionViewController，像普通的CollectionViewController一样进行定义即可，并且实现WaterfallCollectionViewDelegate告诉Item需要现实几列，每隔Item有多高，每个Item的边距是多少。

//
//  WaterfallCollectionViewController.m
//  UICollectionVIewDemo
//
//  Created by DaLei on 2017/6/8.
//  Copyright © 2017年 DaLei. All rights reserved.
//

#import "WaterfallCollectionViewController.h"
#import "WaterfallCollectionViewCell.h"
#import "WaterfallCollectionViewLayout.h"

#define RGBColor(r, g, b) [UIColor colorWithRed:(r)/255.0 green:(g)/255.0 blue:(b)/255.0 alpha:1.0]
#define RandomRGBColor RGBColor(arc4random_uniform(255), arc4random_uniform(255), arc4random_uniform(255))

@interface WaterfallCollectionViewController ()<WaterfallCollectionViewDelegate>

@property(nonatomic,strong)NSArray *productArray;

@end

@implementation WaterfallCollectionViewController

static NSString * const reuseIdentifier = @"WaterfallCollectionViewCell";

-(instancetype)init{
    WaterfallCollectionViewLayout *layout = [WaterfallCollectionViewLayout new];
    layout.delegate = self;
    return [super initWithCollectionViewLayout:layout];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    self.title = @"面皮大师-瀑布流";
    
    // Register cell classes
    [self.collectionView registerNib:[UINib nibWithNibName:NSStringFromClass([WaterfallCollectionViewCell class]) bundle:nil] forCellWithReuseIdentifier:reuseIdentifier];
    self.collectionView.backgroundColor = RGBColor(235, 235, 235);
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
}

-(NSArray *)productArray{
    if (!_productArray) {
        _productArray = @[@"cell1",@"cell2",@"cell3",@"cell4",@"cell5",@"cell6",@"cell7",@"cell1",@"cell2",@"cell3",@"cell4",@"cell5",@"cell6",@"cell7",@"cell1",@"cell2",@"cell3",@"cell4",@"cell5",@"cell6",@"cell7"];
    }
    return _productArray;
}

#pragma mark <UICollectionViewDataSource>


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

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    WaterfallCollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifier forIndexPath:indexPath];
    cell.backgroundColor = RandomRGBColor;
    UIImage *image = [UIImage imageNamed:[self.productArray objectAtIndex:indexPath.item]];
    [cell.image setImage:image];
    return cell;
}

#pragma mark <WaterfallCollectionViewDelegate>

- (CGFloat)waterflowLayout:(WaterfallCollectionViewLayout *)waterflowLayout heightForItemAtIndex:(NSUInteger)index itemWidth:(CGFloat)itemWidth{
    // 获取图片的宽高，根据图片的比例计算Item的高度。
    UIImage *image = [UIImage imageNamed:[self.productArray objectAtIndex:index]];
    CGFloat fixelW = CGImageGetWidth(image.CGImage);
    CGFloat fixelH = CGImageGetHeight(image.CGImage);
    CGFloat itemHeight = fixelH * itemWidth / fixelW;
    return itemHeight + 50;
}

- (NSInteger)columnCountInWaterflowLayout:(WaterfallCollectionViewLayout *)waterflowLayout{
    return 2;
}

- (CGFloat)columnMarginInWaterflowLayout:(WaterfallCollectionViewLayout *)waterflowLayout{
    return 10;
}

- (CGFloat)rowMarginInWaterflowLayout:(WaterfallCollectionViewLayout *)waterflowLayout{
    return 10;
}

- (UIEdgeInsets)edgeInsetsInWaterflowLayout:(WaterfallCollectionViewLayout *)waterflowLayout{
    return UIEdgeInsetsMake(10, 10, 10, 10);
}

@end

运行效果：

瀑布流

学习参考：Developer_CYX

UICollectionViewController基础（1）
UICollectionViewController基础（2）引导页
UICollectionViewController基础（3）瀑布流
UICollectionViewController基础（4）自定义