效果

源碼：https://github.com/YouXianMing/Animations 

//
// GridFlowLayoutViewController.m
// Animations
//
// Created by YouXianMing on 16/5/5.
// Copyright © 2016年 YouXianMing. All rights reserved.
//

#import "GridFlowLayoutViewController.h"
#import "UIView+SetRect.h"
#import "GridLayout.h"
#import "FlowStyleCell.h"
#import "FileManager.h"
#import "NSString+MD5.h"
#import "NSData+JSONData.h"
#import "ResponseData.h"
#import "Math.h"
#import "GCD.h"

static NSString *picturesSource = @"http://www.duitang.com/album/1733789/masn/p/0/50/";

@interface GridFlowLayoutViewController () <UICollectionViewDataSource, UICollectionViewDelegate, GridLayoutDelegate>

@property (nonatomic, strong) UICollectionView *collectionView;
@property (nonatomic)   CGFloat   rowHeight;
@property (nonatomic, strong) NSMutableArray *datas;
@property (nonatomic, strong) ResponseData  *picturesData;
@property (nonatomic, strong) NSMutableArray <WaterfallPictureModel *> *dataSource;

@end

@implementation GridFlowLayoutViewController

- (void)setup {
 
 [super setup];
 
 _dataSource = [NSMutableArray new];
 
 // 初始化布局文件
 CGFloat gap    = 1;
 NSInteger rowCount  = arc4random() % 3 + 2;
 _rowHeight    = (self.contentView.height - (rowCount + 1) * gap) / (CGFloat)rowCount;
 GridLayout *layout  = [GridLayout new];
 layout.manager.edgeInsets = UIEdgeInsetsMake(gap, gap, gap, gap);
 layout.manager.gap  = gap;
 layout.delegate   = self;
 
 NSMutableArray *rowHeights = [NSMutableArray array];
 for (int i = 0; i < rowCount; i++) {
  
  [rowHeights addObject:@(_rowHeight)];
 }
 layout.manager.rowHeights = rowHeights;
 
 self.collectionView        = [[UICollectionView alloc] initWithFrame:self.contentView.bounds
                   collectionViewLayout:layout];
 self.collectionView.delegate      = self;
 self.collectionView.dataSource      = self;
 self.collectionView.backgroundColor    = [UIColor clearColor];
 self.collectionView.showsHorizontalScrollIndicator = NO;
 self.collectionView.alpha       = 0;
 [self.collectionView registerClass:[FlowStyleCell class] forCellWithReuseIdentifier:@"FlowStyleCell"];
 [self.contentView addSubview:self.collectionView];
 
 // 獲取數據
 [GCDQueue executeInGlobalQueue:^{
  
  NSString *string  = [picturesSource lowerMD532BitString];
  NSString *realFilePath = [FileManager theRealFilePath:[NSString stringWithFormat:@"~/Documents/%@", string]];
  NSData *data   = nil;
  
  if ([FileManager fileExistWithRealFilePath:realFilePath] == NO) {
   
   data = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:picturesSource]];
   [data writeToFile:realFilePath atomically:YES];
   
  } else {
   
   data = [NSData dataWithContentsOfFile:realFilePath];
  }
  
  NSDictionary *dataDic = [data toListProperty];
  
  [GCDQueue executeInMainQueue:^{
   
   self.picturesData = [[ResponseData alloc] initWithDictionary:dataDic];
   if (self.picturesData.success.integerValue == 1) {
    
    for (int i = 0; i < self.picturesData.data.blogs.count; i++) {
     
     [_dataSource addObject:self.picturesData.data.blogs[i]];
    }
    
    [_collectionView reloadData];
    [UIView animateWithDuration:0.5f animations:^{
     
     _collectionView.alpha = 1.f;
    }];
   }
  }];
 }];
}

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

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
 
 WaterfallPictureModel *pictureModel = _dataSource[indexPath.row];
 
 FlowStyleCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"FlowStyleCell" forIndexPath:indexPath];
 cell.indexPath  = indexPath;
 cell.data   = pictureModel;
 cell.rowHeight  = _rowHeight;
 [cell loadContent];
 
 return cell;
}

- (CGFloat)itemWidthWithIndexPath:(NSIndexPath *)indexPath {
 
 WaterfallPictureModel *pictureModel = _dataSource[indexPath.row];
 
 return [Math resetFromSize:CGSizeMake(pictureModel.iwd.floatValue, pictureModel.iht.floatValue)
    withFixedHeight:_rowHeight].width;
}

@end 

細節
繼承UICollectionViewLayout

重載UICollectionViewLayout的四個方法

部分實現細節

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