问题：两个CollectionView在使用瀑布流布局时，总是一个设置布局正常，另一个总显示不出来。
原因：The problem was that I was using the same layout object for each collection. In retrospect that makes sense, but you have to make sure you create different layouts for each collectionView.

解决方案

一个collectionView对象需要一个layout对象

#import "ViewController.h"
#import "NDUserCell.h"

#define MAX_JOINED 30

@interface ViewController () <UICollectionViewDelegate, UICollectionViewDataSource>

@property (nonatomic, strong) UICollectionView *topUserCollectionView;

@property (nonatomic, strong) UICollectionView *bottomUserCollectionView;

@property (nonatomic, strong) UIView *backgroundView;

@property (nonatomic, strong) UIView *topView;

@property (nonatomic, strong) UIView *bottomView;

@property (nonatomic, strong) NSMutableArray *allVideoViews;

@property (nonatomic, strong) NSMutableArray *allTopVideoViews;

@property (nonatomic, strong) NSMutableArray *allBottomVideoViews;

@end

static NSString * const topReuseIdentifier = @"top";
static NSString * const bottomReuseIdentifier = @"bottom";

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    [self initUI];
    
    [self registerCell];
    
    [self configVideoViews];
    
}

- (void)reload {
    [self.topUserCollectionView reloadData];
    [self.bottomUserCollectionView reloadData];
}

- (void)registerCell {
    [self.topUserCollectionView registerClass:[NDUserCell class] forCellWithReuseIdentifier:topReuseIdentifier];
    [self.bottomUserCollectionView registerClass:[NDUserCell class] forCellWithReuseIdentifier:bottomReuseIdentifier];
}

- (void)initUI {
    
    self.backgroundView = [UIView new];
    self.backgroundView.backgroundColor = [UIColor greenColor];
    [self.view addSubview:self.backgroundView];
    [self.backgroundView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.edges.equalTo(self.view);
    }];
    
    self.topView = [UIView new];
    [self.backgroundView addSubview:self.topView];
    [self.topView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.leading.width.equalTo(self.backgroundView);
        //加上顶部导航栏和状态栏的高度
        make.top.mas_equalTo(TOP_LAYOUT_HEIGHT);
        //100为小屏变长，上下各10的边距
        make.height.mas_equalTo(120);
    }];
    
    self.bottomView = [UIView new];
    [self.backgroundView addSubview:self.bottomView];
    [self.bottomView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.bottom.leading.width.equalTo(self.backgroundView);
        //100为小屏边长，上下各10的边距
        make.height.mas_equalTo(120);
    }];
    
    
    UICollectionViewFlowLayout *topFlowLayout = [[UICollectionViewFlowLayout alloc] init];
    [topFlowLayout setScrollDirection:UICollectionViewScrollDirectionHorizontal];
    [topFlowLayout setItemSize:CGSizeMake(100.f, 100.f)];//设置cell的尺寸
    topFlowLayout.sectionInset = UIEdgeInsetsMake(10.f, 15.f, 10.f, 15.f);//设置单元格在collectionView中的内边距，距上下各10，左侧15，相隔15
    
////////////////////////////////////////////////////////////////////////////////
//////////////////////////////顶部CollectionView/////////////////////////////////
////////////////////////////////////////////////////////////////////////////////
    _topUserCollectionView = [[UICollectionView alloc] initWithFrame:CGRectMake(5.f, 0, SCREEN_WIDTH, 120.f) collectionViewLayout:topFlowLayout];//设置collectionView距顶部视图左边距5，宽度同屏幕，高度120
    _topUserCollectionView.tag = 101;
    _topUserCollectionView.backgroundColor = [UIColor yellowColor];
    //    [_topUserCollectionView setCollectionViewLayout:flowLayout];
    [_topView addSubview:_topUserCollectionView];
    //设置collectionView距顶部视图左边距5，宽度同屏幕，高度120
    [_topUserCollectionView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.top.trailing.equalTo(self.topView);
        make.leading.equalTo(self.topView).offset(5);
        make.height.mas_equalTo(120.f);
    }];
    _topUserCollectionView.delegate = self;
    _topUserCollectionView.dataSource = self;
    //有导航栏时自动滚动调整，默认为YES
    self.automaticallyAdjustsScrollViewInsets = NO;
    _topUserCollectionView.showsHorizontalScrollIndicator = NO;
////////////////////////////////////////////////////////////////////////////////
//////////////////////////////底部CollectionView/////////////////////////////////
////////////////////////////////////////////////////////////////////////////////
    UICollectionViewFlowLayout *bottomFlowLayout = [[UICollectionViewFlowLayout alloc] init];
    [bottomFlowLayout setScrollDirection:UICollectionViewScrollDirectionHorizontal];
    [bottomFlowLayout setItemSize:CGSizeMake(100.f, 100.f)];//设置cell的尺寸
    bottomFlowLayout.sectionInset = UIEdgeInsetsMake(10.f, 15.f, 10.f, 15.f);//设置单元格在collectionView中的内边距，距上下各10，左侧15，相隔15
    _bottomUserCollectionView = [[UICollectionView alloc] initWithFrame:CGRectMake(5.f, SCREEN_HEIGHT - TOP_LAYOUT_HEIGHT - 120.f, SCREEN_WIDTH, 120.f) collectionViewLayout:bottomFlowLayout];//设置collectionView距顶部视图左边距5，宽度同屏幕，高度120
    _bottomUserCollectionView.tag = 102;
    _bottomUserCollectionView.backgroundColor = [UIColor yellowColor];
    //    [_topUserCollectionView setCollectionViewLayout:flowLayout];
    [_bottomView addSubview:_bottomUserCollectionView];
    //设置collectionView距顶部视图左边距5，宽度同屏幕，高度120
    [_bottomUserCollectionView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.top.trailing.equalTo(self.bottomView);
        make.leading.equalTo(self.bottomView).offset(5);
        make.height.mas_equalTo(120.f);
    }];
    _bottomUserCollectionView.delegate = self;
    _bottomUserCollectionView.dataSource = self;
    _bottomUserCollectionView.showsHorizontalScrollIndicator = NO;
}

- (void)configVideoViews {
    
    self.allTopVideoViews = [NSMutableArray array];
    self.allBottomVideoViews = [NSMutableArray array];
    
    for (int i = 0; i < self.allVideoViews.count; i++) {
        if (i < 25) {
            [self.allTopVideoViews addObject:self.allVideoViews[i]];
        } else {
            [self.allBottomVideoViews addObject:self.allVideoViews[i]];
        }
    }
    
    [self.topUserCollectionView reloadData];
    [self.bottomUserCollectionView reloadData];
    
}

- (NSMutableArray *)allVideoViews {
    if (!_allVideoViews) {
        _allVideoViews = [NSMutableArray array];
        for (int i = 0; i < MAX_JOINED; i++) {
            UIView *view = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];
            view.backgroundColor = [UIColor colorWithRed:arc4random()%256/255.f green:arc4random()%256/255.f blue:arc4random()%256/255.f alpha:.5f];
            [_allVideoViews addObject:view];
        }
    }
    return _allVideoViews;
}


- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
    if (collectionView.tag == 101) {
        LogRed(@"self.allTopVideoViews.count = %lu", self.allTopVideoViews.count);
        return self.allTopVideoViews.count;
    } else if (collectionView.tag == 102) {
        LogRed(@"self.allBottomVideoViews.count = %lu", self.allBottomVideoViews.count);
        return self.allBottomVideoViews.count;
    }
    return 0;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    if (collectionView.tag == 101) {
        NDUserCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:topReuseIdentifier forIndexPath:indexPath];
        cell.videoView = self.allTopVideoViews[indexPath.row];
        return cell;
    } else if (collectionView.tag == 102) {
        NDUserCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:bottomReuseIdentifier forIndexPath:indexPath];
        cell.videoView = self.allBottomVideoViews[indexPath.row];
        return cell;
    }
    return nil;
}

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    return 1;
}

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
    LogBlue(@"点击了一个item = %lu", indexPath.row);
}
@end