错误描述：

*** Assertion failure in -[UICollectionViewData validateLayoutInRect:]
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'UICollectionView received layout attributes for a cell with an index path that does not exist: <NSIndexPath: 0xc000000000200016> {length = 2, path = 0 - 1}'

问题说明：

我在.h中定义了两个collectionView和一个UICollectionViewFlowLayout，然后在.m懒加载两个collectionView都应用了UICollectionViewFlowLayout的实例，在执行2个collectionView相继刷新的过程中，出现了崩溃。

问题原理：

两个collectionView相继刷新时都会用到UICollectionViewFlowLayout实例来验证矩形布局，此时A（collectionView）持有UICollectionViewFlowLayout实例过程中B（collectionView）也要使用UICollectionViewFlowLayout实例，这时就会出现“B（collectionView）接收到具有不存在的索引路径的单元格的布局属性”（系统给出的解释）

深度剖析原理：

实例并初始化a、b、c三个对象，a持有c对象的指针指向c内存，b若在a持有过程中想持有c对象，会出现空指针

解决方案：

同时实例化两个UICollectionViewFlowLayout

代码

.h

@interface AddPurchaseorderPopLayerView : UIView< UICollectionViewDelegate, UICollectionViewDataSource>
@property(nonatomic, strong)UICollectionView *collectionView;
@property(nonatomic, strong)UICollectionViewFlowLayout *flowLayout;
@property(nonatomic, strong)UICollectionView *typeCollectionView;
@property(nonatomic, strong)UICollectionViewFlowLayout *flowLayoutType;   //后来添加的
@end

.m

#import "AddPurchaseorderPopLayerView.h"

@implementation AddPurchaseorderPopLayerView
- (instancetype)initWithFrame:(CGRect)frame{
    if (self = [super initWithFrame:frame]) {
        [self setupView];
    }
    return self;
}

- (void)setupView{
    [self addSubView];
}

- (void)addSubView{
    [self addSubview:self.collectionView];
    [self addSubview:self.typeCollectionView];  
}

- (UICollectionViewFlowLayout *)flowLayout{
    if (!_flowLayout) {
        _flowLayout = [[UICollectionViewFlowLayout alloc] init];
        _flowLayout.itemSize = CGSizeMake(SCREEN_WIDTH/3, 65 * FitHeight);
        _flowLayout.minimumInteritemSpacing = 0;
        _flowLayout.minimumLineSpacing = 0;
        _flowLayout.scrollDirection = UICollectionViewScrollDirectionHorizontal;
        _flowLayout.sectionInset = UIEdgeInsetsMake(0, 0, 0, 0);
        
    }
    return _flowLayout;
}

- (UICollectionView *)collectionView{
    if (!_collectionView) {
        _collectionView = [[UICollectionView alloc] initWithFrame:CGRectMake(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT - 64) collectionViewLayout:self.flowLayout];
        _collectionView.delegate = self;
        _collectionView.dataSource = self;
        _collectionView.pagingEnabled = NO;
        _collectionView.backgroundColor = white_color;
        [_collectionView registerClass:[ProductDetailLadderPriceCollectionViewCell class] forCellWithReuseIdentifier:@"identifier"];
    }
    return _collectionView;
}

//这个是找到问题后加的
- (UICollectionViewFlowLayout *)flowLayoutType{
    if (!_flowLayoutType) {
        _flowLayoutType = [[UICollectionViewFlowLayout alloc] init];
        _flowLayoutType.itemSize = CGSizeMake(SCREEN_WIDTH/3, 65 * FitHeight);
        _flowLayoutType.minimumInteritemSpacing = 0;
        _flowLayoutType.minimumLineSpacing = 0;
        _flowLayoutType.scrollDirection = UICollectionViewScrollDirectionHorizontal;
        _flowLayoutType.sectionInset = UIEdgeInsetsMake(0, 0, 0, 0);

    }
    return _flowLayoutType;
}

//这里面的flowLayoutType原来是flowLayout
- (UICollectionView *)typeCollectionView{
    if (!_typeCollectionView) {
        _typeCollectionView = [[UICollectionView alloc] initWithFrame:CGRectMake(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT - 64) collectionViewLayout:self.flowLayoutType];
        _typeCollectionView.delegate = self;
        _typeCollectionView.dataSource = self;
        _typeCollectionView.pagingEnabled = NO;
        _typeCollectionView.backgroundColor = white_color;
        [_typeCollectionView registerClass:[ProductDetailsUnitCollectionViewCell class] forCellWithReuseIdentifier:@"ProductDetailsUnitCollectionViewCell"];
    }
    return _typeCollectionView;
} 

原来总是把flowLayout写在collectionView懒加载里面了，没出现过这个问题，今天突然实例化了一下，出现了这个问题，特此记录一下，希望对别人有所帮助。