UICollectionView decoration view above some cells when deletesections method called

Question

I have a custom layout inherited from UICollectionViewFlowLayout used to achieve for each section to add backgroundview. The first time to show that there is no problem, but when I use deletesections method to delete the first section, some cells on the display in the decoration view below, the strange thing is to delete the data and then call the reloadData method will normal display.

I have put the code into GitHub:https://github.com/TonightGod/UICollectionView-DecorationView

pictures: after deleteSections method called

code:

@interface MyCustomLayout()
@property(nonatomic,strong) NSMutableArray *decorationViews;
@end


@implementation MyCustomLayout


-(NSMutableArray*)decorationViews
{
    if(!_decorationViews)
    {
        _decorationViews=[NSMutableArray array];
    }
    return _decorationViews;
}


- (void)prepareLayout {
    [super prepareLayout];

    [self.decorationViews removeAllObjects];
    NSInteger sections=[self.collectionView numberOfSections];
    for(NSInteger section=0;section<sections;section++)
    {
        NSInteger items=[self.collectionView numberOfItemsInSection:section];
        if(items>0)
        {
            CGRect firstItemFrame=[self layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:section]].frame;
            CGRect lastItemFrame=[self layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForRow:items-1 inSection:section]].frame;
            UIEdgeInsets sectionInset=self.sectionInset;
            if([self.collectionView.delegate respondsToSelector:@selector(collectionView:layout:insetForSectionAtIndex:)])
            {
                sectionInset = [(id<UICollectionViewDelegateFlowLayout>)self.collectionView.delegate collectionView:self.collectionView layout:self insetForSectionAtIndex:section];
            }

            CGRect sectionFrame=CGRectUnion(firstItemFrame, lastItemFrame);
            sectionFrame.origin.x=0;
            sectionFrame.origin.y-=sectionInset.top;
            if(self.scrollDirection==UICollectionViewScrollDirectionHorizontal)
            {
                sectionFrame.size.width += sectionInset.left + sectionInset.right;
                sectionFrame.size.height = self.collectionView.frame.size.height;
            }else
            {
                sectionFrame.size.width = self.collectionView.frame.size.width;
                sectionFrame.size.height += sectionInset.top + sectionInset.bottom;
            }

            NSString *identifer=[self.delegate collectionView:self.collectionView decorationViewIdentiferAtSection:section];

            if(identifer)
            {
                UICollectionViewLayoutAttributes *decorationAttributes=[UICollectionViewLayoutAttributes layoutAttributesForDecorationViewOfKind:identifer withIndexPath:[NSIndexPath indexPathForItem:0 inSection:section]];
                decorationAttributes.frame=sectionFrame;
                decorationAttributes.zIndex=-1;
                [self.decorationViews addObject:decorationAttributes];
            }
        }else
        {
            continue;
        }

    }

}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSArray *attributes = [super layoutAttributesForElementsInRect:rect];

    NSMutableArray *allAttributes = [NSMutableArray arrayWithArray:attributes];

    [allAttributes addObjectsFromArray:[self.decorationViews filteredArrayUsingPredicate:[NSPredicate predicateWithBlock:^BOOL(id  _Nullable evaluatedObject, NSDictionary<NSString *,id> * _Nullable bindings) {
        UICollectionViewLayoutAttributes *obj=evaluatedObject;
        return CGRectIntersectsRect(rect, obj.frame);
    }]]];

    return allAttributes;
}



@protocol MyCustomLayoutDelegate <UICollectionViewDelegateFlowLayout>

-(NSString*)collectionView:(UICollectionView *)collectionView decorationViewIdentiferAtSection:(NSInteger)section;

@end

@interface MyCustomLayout : UICollectionViewFlowLayout

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