Strange disappearing items when scrolls

Question

Presets,

i have collectionViewFlowLayout subclass with

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
    return YES;
   }

- (NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSArray *arr = [super layoutAttributesForElementsInRect:rect];
    BBLog(@"ARRA:%@", arr);
    for (UICollectionViewLayoutAttributes *attr in arr) {
        if (CGAffineTransformIsIdentity(attr.transform)) {
            attr.transform = CGAffineTransformMakeRotation((CGFloat)M_PI);
        }
    }

    return arr;
}

CollectionView rotate to upside down scroll with

 self.collectionView.transform = CGAffineTransformMakeRotation((CGFloat)M_PI);

But even if jus use native collectionViewFlowLayout without subclassing, a git this error

Problem

I have two messages and more in chat, but when scroll at bottom (top normally) disappear second item.

layoutAttributesForElementsInRect for given rect return two attributes for two indexPaths 0-0 and 0-1, but delegate method

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath

called only for indexPath 0-0

Here images

UPDATE So i found WHY it's happen - this line code

attr.transform = CGAffineTransformMakeRotation((CGFloat)M_PI);

Look if remove transform

Answer 1

I am not entirely sure, but I think that, when You are subclassing the UICollectionViewFlowLayout, You are not supposed to modify the attributes directly, but to make a copy of the attributes, modify it instead and return it.

Short explanation of the top statement : You will have to subclass the UICollectionViewFlowDelegate ( the parent of UICollectionViewDelegateFlowLayout ) and than make your own attributes and modify them as You wish, but this will require of to implement a lot more custom logic.

Also look if You are getting any errors or warnings in the console.

Check out this question : Warning: UICollectionViewFlowLayout has cached frame mismatch for index path 'abc'

Hope that I was at least a little helpful.