For those who stumble here later…. the reason:
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
was not being called was because of the itemSize for the collectionViewFlowLayout’s height was too big.
[self.myCollectionViewFlowLayout setItemSize:CGSizeMake(320, 548)];
If I change the height to 410, it will execute cellForItemAtIndexPath.