Subclassing UICollectionView, skipping indexes - ios

Question

I was trying to subclass collection view layout , in order to get a constant spacing between vertical cells. i have 2 columns and many rows, with dynamic cells height .
The goal is Pinterest like grid.
So i have subclassed the layout class, and now has a constant space between cells, but there is a serious problem caused by that .
When i scroll down, the left cells are not being loaded in time= there are many "holes" so that there is blank space of 3-4 cells, and than- they suddenly appears at once -lately.
so i have this :
1 2
3 4
6
8
than 5 and 7 appears when i scroll down more . i just can't get rid of this !
EDIT: seems that when all cells are in the same size, this will not happens ,so when i return here a constant height :
//cell size
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath;
{
CGSize size=CGSizeMake( imageWidth, scale*height+[Globals sharedGlobals].gridViewStripHeight );
return size;
My subclass(which when not using it, also solves the problem )
//the subclass
- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
NSArray* arr = [super layoutAttributesForElementsInRect:rect];
for (UICollectionViewLayoutAttributes* atts in arr)
{
if (nil == atts.representedElementKind)
{
NSIndexPath* ip = atts.indexPath;
atts.frame = [self layoutAttributesForItemAtIndexPath:ip].frame;
}
}
return arr;
}
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
UICollectionViewLayoutAttributes* atts =[super layoutAttributesForItemAtIndexPath:indexPath];
if (indexPath.item == 0 || indexPath.item == 1) // degenerate case 1, first item of section
return atts;
NSIndexPath* ipPrev =
[NSIndexPath indexPathForItem:indexPath.item-2 inSection:indexPath.section];
CGRect fPrev = [self layoutAttributesForItemAtIndexPath:ipPrev].frame;
CGFloat rightPrev = fPrev.origin.y + fPrev.size.height + 50;
if (atts.frame.origin.y <= rightPrev) // degenerate case 2, first item of line
return atts;
CGRect f = atts.frame;
f.origin.y = rightPrev;
atts.frame = f;
return atts;
}
To use it i have :
UICollectionViewFlowLayout *layout=[[TopAlignedCollectionViewFlowLayout alloc] init]; subclass
CGRect size=CGRectMake( ([UIScreen mainScreen].bounds.size.width-collectionWidth)/2,
upperLineMargin, collectionWidth, [UIScreen mainScreen].bounds.size.height-upperLineMargin);
self.GridView=[[UICollectionView alloc] initWithFrame:size collectionViewLayout:layout];
[self.GridView registerClass:[GridCell class] forCellWithReuseIdentifier:#"Cell"];