GettingStartedWithCustomUICollectionViewLayouts-3

CustomLayout.m

-(NSArray*)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSMutableArray* elementsInRect = [NSMutableArray array];

    //iterate over all cells in this collection
    for(NSUInteger i = 0; i < [self.collectionView numberOfSections]; i++)
    {
        for(NSUInteger j = 0; j < [self.collectionView numberOfItemsInSection:i]; j++)
        {
            //this is the cell at row j in section i
            CGRect cellFrame = CGRectMake(/* calculate your origin x */,
                                          /* calculate your origin y */,
                                          /* calculate your width */,
                                          /* calculate your height */);
 
            //see if the collection view needs this cell
            if(CGRectIntersectsRect(cellFrame, rect))
            {
                //create the attributes object
                NSIndexPath* indexPath = [NSIndexPath indexPathForRow:j inSection:i];
                UICollectionViewLayoutAttributes* attr = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];

                //set the frame for this attributes object
                attr.frame = cellFrame;
                [elementsInRect addObject:attr];
            }
        }
    }
 
    return elementsInRect;
}