UICollectionViewLayout-subclass.m

@implementation VP2CollectionViewLayout

#pragma mark - Called during the layout process

// step 1
- (void)prepareLayout
{
    // if it's not too much data, create all element attributes here
}

// step 2
- (CGSize)collectionViewContentSize
{
    return CGSizeZero;
}

// step 3
- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    // at a minimum, each attribute should have size and position
    // set zIndex if they overlap
    // you can subclass UICollectionViewLayoutAttributes if you need to
    return @[];
}

#pragma mark - Other

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
    // cannot modify attributes here
    return nil;
}

// use for headers and footers
// the elements come from the data source
- (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
{
    return nil;
}

#pragma mark - For insertion/deletion animation

- (UICollectionViewLayoutAttributes *)initialLayoutAttributesForAppearingItemAtIndexPath:(NSIndexPath *)itemIndexPath
{
    return nil;
}

- (UICollectionViewLayoutAttributes *)finalLayoutAttributesForDisappearingItemAtIndexPath:(NSIndexPath *)itemIndexPath
{
    return nil;
}

@end