Apple doc でわかるかもしれませんが、サブクラスは layoutAttributesForElementsInRect と layoutAttributesForItemAtIndexPath の両方をオーバーライドする必要があります。
layoutAttributesForElementsInRect:
Returns the layout attributes for all of the cells and views in the specified rectangle.
- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
Parameters
rect
The rectangle (specified in the collection view’s coordinate system) containing the target views.
Return Value
An array of UICollectionViewLayoutAttributes objects representing the layout information for the cells and views. The default implementation returns nil.
Discussion
Subclasses must override this method and use it to return layout information for all items whose view intersects the specified rectangle. Your implementation should return attributes for all visual elements, including cells, supplementary views, and decoration views.
When creating the layout attributes, always create an attributes object that represents the correct element type (cell, supplementary, or decoration). The collection view differentiates between attributes for each type and uses that information to make decisions about which views to create and how to manage them.
Availability
Available in iOS 6.0 and later.
Declared In
UICollectionViewLayout.h