Best practices to boost smooth scrolling with iOS 10 Pre-Fetching APIs

1. Andrea Prearo
Master Software Engineer - iOS @ Capital One SF
https://github.com/andrea-prearo
https://medium.com/@andrea.prearo
https://twitter.com/andrea_prearo

2. Boost Smooth
Scrolling with iOS 10
Pre-Fetching APIs

3. Choppy Scrolling
Dropped Frame I

4. Choppy Scrolling
Dropped Frame II

5. Dropped Frames
The most common source of dropped frames is loading
expensive data models for a cell from the main thread
Common scenarios:
• Loading images from an URL
• Accessing items from a database or CoreData

6. Cell Lifecycle in iOS9
1. Request the cell: collectionView(_:cellForItemAt:)
2. Display the cell: collectionView(_:willDisplay:forItemAt:)
3. Remove the cell: collectionView(_:didEndDisplaying:forItemAt:)

7. Updates to Cell Lifecycle in iOS10
1. The OS calls collectionView(_:cellForItemAt:) much earlier than it used to:
• Cell loading is performed way before the cell needs to be displayed
• The cell may not end up being displayed at all
2. Cell goes off the visible field:
• collectionView(_:didEndDisplaying:forItemAt:) doesn't force immediate recycle for the cell
• collectionView(_:willDisplay:forItemAt:) may not require to reload cell content

8. Multi-column Layouts Cell Lifecycle
iOS9

9. Multi-column Layouts Cell Lifecycle
iOS10

10. Pre-Fetching API
• Introduced with iOS10
• Adaptive Technology
• Enabled by default
Best Practices
• Set up cell content in collectionView(_:cellForItemAt:)
• Don't use collectionView(_:willDisplay:forItemAt:) and
collectionView(_:didEndDisplaying:forItemAt:)
• Cell may not be displayed even if collectionView(_:cellForItemAt:) gets called (Plan for this!)

11. UICollectionView Pre-Fetching
• collectionView(_:prefetchItemsAt:) (required) — Initiate the
asynchronous loading of cell content (GCD or
OperationQueue)
• collectionView(_:cancelPrefetchingForItemsAt:)
(optional) — Cancel pending cell content loading

12. UITableView Pre-Fetching
• tableView(_:prefetchRowsAt:) (required) — Initiate the
asynchronous loading of cell content (GCD or
OperationQueue)
• tableView(_:cancelPrefetchingForRowsAt:) (optional) —
Cancel pending cell content loading

13. Code samples
All code is available on GitHub:
SmoothScrolling - Collection View
SmoothScrolling - Table View