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In Bite #241, we learned about all the great improvements to UICollectionView's cell lifecycle in iOS 10. Today we'll check out another iOS 10 improvement to UICollectionView: The prefetchDataSource! Let's get started.

UICollectionView gains a new property this year called prefetchDataSource. Just like the existing delegate and dataSource properties, we an simply set it to some object that implements the new UICollectionViewDataSourcePrefetching protocol.

This protocol is brand new in iOS 10, and requires we implement just one new function:

public func collectionView(_ collectionView: UICollectionView, prefetchItemsAt indexPaths: [IndexPath])

When this function is called, we can examine the indexPaths array we're passed in to know which cells are "coming up soon" and thus which cells we should probably begin loading the data for. The indexPaths will be ordered "ascending by geometric distance" which basically means the indexPaths that need to be displayed on screen soonest will be first.

Let's use this new functionality to let our UICollectionView pre-load the images that go in our cells, This way when they scroll on screen, the images will already be visible and won't have to "load in" in front of the user. Classy! 🎩

class SpaceshipsViewController : UICollectionViewController {
  override func viewDidLoad() {

    collectionView?.prefetchDataSource = self

extension SpaceshipsViewController : UICollectionViewDataSourcePrefetching {
  func collectionView(_ collectionView: UICollectionView, prefetchItemsAt indexPaths: [IndexPath]) {
    ImagePreloader.preloadImagesForIndexPaths(indexPaths) // happens on a background queue

  func collectionView(_ collectionView: UICollectionView, cancelPrefetchingForItemsAt indexPaths: [IndexPath]) {
    ImagePreloader.cancelPreloadImagesForIndexPaths(indexPaths) // again, happens on a background queue

That's literally it. We make sure to implement the optional cancelPrefetchingForItems function so that we don't waste resources preloading images the user won't actually see.

UICollectionViewDataSourcePrefetching doesn't replace our existing data loading architecture/functionality. It's essentially a hint as to which cells will be displayed soon, and which ones we might want to begin loading content for.

Finally, just like in Bite #241, we can opt-out of this new prefetching behavior by either simply leaving prefetchDataSource set to nil, or by setting isPrefetchingEnabled to false.

Oh one more thing: All of this works exactly the same on UITableView too now, thanks to the (almost identical) new UITableViewDataSourcePrefetching protocol.