Make blazing fast lists and collection views

// Structuring data

struct DestinationPost: Identifiable {
    // Each post has a unique identifier
    var id: String
    
    var title: String
    var numberOfLikes: Int
    var assetID: Asset.ID
}

// Setting up diffable data source

class DestinationGridViewController: UIViewController {
    // Use DestinationPost.ID as the item identifier
    var dataSource: UICollectionViewDiffableDataSource<Section, DestinationPost.ID>
    
    private func setInitialData() {
        var snapshot = NSDiffableDataSourceSnapshot<Section, DestinationPost.ID>()
        
        // Only one section in this collection view, identified by Section.main
        snapshot.appendSections([.main])
        
        // Get identifiers of all destination posts in our model and add to initial snapshot
        let itemIdentifiers = postStore.allPosts.map { $0.id }
        snapshot.appendItems(itemIdentifiers)
        
        dataSource.apply(snapshot, animatingDifferences: false)
    }
}

// Cell registrations

let cellRegistration = UICollectionView.CellRegistration<DestinationPostCell,
                                                         DestinationPost.ID> {
    (cell, indexPath, postID) in

    let post = self.postsStore.fetchByID(postID)
    let asset = self.assetsStore.fetchByID(post.assetID)
    
    cell.titleView.text = post.region
    cell.imageView.image = asset.image
}

// Cell registrations

let cellRegistration = UICollectionView.CellRegistration<DestinationPostCell,
                                                         DestinationPost.ID> {
    (cell, indexPath, postID) in
    ...
}
   
let dataSource = UICollectionViewDiffableDataSource<Section.ID,
                                                    DestinationPost.ID>(collectionView: cv){
    (collectionView, indexPath, postID) in
  
     return collectionView.dequeueConfiguredReusableCell(using: cellRegistration,
                                                           for: indexPath,
                                                          item: postID)
}

// Existing cell registration

let cellRegistration = UICollectionView.CellRegistration<DestinationPostCell,
                                                         DestinationPost.ID> {
    (cell, indexPath, postID) in

    let post = self.postsStore.fetchByID(postID)
    let asset = self.assetsStore.fetchByID(post.assetID)
    
    cell.titleView.text = post.region
    cell.imageView.image = asset.image
}

// Updating cells asynchronously 

let cellRegistration = UICollectionView.CellRegistration<DestinationPostCell,
                                                         DestinationPost.ID> {
    (cell, indexPath, postID) in

    let post = self.postsStore.fetchByID(postID)
    let asset = self.assetsStore.fetchByID(post.assetID)
    
    if asset.isPlaceholder {
        self.assetsStore.downloadAsset(post.assetID) { asset in
            cell.imageView.image = asset.image
        }
    }
    
    cell.titleView.text = post.region
    cell.imageView.image = asset.image
}

private func setPostNeedsUpdate(id: DestinationPost.ID) {
    var snapshot = dataSource.snapshot()
    snapshot.reconfigureItems([id])
    dataSource.apply(snapshot, animatingDifferences: true)
}

// Updating cells asynchronously

let cellRegistration = UICollectionView.CellRegistration<DestinationPostCell,
                                                         DestinationPost.ID> {
    (cell, indexPath, postID) in

    let post = self.postsStore.fetchByID(postID)
    let asset = self.assetsStore.fetchByID(post.assetID)
    
    if asset.isPlaceholder {
        self.assetsStore.downloadAsset(post.assetID) { _ in
            self.setPostNeedsUpdate(id: post.id)
        }
    }
    
    cell.titleView.text = post.region
    cell.imageView.image = asset.image
}

// Data source prefetching

var prefetchingIndexPaths: [IndexPath: Cancellable]

func collectionView(_ collectionView: UICollectionView,
                    prefetchItemsAt indexPaths [IndexPath]) {
   // Begin download work
    for indexPath in indexPaths {
        guard let post = fetchPost(at: indexPath) else { continue }
        prefetchingIndexPaths[indexPath] = assetsStore.loadAssetByID(post.assetID)
    }
}

func collectionView(_ collectionView: UICollectionView,
                    cancelPrefetchingForItemsAt indexPaths: [IndexPath]) {
    // Stop fetching
    for indexPath in indexPaths {
        prefetchingIndexPaths[indexPath]?.cancel()
    }
}

// Using prepareForDisplay

// Initialize the full image
let fullImage = UIImage()

// Set a placeholder before preparation
imageView.image = placeholderImage

// Prepare the full image
fullImage.prepareForDisplay { preparedImage in
    DispatchQueue.main.async {
       self.imageView.image = preparedImage
    }
}

// Asset downloading – before image preparation

func downloadAsset(_ id: Asset.ID,
                   completionHandler: @escaping (Asset) -> Void) -> Cancellable {
  
    return fetchAssetFromServer(assetID: id) { asset in
        DispatchQueue.main.async {
            completionHandler(asset)
        }
    }
}

// Asset downloading – with image preparation

func downloadAsset(_ id: Asset.ID,
                   completionHandler: @escaping (Asset) -> Void) -> Cancellable {
    // Check for an already prepared image
    if let preparedAsset = imageCache.fetchByID(id) {
        completionHandler(preparedAsset)
        return AnyCancellable {}
    }
    return fetchAssetFromServer(assetID: id) { asset in
        asset.image.prepareForDisplay { preparedImage in
            // Store the image in the cache.
            self.imageCache.add(asset: asset.withImage(preparedImage!))
            DispatchQueue.main.async {
                completionHandler(asset)
            }
        }
    }
}

// Using prepareThumbnail

// Initialize the full image
let profileImage = UIImage(...)

// Set a placeholder before preparation
posterAvatarView.image = placeholderImage

// Prepare the image
profileImage.prepareThumbnail(of: posterAvatarView.bounds.size) { thumbnailImage in
    DispatchQueue.main.async {
        self.posterAvatarView.image = thumbnailImage
    }
}