IceCubesApp/Packages/Timeline/Sources/Timeline/TimelinePrefetcher.swift

import Models
import Nuke
import SwiftUI
import UIKit

final class TimelinePrefetcher: NSObject, ObservableObject, UICollectionViewDataSourcePrefetching {
  private let prefetcher = ImagePrefetcher()

  weak var viewModel: TimelineViewModel?

  func collectionView(_: UICollectionView, prefetchItemsAt indexPaths: [IndexPath]) {
    let imageURLs = getImageURLs(for: indexPaths)
    prefetcher.startPrefetching(with: imageURLs)
  }

  func collectionView(_: UICollectionView, cancelPrefetchingForItemsAt indexPaths: [IndexPath]) {
    let imageURLs = getImageURLs(for: indexPaths)
    prefetcher.stopPrefetching(with: imageURLs)
  }

  private func getImageURLs(for indexPaths: [IndexPath]) -> [URL] {
    guard let viewModel, case let .display(statuses, _) = viewModel.statusesState else {
      return []
    }
    return indexPaths.compactMap {
      $0.row < statuses.endIndex ? statuses[$0.row] : nil
    }.flatMap(getImages)
  }
}

private func getImages(for status: Status) -> [URL] {
  var urls = status.mediaAttachments.compactMap {
    if $0.supportedType == .image {
      return status.mediaAttachments.count > 1 ? $0.previewUrl ?? $0.url : $0.url
    }
    return nil
  }
  if let url = status.card?.image {
    urls.append(url)
  }
  return urls
}