WrappingCompletionHandler.swift

final class ImageFetcher {
  func fetchImages(for identifiers: [String], completionHandler: @escaping (Result<[String: UIImage], Error>) -> Void) {
    // …
  }
}

extension ImageFetcher {
  func fetchImages(for identifiers: [String]) async throws -> [String: UIImage] {
    try await withCheckedThrowingContinuation { continuation in
      fetchImages(for: identifiers) { result in
        continuation.resume(with: result)
      }
    }
  }
}