WrappingAsyncAwait.swift

final class DataFetcher {
  func fetchData(for identifiers: [String]) async throws -> [String: Data] {
    // …
  }
}

extension DataFetcher {
  func fetchData(for identifiers: [String], completionHandler: @escaping (Result<[String: Data], Error>) -> Void) {
    Task {
      do {
        let data = try await fetchData(for: identifiers)
        await MainActor.run {
          completionHandler(.success(data))
        }
      }
      catch {
        await MainActor.run {
          completionHandler(.failure(error))
        }
      }
    }
  }
}