TestAsyncAwait.swift

import Foundation

func checkAsync() async -> [String] {
    (0...10).map({ "\($0)" })
}

func checkAsyncAwait() async -> [String] {
    await checkAsync()
}

@asyncHandler func checkSync() {
    print(await checkAsyncAwait())
}

struct CurrencyInfo: Codable {
    let ccy: String
    let baseCcy: String
    let buy: String
    let sale: String
}

// ‼ Не повторять в реальной жизни.
// Максимально упрощённый и опасный пример
func fetchCurrencies(completion: @escaping ([CurrencyInfo]) -> Void) {
    URLSession.shared.dataTask(with: URL(string: "https://api.privatbank.ua/p24api/pubinfo?exchange&json&coursid=11")!) { data, response, error in
        let decoder = JSONDecoder()
        decoder.keyDecodingStrategy = .convertFromSnakeCase
        completion(try! decoder.decode([CurrencyInfo].self, from: data!))
    }
    .resume()
}

func checkFetchCurrenciesAsyncAwait() async throws -> [CurrencyInfo] {
    return try await withCheckedThrowingContinuation { continuation in
        fetchCurrencies { continuation.resume(returning: $0) }
        // В случае ошибки continuation.resume(throwing: error)
    }
}

runAsyncAndBlock {
    do {
        print(try await checkFetchCurrenciesAsyncAwait())
    } catch {
        print(error)
    }
    print(await checkAsyncAwait())
}