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Last active Sep 21, 2022
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import Foundation
import Combine
enum DataTaskError: Error {
case rateLimit(UUID)
case serverBusy(UUID)
case invalidResponse
// a sample failure JSON
let errorJSON = """
"error": "rateLimit"
""".data(using: .utf8)!
let dataTaskPublisher: AnyPublisher<(data: Data, response: URLResponse), Error> = Deferred {
Future { promise in
print("Network call \(UUID())")
// mock a 429 server response with the `errorJSON` as a response body
let httpResponse = HTTPURLResponse(url: URL(string: "")!, statusCode: 429,
httpVersion: nil, headerFields: nil)!
promise(.success((data: errorJSON, response: httpResponse)))
var cancellables = Set<AnyCancellable>()
// Because I don't like typing long stuff
typealias DataTaskOutput = URLSession.DataTaskPublisher.Output
typealias DataTaskResult = Result<DataTaskOutput, Error>
// analyze the response to ensure we have a response that we want to try
// I transform output -> Result so we can work around the tryCatch later if we encountered a non-retryable error
.tryMap({ (dataTaskOutput: DataTaskOutput) -> DataTaskResult in
// infamous "should never happen" situation
guard let response = dataTaskOutput.response as? HTTPURLResponse else {
throw DataTaskError.invalidResponse
// we want to retry a rate limit error
if response.statusCode == 429 {
throw DataTaskError.rateLimit(UUID())
// we don't want to retry anything else
return .success(dataTaskOutput)
// catch any errors
.catch({ (error: Error) -> AnyPublisher<DataTaskResult, Error> in
switch error {
case DataTaskError.rateLimit(let uuid):
print("caught error: \(uuid)")
// return a Fail publisher that fails after 3 seconds, this means the `retry` will fire after 3s
return Fail(error: error)
.delay(for: 3, scheduler: DispatchQueue.main)
// We encountered a non-retryable error, wrap in Result so the publisher succeeds and we'll extract the error later
return Just(.failure(error))
.setFailureType(to: Error.self)
.tryMap({ result in
// Result -> Result.Success or emit Result.Failure
return try result.get()
.sink(receiveCompletion: { completion in
}, receiveValue: { value in
.store(in: &cancellables)
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lobianco commented May 10, 2021

This is so clever 👏

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