Combine framework retry after delay?

Question

I see how to use .retry directly, to resubscribe after an error, like this:

    URLSession.shared.dataTaskPublisher(for:url)
        .retry(3)

But that seems awfully simple-minded. What if I think that this error might go away if I wait awhile? I could insert a .delay operator, but then the delay operates even if there is no error. And there doesn't seem to be a way to apply an operator conditionally (i.e. only when there's an error).

I see how I could work around this by writing a RetryWithDelay operator from scratch, and indeed such an operator has been written by third parties. But is there a way to say "delay if there's an error", purely using the operators we're given?

My thought was that I could use .catch, because its function runs only if there is an error. But the function needs to return a publisher, and what publisher would we use? If we return somePublisher.delay(...) followed by .retry, we'd be applying .retry to the wrong publisher, wouldn't we?

Answer 1

I found a few quirks with the implementations in the accepted answer.

• Firstly the first two attempts will be fired off without a delay since the first delay will only take effect after the second attempt.

• Secondly if any one of the retry attempts succeed, the output value will also delayed which seems unnecessary.

• Thirdly the extension is not flexible enough to allow the user to decide which scheduler it would like the retry attempts to be dispatched to.

After some tinkering back and forth I ended up with a solution like this:

public extension Publisher {
    /**
     Creates a new publisher which will upon failure retry the upstream publisher a provided number of times, with the provided delay between retry attempts.
     If the upstream publisher succeeds the first time this is bypassed and proceeds as normal.

     - Parameters:
        - retries: The number of times to retry the upstream publisher.
        - delay: Delay in seconds between retry attempts.
        - scheduler: The scheduler to dispatch the delayed events.

     - Returns: A new publisher which will retry the upstream publisher with a delay upon failure.

     ~~~
     let url = URL(string: "https://api.myService.com")!

     URLSession.shared.dataTaskPublisher(for: url)
         .retryWithDelay(retries: 4, delay: 5, scheduler: DispatchQueue.global())
         .sink { completion in
             switch completion {
             case .finished:
                 print("Success 😊")
             case .failure(let error):
                 print("The last and final failure after retry attempts: \(error)")
             }
         } receiveValue: { output in
             print("Received value: \(output)")
         }
         .store(in: &cancellables)
     ~~~
     */
    func retryWithDelay<S>(
        retries: Int,
        delay: S.SchedulerTimeType.Stride,
        scheduler: S
    ) -> AnyPublisher<Output, Failure> where S: Scheduler {
        self
            .delayIfFailure(for: delay, scheduler: scheduler)
            .retry(retries)
            .eraseToAnyPublisher()
    }

    private func delayIfFailure<S>(
        for delay: S.SchedulerTimeType.Stride,
        scheduler: S
    ) -> AnyPublisher<Output, Failure> where S: Scheduler {
        self.catch { error in
            Future { completion in
                scheduler.schedule(after: scheduler.now.advanced(by: delay)) {
                    completion(.failure(error))
                }
            }
        }
        .eraseToAnyPublisher()
    }
}

Answer 2

Using .catch is indeed the answer. We simply make a reference to the data task publisher and use that reference as the head of both pipelines — the outer pipeline that does the initial networking, and the inner pipeline produced by the .catch function.

Let's start by creating the data task publisher and stop:

let pub = URLSession.shared.dataTaskPublisher(for: url).share()

Now I can form the head of the pipeline:

let head = pub.catch {_ in pub.delay(for: 3, scheduler: DispatchQueue.main)}
    .retry(3)

That should do it! head is now a pipeline that inserts a delay operator only just in case there is an error. We can then proceed to form the rest of the pipeline, based on head.

Observe that we do indeed change publishers; if there is a failure and the catch function runs, the pub which is the upstream of the .delay becomes the publisher, replacing the pub we started out with. However, they are the same object (because I said share), so this is a distinction without a difference.