Loop over Publisher Combine framework - swift

Question

I have the following function to perform an URL request:
final class ServiceManagerImpl: ServiceManager, ObservableObject {
private let session = URLSession.shared
func performRequest<T>(_ request: T) -> AnyPublisher<String?, APIError> where T : Request {
session.dataTaskPublisher(for: self.urlRequest(request))
.tryMap { data, response in
try self.validateResponse(response)
return String(data: data, encoding: .utf8)
}
.mapError { error in
return self.transformError(error)
}
.eraseToAnyPublisher()
}
}
Having these 2 following functions, I can now call the desired requests from corresponded ViewModel:
final class AuditServiceImpl: AuditService {
private let serviceManager: ServiceManager = ServiceManagerImpl()
func emptyAction() -> AnyPublisher<String?, APIError> {
let request = AuditRequest(act: "", nonce: String.randomNumberGenerator)
return serviceManager.performRequest(request)
}
func burbleAction(offset: Int) -> AnyPublisher<String?, APIError> {
let request = AuditRequest(act: "burble", nonce: String.randomNumberGenerator, offset: offset)
return serviceManager.performRequest(request)
}
}
final class AuditViewModel: ObservableObject {
#Published var auditLog: String = ""
private let auditService: AuditService = AuditServiceImpl()
init() {
let timer = Timer(timeInterval: 5, repeats: true) { _ in
self.getBurbles()
}
RunLoop.main.add(timer, forMode: .common)
}
func getBurbles() {
auditService.emptyAction()
.flatMap { [unowned self] offset -> AnyPublisher<String?, APIError> in
let currentOffset = Int(offset?.unwrapped ?? "") ?? 0
return self.auditService.burbleAction(offset: currentOffset)
}
.receive(on: RunLoop.main)
.sink(receiveCompletion: { [unowned self] completion in
print(completion)
}, receiveValue: { [weak self] burbles in
self?.auditLog = burbles!
})
.store(in: &cancellableSet)
}
}
Everything is fine when I use self.getBurbles() for the first time. However, for the next calls, print(completion) shows finished, and the code doesn't perform self?.auditLog = burbles!
I don't know how can I loop over the getBurbles() function and get the response at different intervals.
Edit
The whole process in a nutshell:
I call getBurbles() from class initializer
getBurbles() calls 2 nested functions: emptyAction() and burbleAction(offset: Int)
Those 2 functions generate different requests and call performRequest<T>(_ request: T)
Finally, I set the response into auditLog variable and show it on the SwiftUI layer

Answer 1

There are at least 2 issues here.
First when a Publisher errors it will never produce elements again. That's a problem here because you want to recycle the Publisher here and call it many times, even if the inner Publisher fails. You need to handle the error inside the flatMap and make sure it doesn't propagate to the enclosing Publisher. (ie you can return a Result or some other enum or tuple that indicates you should display an error state).
Second, flatMap is almost certainly not what you want here since it will merge all of the api calls and return them in arbitrary order. If you want to cancel any existing requests and only show the latest results then you should use .map followed by switchToLatest.