Swift combine chain requests - swift

Question

I'm attempting to use combine to chain two requests together. The code is pretty rough, but I need to call two api requests. One to get the schedule data than one for live data. I'm able to get the live data (second request) but how do I get the schedule data (first request)? I'm having a hard time understanding how to use combine to chain two requests together, this is my first need to use combine for a widget I'm working on. I'm still fresh to Swift, so my terminology may be lacking.
My last code example wasn't correct and my question was unclear. I have two publishers and the second one depends on the first one. My understanding is still unclear on how to handle the data from my first publisher as well as in .flatMap for the second data. Does it need to be ObservableObject class and have #Published variables for the data? Do I use .assign or .sink to get data from my codable data Schedule and Live? Articles seem a bit too advance for myself as they create custom extensions and changing the API data to nested types.
New example code
import Foundation
import Combine
class DataGroup {
// How to get data from Schedule and Live codable data, do I use a variable and .assign or .sink?
// Where do I put the subscriber?
func requestSchedule(_ teamID : Int) -> AnyPublisher<Schedule, Error> {
let url = URL(string: "https://statsapi.web.nhl.com/api/v1/schedule?teamId=\(teamID)")!
return URLSession
.shared.dataTaskPublisher(for: url)
.map(\.data)
.decode(type: Schedule.self, decoder: JSONDecoder())
.flatMap {self.fetchLiveFeed($0.dates.first?.games.first?.link ?? "")}
/*
.flatMap {URLSession.shared.dataTaskPublisher(for: URL(string: $0.dates.first?.games.first?.link ?? "")!)}
*/
.eraseToAnyPublisher()
}
// Remove and put into flatMap URLSession.shared.dataTaskPublisher?
func fetchLiveFeed(_ link: String) -> AnyPublisher<Live, Error> {
let url = URL(string: "https://statsapi.web.nhl.com\(link)")!
return URLSession.shared.dataTaskPublisher(for: url)
.map(\.data)
.decode(type: Live.self, decoder: JSONDecoder())
.eraseToAnyPublisher()
}
}
OLD
import Foundation
import Combine
class CombineData {
var schedule: Schedule? // Get schedule data alongside live data
var live: Live?
private var cancellables = Set<AnyCancellable>()
func fetchSchedule(_ teamID: Int, _ completion: #escaping (/* Schedule, */Live) -> Void) {
let url = URL(string: "https://statsapi.web.nhl.com/api/v1/schedule?teamId=\(teamID)")!
URLSession.shared.dataTaskPublisher(for: url)
.map { $0.data }
.decode(type: Schedule.self, decoder: JSONDecoder())
.flatMap { self.fetchLiveFeed($0.dates.first?.games.first?.link ?? "") }
.receive(on: DispatchQueue.main)
.sink(receiveCompletion: { _ in }) { data in
// How to get both schedule data and live data here?
//self.schedule = ?
self.live = data
print(data)
completion(self.schedule!, self.live!)
}.store(in: &cancellables)
}
func fetchLiveFeed(_ link: String) -> AnyPublisher<Live, Error> {
let url = URL(string: "https://statsapi.web.nhl.com\(link)")!
return URLSession.shared.dataTaskPublisher(for: url)
.map(\.data)
.decode(type: Live.self, decoder: JSONDecoder())
.eraseToAnyPublisher()
}
}

Answer 1

The general idea is to use a flatMap for chaining, which is what you did, but if you also need the original value, you can return a Zip publisher (with a .zip operator) that puts two results into a tuple.
One of the publishers is the second request, and the other should just emit the value. You can typically do this with Just(v), but you have to make sure that its failure type (which is Never) matches with the other publisher. You can match its failure type with .setFailureType(to:):
publisher1
.flatMap { one in
Just(one).setFailureType(to: Error.self) // error has to match publisher2
.zip(publisher2(with: one))
}
.sink(receiveCompletion: { completion in
// ...
}, receiveValue: { (one, two) in
// ...
})
Alternatively, you can use Result.Publisher which would infer the error (but might look somewhat odd):
.flatMap { one in
Result.Publisher(.success(one))
.zip(publisher2)
}
So, in your case it's going to be something like this:
URLSession.shared.dataTaskPublisher(for: url)
.map(\.data)
.decode(type: Schedule.self, decoder: JSONDecoder())
.flatMap {
Result.Publisher(.success($0))
.zip(self.fetchLiveFeed($0.dates.first?.games.first?.link ?? ""))
}
.sink(receiveCompletion: { completion in
// ...
}, receiveValue: { (schedule, live) in
// ...
})
.store(in: &cancellables)