Cold publisher Sink attempt after failure.

cold_publisher_sink_after_failure.swift

enum SomeError: Error {
	case anError
}

let requestURL = URL(string: "https://picsum.photos/v2/list")!

struct Photo: Decodable {
	var id: String
}

var attemptCount = 1

let coldPublisher: AnyPublisher<[Photo], Error> = URLSession.shared
	.dataTaskPublisher(for: requestURL)
	.map(\.data)
	.decode(type: [Photo].self, decoder: JSONDecoder())
	.tryMap {
		defer { attemptCount += 1 }
		if attemptCount.isMultiple(of: 2) {
			return $0
		} else {
			throw SomeError.anError
		}
	}
	.eraseToAnyPublisher()

let sink = Subscribers.Sink<[Photo], Error>( /// (1) `Sink` implicitly requests unlimited demand.
	receiveCompletion: { print($0) },
	receiveValue: { print($0.map(\.id)) }
)

coldPublisher
	.subscribe(sink)

coldPublisher
	.subscribe(sink)

/// (2) Output, only the first failure logged and the second attempt is ignored:
/// ```none
/// failure(__lldb_expr_47.SomeError.anError)
/// ```