Cold publisher double `Sink` with distinct subscribers.

cold_publisher_double_sink.swift

import PlaygroundSupport /// (1) We’ll need this import to allow for indefinite execution.

PlaygroundPage.current.needsIndefiniteExecution = true

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 sink1 = Subscribers.Sink<[Photo], Error>(
	receiveCompletion: { print("1: \($0)") },
	receiveValue: { print("1: \($0.map(\.id))") }
)

let sink2 = Subscribers.Sink<[Photo], Error>(
	receiveCompletion: { print("2: \($0)") },
	receiveValue: { print("2: \($0.map(\.id))") }
)

coldPublisher
	.subscribe(sink1)

DispatchQueue.main.asyncAfter(deadline: .now() + 2) { /// (2) Tacking on some delay to make things more clear.
	coldPublisher
		.subscribe(sink2)
}

/// (3) Output:
/// ```none
/// 1: failure(__lldb_expr_55.SomeError.anError)
/// 2: ["0", "1", "10", "100", "1000", "1001", "1002", "1003", "1004", "1005", "1006", "1008", "1009", "101", "1010", "1011", "1012", "1013", "1014", "1015", "1016", "1018", "1019", "102", "1020", "1021", "1022", "1023", "1024", "1025"]
/// 2: finished
/// ```