Why doesn't URLSession.DataTaskPublisher ever publish values?

Question

In Xcode 11 beta 5 or 6 my existing code that relied on URLSession.DataTaskPublisher stopped working. It seems like DataTaskPublisher is never publishing any values but I can't work out why.

I've tried with .sink and .handleEvents as subscribers. I've tested .sink with a Just publisher and confirmed it receives a value there.

I've also tried both giving the DataTaskPublisher a URL and giving it a URLRequest. I've tried a request to an API including an authorization header, as well as basic requests to google.com and apple.com. I've tried using URLSession.shared and creating a new instance of URLSession. I've also tried with and without map and decode operators.

I've used XCTest expectations to confirm that the test times out every single time, even if I give it a 4-minute timeout.

I just made a new example project and replicated the problem with the following code in the root view controller:

override func viewDidLoad() {
        super.viewDidLoad()

        print("view did load")

        URLSession.shared.dataTaskPublisher(for: URL(string: "http://apple.com")!)
            .handleEvents(receiveSubscription: { (sub) in
                print(sub)
            }, receiveOutput: { (response) in
                print(response)
            }, receiveCompletion: { (completion) in
                print(completion)
            }, receiveCancel: {
                print("cancel")
            }, receiveRequest: { (demand) in
                print(demand)
            })
    }

The project prints "view did load" but nothing else ever prints. Any ideas about where I'm going wrong here? Thanks!

Answer 1

I think that there are two problems with your code, firstly you only have a publisher (handleEvent returns a publisher) and secondly that publisher goes out of scope and disappears. This works although it isn't exactly elegant.

import Combine
import SwiftUI

var pub: AnyPublisher<(data: Data, response: URLResponse), URLError>? = nil
var sub: Cancellable? = nil

var data: Data? = nil
var response: URLResponse? = nil

func combineTest() {
    guard let url = URL(string: "https://apple.com") else {
        return
    }
    pub = URLSession.shared.dataTaskPublisher(for: url)
            .print("Test")
            .eraseToAnyPublisher()
    sub = pub?.sink(
        receiveCompletion: { completion in
            switch completion {
            case .finished:
                break
            case .failure(let error):
                fatalError(error.localizedDescription)
            }
        },
        receiveValue: { data = $0.data; response = $0.response }
    )
}

struct ContentView: View {
    var body: some View {
        Button(
            action: { combineTest() },
            label: { Text("Do It").font(.largeTitle) }
        )
    }
}

I did it in SwiftUI so that I would have less to worry about and I used 3 variables so that I could follow better. You need to use the 2 parameter sink as the publisher's error isn't Never. Finally the print() is just for test and works really well.