When to use weak self with URLSession.shared.dataTask - swift

Question

I have the following, but it completes using a closure. The question is whether the code will cause a memory cycle without the use of [weak self].
class ViewModel {
init() {}
var completion: ((Users) -> Void)?
func downloadFiles() {
guard let url = URL(string: "https://reqres.in/api/users?page=2") else {return}
let task = URLSession.shared.dataTask(
with: url,
completionHandler: { data, response, _ in // add [weak self]
guard let data = data,
let httpResponse = response as? HTTPURLResponse,
200..<300 ~= httpResponse.statusCode
else { return }
if let decoded = try? JSONDecoder().decode(Users.self, from: data) {
self.completion?(decoded)
}
})
task.resume()
}
}
using the following viewcontroller
class ViewController: UIViewController {
private let viewModel: ViewModel
override func viewDidLoad() {
super.viewDidLoad()
}
override func loadView() {
let view = UIView()
view.backgroundColor = .red
self.view = view
}
init(viewModel: ViewModel) {
self.viewModel = viewModel
super.init(nibName: nil, bundle: nil)
}
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
viewModel.completion = {
print($0)
}
viewModel.downloadFiles()
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
and model
struct Users: Decodable {
let page: Int
let per_page: Int
let total: Int
let total_pages: Int
let data: [UserData]
}
struct UserData: Decodable {
let id: Int
let email: String
let first_name: String
let last_name: String
let avatar: String
}
So should [weak self] be used, and if not why not. This is a minimum example, and as such the memory debugger doesn't help me out here, but I want to know for the general case.

Answer 1

You need to use [weak self]. There are times when you won't end up with a memory leak not using [weak self], but you can't rely on that. Suppose your request takes too long to load, and the user clicks the back button. Your ViewController will wan to deinitalize, and clear itself from memory, but it can't. It can't because it has a strong reference to your ViewModel which is still downloading and has a strong reference there also. So basically you end up with a memory leak, that is your ViewController remains in memory, which you can't clear anymore.

Answer 2

In your code URLSessionDataTask completion block holds reference to view model. This means that it won't be released until task completes. There is no memory leak because you don't have situation where object A holds object B and object B holds object A.
However, we can ask a question: Do you need results of your data download when view controller is released and thus should be view model. If not you should go with [weak self] for view model