iOS Swift Combine: cancel a Set

Question

If I have stored a cancellable set into a ViewController:

private var bag = Set<AnyCancellable>()

Which contains multiple subscription.

1 - Should I cancel subscription in deinit? or it does the job automatically?

2 - If so, how can I cancel all the stored subscriptions?

bag.removeAll() is enough?

or should I iterate through the set and cancel all subscription one by one?

for sub in bag {
   sub.cancel()
}

Apple says that the subscription is alive until the stored AnyCancellable is in memory. So I guess that deallocating the cancellables with bag.removeAll() should be enough, isn't it?

Answer 1

On deinit your ViewController will be removed from memory. All of its instance variables will be deallocated.

The docs for Combine > Publisher > assign(to:on:) say:

An AnyCancellable instance. Call cancel() on this instance when you no longer want the publisher to automatically assign the property. Deinitializing this instance will also cancel automatic assignment.

1 - Should I cancel subscription in deinit? or it does the job automatically?

You don't need to, it does the job automatically. When your ViewController gets deallocated, the instance variable bag will also be deallocated. As there is no more reference to your AnyCancellable's, the assignment will end.

2 - If so, how can I cancel all the stored subscriptions?

Not so. But often you might have some subscriptions that you want to start and stop on, say, viewWillAppear/viewDidDissapear, for example. In this case your ViewController is still in memory.

So, in viewDidDissappear, you can do bag.removeAll() as you suspected. This will remove the references and stop the assigning.

Here is some code you can run to see .removeAll() in action:

var bag = Set<AnyCancellable>()

func testRemoveAll() {
  Timer.publish(every: 1, on: .main, in: .common).autoconnect()
    .sink { print("===== timer: \($0)") }
    .store(in: &bag)

  Timer.publish(every: 10, on: .main, in: .common).autoconnect()
    .sink { _ in self.bag.removeAll() }
    .store(in: &bag)
}

The first timer will fire every one second and print out a line. The second timer will fire after 10 seconds and then call bag.removeAll(). Then both timer publishers will be stopped.

https://developer.apple.com/documentation/combine/publisher/3235801-assign

Answer 2

if you happened to subscribe to a publisher from your View controller, likely you will capture self in sink, which will make a reference to it, and won't let ARC remove your view controller later if the subscriber didn't finish, so it's, advisable to weakly capture self

---

so instead of:

   ["title"]
      .publisher
      .sink { (publishedValue) in
        self.title.text = publishedValue
    }

.store(in: &cancellable)

you should use a [weak self]:

   ["title"]
      .publisher
      .sink { [weak self] (publishedValue) in
        self?.title.text = publishedValue
    }

.store(in: &cancellable)

thus, when View controller is removed, you won't have any retain cycle or memory leaks.

Answer 3

Try creating a pipeline and not storing the cancellable in some state variable. You’ll find that the pipeline stops as soon as it encounters an async operation. That’s because the Cancellable was cleaned up by ARC and it was thus automatically cancelled. So you don’t need to call cancel on a pipeline if you release all references to it.

From the documentation:

An AnyCancellable instance automatically calls cancel() when deinitialized.

Answer 4

I test this code

let cancellable = Set<AnyCancellable>()

Timer.publish(every: 1, on: .main, in: .common).autoconnect()
  .sink { print("===== timer: \($0)") }
  .store(in: &cancellable)
  
cancellable.removeAll() // just remove from Set. not cancellable.cancel()

---

so I use this extension.

import Combine

typealias CancelBag = Set<AnyCancellable>

extension CancelBag {
  mutating func cancelAll() {
    forEach { $0.cancel() }
    removeAll()
  }
}

Answer 5

Create a Cancellable+Extensions.swift

import Combine

typealias DisposeBag = Set<AnyCancellable>

extension DisposeBag {
    mutating func dispose() {
        forEach { $0.cancel() }
        removeAll()
    }
}

In your implementation class, in my case CurrentWeatherViewModel.swift simply add disposables.dispose() to remove Set of AnyCancellable

import Combine
import Foundation

final class CurrentWeatherViewModel: ObservableObject {
    @Published private(set) var dataSource: CurrentWeatherDTO?

    let city: String
    private let weatherFetcher: WeatherFetchable
    private var disposables = Set<AnyCancellable>()

    init(city: String, weatherFetcher: WeatherFetchable = WeatherNetworking()) {
        self.weatherFetcher = weatherFetcher
        self.city = city
    }

    func refresh() {
        disposables.dispose()

        weatherFetcher
            .currentWeatherForecast(forCity: city)
            .map(CurrentWeatherDTO.init)
            .receive(on: DispatchQueue.main)
            .sink(receiveCompletion: { [weak self] value in
                guard let self = self else { return }
                switch value {
                case .failure:
                    self.dataSource = nil
                case .finished:
                    break
                }
                }, receiveValue: { [weak self] weather in
                    guard let self = self else { return }
                    self.dataSource = weather
            })
            .store(in: &disposables)
    }
}