iOS Swift Combine: cancel a Set<AnyCancellable>

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