What happens to a promise that's abandoned?

Question

I have the following code defined in a view controller.

    _ = accountService.getAccount()

      .then { user -> Void in

         self.txtBxUser.text = user.username
         self.txtBxEmail.text = user.email
   }

getAccount makes a REST API request.

If the user dismisses the view controller before the call has returned, what happens to the call back chain? Does it still run given that, I presume, it's still referenced?

Answer 1

If the user dismisses the view controller before the call has returned, what happens to the call back chain? Does it still run given that, I presume, it's still referenced?

Yes, it does still run.

Be forewarned that the reference to self in the closure means that it is also keeping a reference to the view controller until that then closure finishes running. For that reason, if there's a chance the view controller could have been dismissed, you might want to use a weak reference:

_ = accountService.getAccount().then { [weak self] user -> Void in
    self?.txtBxUser.text = user.username
    self?.txtBxEmail.text = user.email
}

Ideally, you should also make getAccount cancellable and cancel it in the view controller's deinit.

(Note, in FAQ - Should I be Concerned About Retain Cycles, the PromiseKit documentation points out that you don't need weak references, which is correct. It's just a question of whether or not you mind if the deallocation of the dismissed view controller is deferred until after the promise is fulfilled.)