Why is an old view model responding to a notification? - swift

Question

I’m creating a weightlifting calculator application (Swift 4) using MVVM and have been trying for 2 days to figure out why a view model that should have died is still responding to a UserDefaults.defaultsDidChange event notification.
I launch the app:
At launch, in the AppDelegate, I create a new lift event object and use it to initialize a new CalculatorLiftEventViewModelFromLiftEvent for the `CalculatorViewController':
I calculate a lift and save it
I tap the + button to create a new lift:
this causes a new, empty lift event object to be created
this new lift event object is used to initialize a new CalculatorLiftEventViewModelFromLiftEvent object
this new CalculatorLiftEventViewModelFromLiftEvent is then assigned to the CalculatorViewController's viewModel property, replacing the one created when the app launched
the values on the calculator screen are zeroed out, ready for a new lift event to be entered
I tap the Settings button to go to Settings where I change the Formula associated with the current lift event.
The new Formula is saved as the default and the UserDefaults.defaultsDidChange notification is fired
HERE’S THE PART I CAN’T FIGURE OUT: the original view model is still alive and it’s still listening for UserDefault notifications. When I close the Settings screen and go back to the Calculator view, the values from the prior lift event that had been cleared out now reappear.
Here’s what happens when the + (new) button on the Calculator screen is tapped:
#objc fileprivate func onNewButtonTapped(_ sender: UIBarButtonItem) {
let newLiftEvent = dataManager.createNewLiftEvent()
viewModel = CalculatorLiftEventViewModelFromLiftEvent(withLiftEvent: newLiftEvent, dataManager: dataManager)
setupView()
}
Here’s how the CalculatorLiftEventViewModelFromLiftEvent is initialized:
init(withLiftEvent liftEvent: LiftEventRepresentable, dataManager: CoreDataHelper) {
self.modelLiftEvent = liftEvent
self.liftName = Dynamic("\(modelLiftEvent.lift.liftName)")
self.weightLiftedTextField = Dynamic(modelLiftEvent.liftWeight.value)
self.repetitionsTextField = Dynamic("\(modelLiftEvent.repetitions)")
self.oneRepMaxTextField = Dynamic(modelLiftEvent.oneRepMax.value)
self.unitsTextField = Dynamic("\(UserDefaults.weightUnit())")
self.weightPercentages = Dynamic( [ : ] )
self.dataManager = dataManager
super.init()
subscribeToNotifications()
}
UPDATE: Here are the deinit and the addObservers in CalculatorLiftEventViewModelFromLiftEvent. Notice I'm not using block-based observations.
deinit {
print("I got to the deinit method")
unsubscribeFromNotifications()
}
func subscribeToNotifications() {
NotificationCenter.default.addObserver(self,
selector: #selector(liftNameDidChangeNotification(_:)),
name: NSNotification.Name(rawValue: LiftEventNotifications.LiftNameDidChangeNotification),
object: nil)
NotificationCenter.default.addObserver(self,
selector: #selector(weightUnitDidChangeNotification(_:)),
name: NSNotification.Name(rawValue: LiftEventNotifications.WeightUnitDidChangeNotification),
object: nil)
NotificationCenter.default.addObserver(self,
selector: #selector(roundingOptionDidChangeNotification(_:)),
name: NSNotification.Name(rawValue: UserDefaultsNotifications.roundingOptionDidChangeNotification),
object: nil)
NotificationCenter.default.addObserver(self, selector: #selector(self.defaultsDidChange), name: UserDefaults.didChangeNotification,
object: nil)
}
--- END UPDATE
I pass the modelLiftEvent when segueing to the SettingsViewController:
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if let identifier = segue.identifier {
switch identifier {
case a:...
case b:...
case "SettingsSegue":
if let nav = segue.destination as? UINavigationController {
let destinationViewController = nav.topViewController as! SettingsViewController
destinationViewController.dismissalDelegate = self
let settingsViewModel = SettingsViewModelFromLiftEvent(withLiftEvent: self.viewModel.modelLiftEvent)
destinationViewController.settingsViewModel = settingsViewModel
destinationViewController.dataManager = dataManager
settingsViewModel.dataManager = dataManager
}
Finally, in CalculatorLiftEventViewModelFromLiftEvent, I’ve put a break point here because this is called when the view model hears the UserDefaults.defaultsDidChange notification. At this point, I have also verified that this CalculatorLiftEventViewModelFromLiftEvent is the old one, not the new one created when I tapped the + button:
#objc func defaultsDidChange(_ notification: Notification) {
let oneRepMax = modelLiftEvent.calculateOneRepMax()
guard oneRepMax.value != 0.0 else { return }
let weightPercentages = getWeightPercentages(weight: oneRepMax.value)
self.weightPercentages.value = weightPercentages
weightLiftedTextField.value = modelLiftEvent.liftWeight.value
repetitionsTextField.value = "\(modelLiftEvent.repetitions)"
oneRepMaxTextField.value = modelLiftEvent.oneRepMax.value
}
I've read through a bunch of documentation about the life cycle of objects but haven't found anything that helps. I expect that when the new CalculatorLiftEventViewModelFromLiftEvent is created and assigned to the `CalculatorViewController''s viewModel property, it would replace the reference to the old one and it would cease to exist. Evidently, that's not what's happening.
Does anyone have any idea why when I go from the Calculator view (step 3) that has no values (except for 0.0) to the Settings and then come back, the prior lift event values are displayed?

Answer 1

I've fixed the problem of the prior liftEvent being displayed after clearing the calculator, changing the default formula, and coming back to the calculator screen.
On CalculatorViewController, when the + button is tapped, instead of creating a new viewModel and assigning it to the viewModel property, I'm asking my AppDelegate to create both a new CalculatorViewController and CalculatorLiftEventViewModelFromLiftEvent by using the launchCalculatorViewController method which does this when the app launches.
The original code in CalculatorViewController:
#objc fileprivate func onNewButtonTapped(_ sender: UIBarButtonItem) {
let newLiftEvent = dataManager.createNewLiftEvent()
viewModel = CalculatorLiftEventViewModelFromLiftEvent(withLiftEvent: newLiftEvent, dataManager: dataManager)
self.percentagesTableView.reloadData()
setupView()
}
Now the new code in CalculatorViewController:
#objc fileprivate func onNewButtonTapped(_ sender: UIBarButtonItem) {
(UIApplication.shared.delegate as? AppDelegate)?.launchCalculatorViewController()
}
and in AppDelegate:
func launchCalculatorViewController() {
self.window = UIWindow(frame: UIScreen.main.bounds)
let mainStoryboard: UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
if let initialViewController: CalculatorViewController = mainStoryboard.instantiateInitialViewController() as? CalculatorViewController {
self.window?.rootViewController = initialViewController
let liftEvent = dataManager.createNewLiftEvent()
let viewModel = CalculatorLiftEventViewModelFromLiftEvent(withLiftEvent: liftEvent, dataManager: dataManager)
initialViewController.viewModel = viewModel
initialViewController.dataManager = dataManager
self.window?.makeKeyAndVisible()
}
}
Unfortunately, I determined that CalculatorLiftEventViewModelFromLiftEvent objects are never being deallocated which tells me I've got a strong reference cycle that won't let go:
That will have to be another SO question.