In summary
If a user takes a photo with an overlay on VC1 it triggers a notification. The corresponding observer gets removed by my method as a I present VC2.
If the user doesn't take a photo on VC1, the notification observer isn't trigger. The same removal method is called as VC2 is presented but the observer stays active and causes unwanted behaviours with VC2 photo capture.
Full explanation
I have a registration form where a user can take an optional photo of their face. I'm using a simple UIImagePickerController and presenting a 'passport' style overlay.
if sourceType == .camera {
imagePickerController.cameraDevice = .front
let overlay = PassportOverlayView(frame: imagePickerController.view.frame)
imagePickerController.cameraOverlayView = overlay
}
The overlay covers the 'retake' and 'choose' buttons of the UIImagePickerController so I observe the following NSNotifications to remove the overlay when a photo taken, and re-add it if required should the user wish to retake their photo.
NotificationCenter.default.addObserver(forName: NSNotification.Name(rawValue: "_UIImagePickerControllerUserDidCaptureItem"), object:nil, queue:nil, using: { note in
self.imagePickerController.cameraOverlayView = nil
})
NotificationCenter.default.addObserver(forName: NSNotification.Name(rawValue: "_UIImagePickerControllerUserDidRejectItem"), object:nil, queue:nil, using: { note in
self.imagePickerController.cameraOverlayView = PassportOverlayView(frame: self.imagePickerController.view.frame)
})
Everything works as intended. I remove the notifications before the next UIViewController is pushed onto the stack.
func removeObservers(){
print("remove Observers")
NotificationCenter.default.removeObserver(self, name: NSNotification.Name(rawValue: "_UIImagePickerControllerUserDidCaptureItem"), object: nil)
NotificationCenter.default.removeObserver(self, name: NSNotification.Name(rawValue: "_UIImagePickerControllerUserDidRejectItem"), object: nil)
}
If the user has taken a photo and therefore called the observer adding a photo on the next UIViewController, which doesn't require an overlay, works fine.
If however, the user doesn't take a photo on the first UIViewController [which is perfectly fine for the purposes of my app] they run into an issue on the second UIViewController.
In both scenerios the removeObservers() function is called. "remove Observers" is printed to the console on both occasions. Yet when the users tries to take a photo from within the second UIViewController the app crashes as it Unexpectedly found nil while implicitly unwrapping an Optional value and points me to the self.imagePickerController.cameraOverlayView = nil line of the _UIImagePickerControllerUserDidCaptureItem notification.
I understand the error, it's trying to remove a cameraOverlayView that isn't there. What I can't understand is why the observer is still there when I believe it's already been removed. If the user takes the first photo with the cameraOverlayView and triggers the _UIImagePickerControllerUserDidCaptureItem notification there isn't an issue. The observer is removed and subsequent UIImagePickerControllers don't have the same issue.
Any help would be greatly appreciated.
I think I know what your problem is. A retain cycle is causing the observer to not be removed successfully. The reason why you have a retain cycle is because you are using a strong reference to self in the closure where you set your notifications. Here is how you fix it:
NotificationCenter.default.addObserver(forName: NSNotification.Name(rawValue: "_UIImagePickerControllerUserDidCaptureItem"), object:nil, queue:nil, using: { [unowned self] note in
self.imagePickerController.cameraOverlayView = nil
})
NotificationCenter.default.addObserver(forName: NSNotification.Name(rawValue: "_UIImagePickerControllerUserDidRejectItem"), object:nil, queue:nil, using: { [unowned self] note in
self.imagePickerController.cameraOverlayView = PassportOverlayView(frame: self.imagePickerController.view.frame)
})
Again, you shouldn't have to worry about removing the observers, so try to run your code without removeObserver and it should work.
Hope this helps.
Related
I am writing several Swift multiplayer games based on the Ray Wenderlich tutorial for Nine Knights. (https://www.raywenderlich.com/7544-game-center-for-ios-building-a-turn-based-game)
I use pretty much the same GameCenterHelper file except that I change to a segue instead of present scene since I am using UIKit instead of Sprite Kit with the following important pieces:
present match maker:
func presentMatchmaker() {
guard GKLocalPlayer.local.isAuthenticated else {return}
let request = GKMatchRequest()
request.minPlayers = 2
request.maxPlayers = 2
request.inviteMessage = "Would you like to play?"
let vc = GKTurnBasedMatchmakerViewController(matchRequest: request)
vc.turnBasedMatchmakerDelegate = self
currentMatchmakerVC = vc
print(vc)
viewController?.present(vc, animated: true)
}
the player listener function:
extension GameCenterHelper: GKLocalPlayerListener {
func player(_ player: GKPlayer, receivedTurnEventFor match: GKTurnBasedMatch, didBecomeActive: Bool) {
if let vc = currentMatchmakerVC {
currentMatchmakerVC = nil
vc.dismiss(animated: true)
}
guard didBecomeActive else {return}
NotificationCenter.default.post(name: .presentGame, object: match)
}
}
The following extension for Notification Center:
extension Notification.Name {
static let presentGame = Notification.Name(rawValue: "presentGame")
static let authenticationChanged = Notification.Name(rawValue: "authenticationChanged")
}
In the viewdidload of the menu I call the following:
override func viewDidLoad() {
super.viewDidLoad()
createTitleLabel()
createGameImage()
createButtons()
GameCenterHelper.helper.viewController = self
GameCenterHelper.helper.currentMatch = nil
NotificationCenter.default.addObserver(self, selector: #selector(authenticationChanged(_:)), name: .authenticationChanged, object: nil)
NotificationCenter.default.addObserver(self, selector: #selector(presentGame(_:)), name: .presentGame, object: nil)
}
and tapping the multi device buttons calls the following:
#objc func startMultiDeviceGame() {
multiPlayer = true
GameCenterHelper.helper.presentMatchmaker()
}
and the notifications call the following:
#objc func presentGame(_ notification: Notification) {
// 1
print("present game")
guard let match = notification.object as? GKTurnBasedMatch else {return}
loadAndDisplay(match: match)
}
// MARK: - Helpers
private func loadAndDisplay(match: GKTurnBasedMatch) {
match.loadMatchData { [self] data, error in
if let data = data {
do {
gameModel = try JSONDecoder().decode(GameModel.self, from: data)
} catch {gameModel = GameModel()}
} else {gameModel = GameModel()}
GameCenterHelper.helper.currentMatch = match
print("load and display")
performSegue(withIdentifier: "gameSegue", sender: nil)
}
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
print("prepare to segue")
if let vc = segue.destination as? GameVC {vc.gameModel = gameModel}
}
Which is hopefully easy to follow.
The game starts and the menu scene adds the observer for present game
The player taps multi device, which presents the matchmaker
The player selects their game from the match maker, which I think activates the player listener function
This posts to the Notification Center for present game
The notification center observer calls present game, which calls load and display, with a little help from prepare segue
My issue is that the first time I do this it works perfectly, and per the framework from that tutorial that I can't figure out how to change (an issue for a different question I think) after a player takes their turn they are returned to the menu. The second time they enter present matchmaker and select a game the present game function is called twice, and the third time they take their turn without shutting down the app it is called 3 times, etc. (I have the print statements in both the present game and load and display functions and they are called back to back the 2nd time through and back to back to back the 3rd time etc. even though they are only called once the first time a game is selected from the matchmaker)
Console messages
present matchmaker true
<GKTurnBasedMatchmakerViewController: 0x104810000>
present game
present game
present game
load and display
prepare to segue
load and display
prepare to segue
load and display
prepare to segue
2021-03-20 22:32:26.838680-0600 STAX[4997:435032] [Presentation] Attempt to present <STAX.GameVC: 0x103894c00> on <Game.MenuVC: 0x103814800> (from < Game.MenuVC: 0x103814800>) whose view is not in the window hierarchy.
(419.60100000000006, 39.0)
2021-03-20 22:32:26.877943-0600 STAX[4997:435032] [Presentation] Attempt to present <STAX.GameVC: 0x103898e00> on < Game.MenuVC: 0x10501c800> (from < Game.MenuVC: 0x10501c800>) whose view is not in the window hierarchy.
I had thought that this was due to me not removing the Notification Center observers, but I tried the following in the view did load for the menu screen (right before I added the .presentGame observer):
NotificationCenter.default.removeObserver(self, name: .presentGame, object: nil)
and that didn't fix the issue, so I tried the following (in place of the above):
NotificationCenter.default.removeObserver(self)
and that didn't work so I tried them each, one at a time in the view did disappear of the game view controller (which I didn't think would work since self refers to the menu vc, but I was getting desperate) and that didn't work either.
I started thinking that maybe I'm not adding multiple observers that are calling present game more than once, since the following didn't work at all the second time (I'm just using a global variable to keep track of the first run through that adds the observers and then not adding them the second time):
if addObservers {
NotificationCenter.default.addObserver(self, selector: #selector(authenticationChanged(_:)), name: .authenticationChanged, object: nil)
NotificationCenter.default.addObserver(self, selector: #selector(presentGame(_:)), name: .presentGame, object: nil)
addObservers = false
}
since it is trying to add a view that is not in the view hierarchy. (Although the background music for that screen starts playing, the menu remains and the game board is not shown...)
I wasn't sure if I'm removing the Notification Center observers incorrectly or if they aren't really the source of the problem so I decided to ask for help :)
Thank you!
I figured it out. I was trying to remove the Notifications from a deallocated instance of the view controller per the below link (The bottom most answer):
How to avoid adding multiple NSNotification observer?
The correct way to remove the notifications is in the view will disappear function like this:
override func viewWillDisappear(_ animated: Bool) {
NotificationCenter.default.removeObserver(self, name: Notification.Name.presentGame, object: nil)
NotificationCenter.default.removeObserver(self, name: Notification.Name.authenticationChanged, object: nil)
}
After implementing that I stopped making multiple calls to the notification center.
In my app, there is a ViewController.swift file and a popupViewController.swift file. Inside the app, when I open the popupViewController with storyboard segue as presentModally and then come back from popupViewController to ViewController with the code dismiss(), the methods viewDidLoad, viewWillAppear, viewDidAppear, ViewWillLayoutSubviews etc. nothing works, they execute just once and don't repeat when I go and return back. So, I want to execute the code every time when viewController.swift is active. I couldn't find a useful info in stackoverflow about this.
Meanwhile, I don't know much about notification and observers(if certainly needed), therefore, can you tell step by step in detail how to do that in Swift (not objective-c)? I mean how to determine if current view controller is active.
Edit: I am navigating from StoryBoard segue, presentModally. There is no Navigation Controller in storyboard.
I tried some codes but nothing happens. The point I came so far is:
override func viewDidLoad() {
super.viewDidLoad()
NotificationCenter.default.addObserver(self, selector:#selector(appWillEnterForeground), name:UIApplication.willEnterForegroundNotification, object: nil)
}
#objc func appWillEnterForeground() {
print("asdad") //nothing happens
if self.viewIfLoaded?.window != nil {
// viewController is visible
print("CURRENT VİEW CONTROLLER") //nothing happens
}
}
As mention in my comments, I don't use storyboards. There may be a way to create an unwind segue - or maybe not - but [here's a link][1] that may help you with a storyboard-only way of fixing your issue. A quick search on "modal" turned up 9 hits, and the second one starts going into details.
I'm thinking the issue is with what modality is. Basically, your first view controller, which properly executed viewDidAppear, is still visible. So it's effectively not executing viewDidDisappear when your second VC is presented.
You might want to change your concept a bit - an application window (think AppDelegate and/or SceneDelegate become active, where a UIViewController has a is initialized and deinitialized, along with a root UIView that is loaded, appears* and disappears*. This is important, because what you want to do is send your notification from the modal VC's viewDidDisappear override.
First, I find it easiest to put all your notication definitions in an extension:
extension Notification.Name {
static let modalHasDisappeared = Notification.Name("ModalHasDisappeared")
}
This helps not only reduce string typos but also is allows Xcode's code completion to kick in.
Next, in your first view controller, ad an observer to this notification:
init() {
super.init(nibName: nil, bundle: nil)
NotificationCenter.default.addObserver(self, selector: #selector(modalHasDisappeared), name: .modalHasDisappeared, object: nil)
}
required init?(coder: NSCoder) {
super.init(coder: coder)
NotificationCenter.default.addObserver(self, selector: #selector(modalHasDisappeared), name: .modalHasDisappeared, object: nil)
}
#objc func modalHasDisappeared() {
print("modal has disappeared")
}
I've added both forms of init for clarity. Since you are using a storyboard, I'd expect that init(coder:) is the one you need.
Finally, just send the notification when the modal has disappeared:
override func viewDidDisappear(_ animated: Bool) {
super.viewDidDisappear(animated)
NotificationCenter.default.post(name: .modalHasDisappeared, object: nil, userInfo: nil)
}
This sends no data, just the fact that the modal has disappeared. If you want to send data - say, a string or a table cell value, change the object parameter to it:
NotificationCenter.default.post(name: .modalHasDisappeared, object: myLabel, userInfo: nil)
And make the following changes in your first VC:
NotificationCenter.default.addObserver(self, selector: #selector(modalHasDisappeared(_:)), name: .modalHasDisappeared, object: nil)
#objc func modalHasDisappeared(_ notification:Notification) {
let label = notification.object as! UILabel!
print(label.text)
}
Last notes:
To repeat, note that by declaring an extension to Notification.Name, I've only have one place where I'm declaring a string.
There is no code in AppDelegate or SceneDelegate, nor any references to `UIApplication(). Try to think of the view (and view controller) as appearing/disappearing, not background/foreground.
While the first view is visually in the background, it's still visible. So the trick is to code against the modal view disappearing instead.
In my first view controller, I post the notification with the following code:
NotificationCenter.default.post(name: Notification.Name("date"), object: formattedDate)
I then "receive" the notification in a second view controller with the following code:
func receiveNotification () {
NotificationCenter.default.addObserver(self, selector: #selector(self.didGetTheDate(_:)), name: NSNotification.Name("date"), object: nil)
}
#objc
func didGetTheDate(_ notification: Notification) {
print("In did get date")
date = notification.object as! String
}
However, the function "didGetTheDate" never gets called.
I have triple checked that the function "receiveNotification" gets called as I have added print statements to check this.
Can somebody please help me with this.
NSNotificacionCenter is a variation of the Observer Pattern, you can read about it here
This means that you will have to register an Observer before posting any notification. If you post anything before that, the NSNotificationCenter will look at the observer for name and will see that nobody is listening for it, thus nothing will happen.
I am having a little bit of trouble. I am trying to use the notification center to alert the app that some content is done loading. After the user signs up or logs in successfully I create the view controller and then and then make it the root view controller using the blow
func finishLoggingIn() {
// print("Finish logging in from LoginController")
let homeController = HomeViewController()
self.loginButton.stopAnimation(animationStyle: .expand, completion: {
self.view.window?.rootViewController = homeController
self.view.window?.makeKeyAndVisible()
})
}
The login button just creates a loading animation on the button for UI purposes.
When I first enter the controller I add the observer for the notification.
let MainVCSetup = Notification.Name("mainVCComplete")
NotificationCenter.default.addObserver(self, selector: #selector(handleRootViewSwitch), name: MainVCSetup, object: nil)
When the content in my mainVC is done loading I post this same notification to the Notification Center like so
NotificationCenter.default.post(name: MainVCSetup, object: nil)
However this function never fires off no matter what I do
#objc func handleRootViewSwitch(){
print("Trying to handle root view switch attack")
NotificationCenter.default.removeObserver(self, name: MainVCSetup, object: nil)
}
If anyone notices where I went wrong I would greatly appreciate it.
I have a few notifications that were created using block / trailing closure syntax which look like this:
NotificationCenter.default.addObserver(forName: .NSManagedObjectContextObjectsDidChange, object: moc, queue: nil) { note in
// implementation
}
Which I was later removing by name, like this:
NotificationCenter.default.removeObserver(self, name: NSNotification.Name.NSManagedObjectContextObjectsDidChange, object: moc)
My Question
Is this adequate? Or do I absolutely need to save the NSObjectProtocol to it's own property and remove that property with the following syntax?
NotificationCenter.default.removeObserver(didChangeNotification)
You absolutely need to store the return value in a property and remove that later on.
From https://developer.apple.com/reference/foundation/nsnotificationcenter/1411723-addobserverforname:
Return Value
An opaque object to act as the observer.
When you call any one of the removeObserver methods, the first parameter is the observer to remove. When you set up a block to respond to a notification, self is not the observer, NSNotificationCenter creates its own observer object behind the scenes and returns it to you.
Note: as of iOS 9, you are no longer required to call removeObserver from dealloc/deinit, as that will happen automatically when the observer goes away. So, if you're only targeting iOS 9, this may all just work, but if you're not retaining the returned observer at all, the notification could be removed before you expect it to be. Better safe than sorry.
To add to #Dave's answer, it looks like documentation isn't always 100% accurate. According to this article by Ole Begemann there is a contradiction in the doc and self-removing magic was not happening as of iOS 11.2 in his test app.
So that the answer is still "Yes, one needs to remove that observer manually" (and yes, self is not the observer, the result of addObserver() method is the observer).
Here an example with code, for how a correct implementation looks like:
Declare the variable that gets returned when you add the observer in your class A (the receiver of the notification or observer):
private var fetchTripsNotification: NSObjectProtocol?
In your init method add yourself as an observer:
init() {
fetchTripsNotification = NotificationCenter.default.addObserver(forName: .needsToFetchTrips, object: nil, queue: nil) { [weak self] _ in
guard let `self` = self else {
return
}
self.fetchTrips()
}
}
In the deinit method of your class, make sure to remove the observer:
deinit { NotificationCenter.default.removeObserver(fetchTripsNotification as Any) }
In your class B (the poster of the notification) trigger the notification like usually:
NotificationCenter.default.post(name: .needsToFetchTrips, object: nil)