What are proper places to detect foreground/background delegates in iOS? - ios

Question

Backgound:
I am working in an iOS application. We have around 100 ViewControllers and all of them in our application are inherited from BaseViewController from the beginning. Currently while refactoring, I see many view controllers require to detect willEnterForegroundNotification[1] and didEnterBackgroundNotification[2]
delegates to do some internal tasks. Almost 20~25 view controllers are setting their own notification observers to the delegates on their viewDidLoad. I was thinking to move this detection task to central BaseViewController for code clarity.
My Proposed Solution:
My intended design is like below,
class BaseViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let notificationCenter = NotificationCenter.default
notificationCenter.addObserver(self, selector: #selector(appMovedToForeground), name: Notification.Name.UIApplicationWillEnterForeground, object: nil)
notificationCenter.addObserver(self, selector: #selector(appMovedToBackground), name: Notification.Name.UIApplicationDidEnterBackground, object: nil)
}
func appMovedToBackground() {
print("App moved to Background!")
}
func appMovedToForeground() {
print("App moved to ForeGround!")
}
}
class MyViewController: BaseViewController {
override func appMovedToBackground() {
print(“Do whatever need to do for current view controllers on appMovedToBackground”)
}
override func appMovedToForeground() {
print(“Do whatever need to do for current view controllers on appMovedToForeground”)
}
}
I see that if I move this detection into BaseViewController many tasks of custom observer handling are reduced from child view controllers. Child ViewControllers (i.e. MyViewController in example code) only need to use these two functions appMovedToBackground and appMovedToForeground when they require.
Issues:
However, I am still concern about one thing. As I am moving the observer setting part into BaseViewController, thus all the ViewControllers (approx 100 of them in my project) will register the observer in their default viewDidLoad and many of them won’t even use them in reality. I am afraid this design might heavily costs app performance. Is my intended design acceptable when trading of between performance vs code clarity and maintainability in such situation? Is there any better design in my case?
Reference:
[1] willEnterForegroundNotification - Posted when the app enters the background.
[2] didEnterBackgroundNotification - Posted shortly before an app leaves the background state on its way to becoming the active app.

Answer 1

You can declare a protocol lets call it BGFGObserver.
Let each VC which needs to observe for foreground, background confirm to this protocol.
In base class check if self confirms to BGFGObserver, if yes then only register as observer.
In BGFGObserver you will need to have the methods to handle background and foreground.

Answer 2

Notification is one to many communication. If you really don't this functionality. you can use the protocol delegate method. you can assign a delegate, only when you need it.
and to solve your problem, you can move your observer to didSet of delegate variable. So, only when you assign a delegate, that time only observers will be added. if you don't set it, it will not be added for that viewController.
#objc protocol AppActivityTracker{
func appMovedToBackground()
func appMovedToForeground()
}
class BaseViewController: UIViewController {
var activityDelegate : AppActivityTracker? {
didSet{
//MARK:-Observer will be added only when you assign delegate.
let notificationCenter = NotificationCenter.default
notificationCenter.addObserver(self, selector: #selector(activityDelegate?.appMovedToForeground), name: UIApplication.didBecomeActiveNotification, object: nil)
notificationCenter.addObserver(self, selector: #selector(activityDelegate?.appMovedToBackground), name: UIApplication.didEnterBackgroundNotification, object: nil)
}
}
override func viewDidLoad() {
super.viewDidLoad()
}
}
class MyViewController: BaseViewController {
override func viewDidLoad() {
super.viewDidLoad()
//MARK:- Assign delegate only when you need observers
activityDelegate = self
}
}
//MARK:- Assign delegate only when you need observers
extension MyViewController : AppActivityTracker{
func appMovedToBackground() {
print("Do whatever need to do for current view controllers on appMovedToBackground")
}
func appMovedToForeground() {
print("Do whatever need to do for current view controllers on appMovedToForeground")
}
}
Discussion from comment:-
Adding observer on viewWillAppear and removing observer on viewDidDisappear.
#objc protocol AppActivityTracker{
func appMovedToBackground()
func appMovedToForeground()
}
class BaseViewController: UIViewController {
var activityDelegate : AppActivityTracker? {
didSet{
if activityDelegate != nil{
addOberservers()
}
else{
removeOberservers()
}
}
}
func addOberservers(){
let notificationCenter = NotificationCenter.default
notificationCenter.addObserver(self, selector: #selector(activityDelegate?.appMovedToForeground), name: UIApplication.didBecomeActiveNotification, object: nil)
notificationCenter.addObserver(self, selector: #selector(activityDelegate?.appMovedToBackground), name: UIApplication.didEnterBackgroundNotification, object: nil)
}
func removeOberservers(){
let notificationCenter = NotificationCenter.default
notificationCenter.removeObserver(self, name: UIApplication.didBecomeActiveNotification, object: nil)
notificationCenter.removeObserver(self, name: UIApplication.didEnterBackgroundNotification, object: nil)
}
}
class MyViewController: BaseViewController {
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
//MARK:- Assign delegate only when you need observers
self.activityDelegate = self
}
override func viewDidDisappear(_ animated: Bool) {
super.viewDidDisappear(animated)
//MARK:- Removing observer on view will disappear.
self.activityDelegate = nil
}
}
//MARK:- Assign delegate only when you need observers
extension MyViewController : AppActivityTracker{
func appMovedToBackground() {
print("Do whatever need to do for current view controllers on appMovedToBackground")
}
func appMovedToForeground() {
print("Do whatever need to do for current view controllers on appMovedToForeground")
}
}

Answer 3

You can do this in AppDelegate or create a separate class that is held in AppDelegate to specifically do this.