Why does viewWillAppear not get called when an app comes back from the background

Question

I m writing an app and I need to change the view if the user is looking at the app while talking on the phone   I ve implemented the following method   -  void viewWillAppear  BOOL animated        super viewWillAppear animated       NSLog   viewWillAppear          sv frame   CGRectMake 0 0  0 0  320 0  self view bounds size height       But it s not being called when the app returns to the foreground   I know that I can implement     NSNotificationCenter defaultCenter  addObserver self selector  selector statusBarFrameChanged   name UIApplicationDidChangeStatusBarFrameNotification object nil     but I don t want to do this  I d much rather put all my layout information in the viewWillAppear  method  and let that handle all possible scenarios   I ve even tried to call viewWillAppear  from applicationWillEnterForeground   but I can t seem to pinpoint which is the current view controller at that point   Does anybody know the proper way to deal with this  I m sure I m missing an obvious solution

User · Accepted Answer

The method viewWillAppear should be taken in the context of what is going on in your own application  and not in the context of your application being placed in the foreground when you switch back to it from another app   In other words  if someone looks at another application or takes a phone call  then switches back to your app which was earlier on backgrounded  your UIViewController which was already visible when you left your app  doesn t care  so to speak -- as far as it is concerned  it s never disappeared and it s still visible -- and so viewWillAppear isn t called   I recommend against calling the viewWillAppear yourself -- it has a specific meaning which you shouldn t subvert  A refactoring you can do to achieve the same effect might be as follows   -  void viewWillAppear  BOOL animated        super viewWillAppear animated        self doMyLayoutStuff self      -  void doMyLayoutStuff  id sender          stuff     Then also you trigger doMyLayoutStuff from the appropriate notification     NSNotificationCenter defaultCenter  addObserver self selector  selector doMyLayoutStuff   name UIApplicationDidChangeStatusBarFrameNotification object self     There s no out of the box way to tell which is the  current  UIViewController by the way  But you can find ways around that  e g  there are delegate methods of UINavigationController for finding out when a UIViewController is presented therein  You could use such a thing to track the latest UIViewController which has been presented   Update  If you layout out UIs with the appropriate autoresizing masks on the various bits  sometimes you don t even need to deal with the  manual  laying out of your UI - it just gets dealt with

User · Answer

viewWillAppear animated   one of the most confusing methods in the iOS SDKs in my opinion  is never be invoked in such a situation  i e   application switching  That method is only invoked according to the relationship between the view controller s view and the application s window  i e   the message is sent to a view controller only if its view appears on the application s window  not on the screen   When your application goes background  obviously the topmost views of the application window are no longer visible to the user  In your application window s perspective  however  they are still the topmost views and therefore they did not disappear from the window  Rather  those views disappeared because the application window disappeared  They did not disappeared because they disappeared from the window    Therefore  when the user switches back to your application  they obviously seem to appear on the screen  because the window appears again  But from the window s perspective  they haven t disappeared at all  Therefore the view controllers never get the viewWillAppear animated message

User · Answer

It s even easier with SwiftUI  var body  some View            Text  quot Hello World quot        onReceive NotificationCenter default publisher for  UIApplication willResignActiveNotification       in         print  quot Moving to background  quot              onReceive NotificationCenter default publisher for  UIApplication willEnterForegroundNotification       in         print  quot Moving back to foreground  quot

User · Answer

Swift  Short answer  Use a NotificationCenter observer rather than viewWillAppear   override func viewDidLoad         super viewDidLoad           set observer for UIApplication willEnterForegroundNotification     NotificationCenter default addObserver self  selector   selector willEnterForeground   name  UIApplication willEnterForegroundNotification  object  nil         my selector that was defined above  objc func willEnterForeground            do stuff       Long answer  To find out when an app comes back from the background  use a NotificationCenter observer rather than viewWillAppear  Here is a sample project that shows which events happen when   This is an adaptation of this Objective-C answer    import UIKit class ViewController  UIViewController           MARK  - Overrides      override func viewDidLoad             super viewDidLoad           print  view did load               add notification observers         NotificationCenter default addObserver self  selector   selector didBecomeActive   name  UIApplication didBecomeActiveNotification  object  nil          NotificationCenter default addObserver self  selector   selector willEnterForeground   name  UIApplication willEnterForegroundNotification  object  nil              override func viewWillAppear   animated  Bool            print  view will appear              override func viewDidAppear   animated  Bool            print  view did appear                 MARK  - Notification oberserver methods       objc func didBecomeActive             print  did become active               objc func willEnterForeground             print  will enter foreground              On first starting the app  the output order is   view did load view will appear did become active view did appear   After pushing the home button and then bringing the app back to the foreground  the output order is   will enter foreground did become active    So if you were originally trying to use viewWillAppear then UIApplication willEnterForegroundNotification is probably what you want   Note  As of iOS 9 and later  you don t need to remove the observer  The documentation states      If your app targets iOS 9 0 and later or macOS 10 11 and later  you   don t need to unregister an observer in its dealloc method

User · Answer

Just trying to make it as easy as possible see code below   -  void viewDidLoad       self appWillEnterForeground     register For Application Will enterForeground     -  id appWillEnterForeground    Application will enter foreground         NSNotificationCenter defaultCenter  addObserver self                                              selector  selector allFunctions                                                   name UIApplicationWillEnterForegroundNotification                                                object nil       return self      - void  allFunctions    call any functions that need to be run when application will enter foreground      NSLog   calling all functions   application just came back from foreground

User · Answer

Swift 4 2   5  override func viewDidLoad         super viewDidLoad       NotificationCenter default addObserver self  selector   selector willEnterForeground                                              name  Notification Name UIApplicationWillEnterForeground                                             object  nil      objc func willEnterForeground           do what s needed

User · Answer

Use Notification Center in the viewDidLoad  method of your ViewController to call a method and from there do what you were supposed to do in your viewWillAppear  method  Calling viewWillAppear  directly is not a good option   -  void viewDidLoad        super viewDidLoad       NSLog   view did load           NSNotificationCenter defaultCenter  addObserver self          selector  selector applicationIsActive            name UIApplicationDidBecomeActiveNotification          object nil          NSNotificationCenter defaultCenter  addObserver self          selector  selector applicationEnteredForeground            name UIApplicationWillEnterForegroundNotification         object nil      -  void applicationIsActive  NSNotification   notification       NSLog   Application Did Become Active       -  void applicationEnteredForeground  NSNotification   notification       NSLog   Application Entered Foreground

[ios] Why does viewWillAppear not get called when an app comes back from the background?

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