back button callback in navigationController in iOS

Question

I have pushed a view onto the navigation controller and when I press the back button it goes to the previous view automatically  I want to do a few things when back button is pressed before popping the view off the stack  Which is the back button callback function

User · Answer

This is the correct way to detect this    -  void willMoveToParentViewController  UIViewController   parent      if  parent    nil             do stuff            this method is called when view is pushed as well  So checking parent  nil is for popping view controller from stack

User · Answer

Maybe it s a little too late  but I also wanted the same behavior before  And the solution I went with works quite well in one of the apps currently on the App Store  Since I haven t seen anyone goes with similar method  I would like to share it here  The downside of this solution is that it requires subclassing UINavigationController  Though using Method Swizzling might help avoiding that  I didn t go that far   So  the default back button is actually managed by UINavigationBar  When a user taps on the back button  UINavigationBar ask its delegate if it should pop the top UINavigationItem by calling navigationBar   shouldPop    UINavigationController actually implement this  but it doesn t publicly declare that it adopts UINavigationBarDelegate  why     To intercept this event  create a subclass of UINavigationController  declare its conformance to UINavigationBarDelegate and implement navigationBar   shouldPop    Return true if the top item should be popped  Return false if it should stay   There are two problems  The first is that you must call the UINavigationController version of navigationBar   shouldPop   at some point  But UINavigationBarController doesn t publicly declare it conformance to UINavigationBarDelegate  trying to call it will result in a compile time error  The solution I went with is to use Objective-C runtime to get the implementation directly and call it  Please let me know if anyone has a better solution   The other problem is that navigationBar   shouldPop   is called first follows by popViewController animated   if the user taps on the back button  The order reverses if the view controller is popped by calling popViewController animated    In this case  I use a boolean to detect if popViewController animated   is called before navigationBar   shouldPop   which mean that the user has tapped on the back button   Also  I make an extension of UIViewController to let the navigation controller ask the view controller if it should be popped if the user taps on the back button  View controllers can return false and do any necessary actions and call popViewController animated   later   class InterceptableNavigationController  UINavigationController  UINavigationBarDelegate          If a view controller is popped by tapping on the back button   navigationBar     shouldPop    is called first follows by  popViewController animated            If it is popped by calling to  popViewController animated     the order reverses and we need this flag to check that      private var didCallPopViewController   false      override func popViewController animated  Bool  - gt  UIViewController            didCallPopViewController   true         return super popViewController animated  animated             func navigationBar   navigationBar  UINavigationBar  shouldPop item  UINavigationItem  - gt  Bool              If this is a subsequence call after  popViewController animated     we should just pop the view controller right away          if didCallPopViewController               return originalImplementationOfNavigationBar navigationBar  shouldPop  item                        The following code is called only when the user taps on the back button           guard let vc   topViewController  item    vc navigationItem else               return false                    if vc shouldBePopped self                return originalImplementationOfNavigationBar navigationBar  shouldPop  item            else               return false                      func navigationBar   navigationBar  UINavigationBar  didPop item  UINavigationItem            didCallPopViewController   false                Since  UINavigationController  doesn t publicly declare its conformance to  UINavigationBarDelegate           trying to called  navigationBar   shouldPop    will result in a compile error          So  we ll have to use Objective-C runtime to directly get super s implementation of  navigationBar   shouldPop    and call it      private func originalImplementationOfNavigationBar   navigationBar  UINavigationBar  shouldPop item  UINavigationItem  - gt  Bool           let sel    selector UINavigationBarDelegate navigationBar   shouldPop            let imp   class getMethodImplementation class getSuperclass InterceptableNavigationController self   sel          typealias ShouldPopFunction    convention c   AnyObject  Selector  UINavigationBar  UINavigationItem  - gt  Bool         let shouldPop   unsafeBitCast imp  to  ShouldPopFunction self          return shouldPop self  sel  navigationBar  item           extension UIViewController        objc func shouldBePopped   navigationController  UINavigationController  - gt  Bool           return true           And in you view controllers  implement shouldBePopped      If you don t implement this method  the default behavior will be to pop the view controller as soon as the user taps on the back button just like normal   class MyViewController  UIViewController       override func shouldBePopped   navigationController  UINavigationController  - gt  Bool           let alert   UIAlertController title   Do you want to go back                                          message   Do you really want to go back  Tap on   Yes   to go back  Tap on   No   to stay on this screen                                          preferredStyle   alert          alert addAction UIAlertAction title   No   style   cancel  handler  nil           alert addAction UIAlertAction title   Yes   style   default  handler      in             navigationController popViewController animated  true                      present alert  animated  true  completion  nil          return false           You can look at my demo here

User · Answer

If you can t use  viewWillDisappear  or similar method  try to subclass UINavigationController  This is the header class    import  lt Foundation Foundation h gt   class MyViewController    interface CCNavigationController   UINavigationController   property  nonatomic  strong  MyViewController  viewController    end   Implementation class    import  CCNavigationController h   import  MyViewController h    implementation CCNavigationController      -  UIViewController   popViewControllerAnimated  BOOL animated         This is the moment for you to do whatever you want       self viewController doCustomMethod       return  super popViewControllerAnimated animated       end   In the other hand  you need to link this viewController to your custom NavigationController  so  in your viewDidLoad method for your regular viewController do this    implementation MyViewController       -  void viewDidLoad                super viewDidLoad             CCNavigationController  self navigationController  viewController   self

User · Answer

For  BEFORE popping the view off the stack     -  void willMoveToParentViewController  UIViewController   parent      if  parent    nil           NSLog   do whatever you want here

User · Answer

it s probably better to override the backbutton so you can handle the event before the view is popped for things such as user confirmation   in viewDidLoad create a UIBarButtonItem and set self navigationItem leftBarButtonItem to it passing in a sel  -  void  viewDidLoad      change the back button to cancel and add an event handler UIBarButtonItem  backButton     UIBarButtonItem alloc  initWithTitle     back    style UIBarButtonItemStyleBordered target self action  selector handleBack      self navigationItem leftBarButtonItem   backButton   backButton release      -  void  handleBack  id sender      pop to root view controller  self navigationController popToRootViewControllerAnimated YES        Then you can do things like raise an UIAlertView to confirm the action  then pop the view controller  etc   Or instead of creating a new backbutton  you can conform to the UINavigationController delegate methods to do actions when the back button is pressed

User · Answer

William Jockusch s answer solve this problem with easy trick   - void  viewWillDisappear  BOOL animated       if   self navigationController viewControllers indexOfObject self   NSNotFound              back button was pressed   We know this is true because self is no longer           in the navigation stack               super viewWillDisappear animated

User · Answer

In my opinion the best solution   -  void didMoveToParentViewController  UIViewController   parent       if    parent isEqual self parentViewController              NSLog   Back pressed              But it only works with iOS5

User · Answer

I end up with this solutions  As we tap back button viewDidDisappear method called  we can check by calling isMovingFromParentViewController selector which return true  we can pass data back  Using Delegate  hope this help someone   - void viewDidDisappear  BOOL animated       if  self isMovingToParentViewController               if  self isMovingFromParentViewController             moving back           pass to viewCollection delegate and update UI          self delegateObject passBackSavedData self dataModel

User · Answer

This is what it works for me in Swift   override func viewWillDisappear   animated  Bool        if self navigationController  viewControllers index of  self     nil              back button pressed or back gesture performed            super viewWillDisappear animated

User · Answer

Here s another way I implemented  didn t test it with an unwind segue but it probably wouldn t differentiate  as others have stated in regards to other solutions on this page  to have the parent view controller perform actions before the child VC it pushed gets popped off the view stack  I used this a couple levels down from the original UINavigationController    This could also be used to perform actions before the childVC gets pushed  too   This has the added advantage of working with the iOS system back button  instead of having to create a custom UIBarButtonItem or UIButton    Have your parent VC adopt the UINavigationControllerDelegate protocol and register for delegate messages   MyParentViewController   UIViewController  lt UINavigationControllerDelegate gt   - void viewDidLoad       self navigationcontroller delegate   self     Implement this UINavigationControllerDelegate instance method in MyParentViewController   -  id lt UIViewControllerAnimatedTransitioning gt  navigationController  UINavigationController   navigationController animationControllerForOperation  UINavigationControllerOperation operation fromViewController  UIViewController   fromVC toViewController  UIViewController   toVC          Test if operation is a pop  can also test for a push  i e   do something before the ChildVC is pushed     if  operation    UINavigationControllerOperationPop               Make sure it s the child class you re looking for         if   fromVC isKindOfClass  ChildViewController class                     Can handle logic here or send to another method  can also access all properties of child VC at this time             return  self didPressBackButtonOnChildViewControllerVC fromVC                          If you don t want to specify a nav controller transition     return nil     If you specify a specific callback function in the above UINavigationControllerDelegate instance method  - id  lt UIViewControllerAnimatedTransitioning gt  didPressBackButtonOnAddSearchRegionsVC  UIViewController   fromVC       ChildViewController  childVC   ChildViewController new      childVC    ChildViewController   fromVC          childVC propertiesIWantToAccess go here         If you don t want to specify a nav controller transition     return nil

User · Answer

If you re using a Storyboard and you re coming from a push segue  you could also just override shouldPerformSegueWithIdentifier sender

User · Answer

There s a more appropriate way than asking the viewControllers  You can make your controller a delegate of the navigationBar that has the back button  Here s an example  In the implementation of the controller where you want to handle the press of the back button  tell it that it will implement the UINavigationBarDelegate protocol    interface MyViewController     lt UINavigationBarDelegate gt    Then somewhere in your initialization code  probably in viewDidLoad  make your controller the delegate of its navigation bar   self navigationController navigationBar delegate   self    Finally  implement the shouldPopItem method  This method gets called right when the back button is pressed  If you have multiple controllers or navigation Items in the stack  you ll probably want to check which of those navigation items is getting popped  the item parameter   so that you only do your custom stuff when you expect to  Here s an example   - BOOL navigationBar  UINavigationBar   navigationBar shouldPopItem  UINavigationItem   item       NSLog   Back button got pressed           if you return NO  the back button press is cancelled     return YES

[ios] back button callback in navigationController in iOS

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