ios - Tabbed view controller, scrollToItem in collection view not working after switching tabs -

i have tabbed bar view controller. main view controller has collection view. want animate list of cells contained in array so:

// called in viewdidappear func animatenewachievements() {         if let indexpath = newachievementindexpaths.first {             collectionview.scrolltoitem(at: indexpath, at: .bottom, animated: true)         }     } // delegate method func scrollviewdidendscrollinganimation(_ scrollview: uiscrollview) {         if let indexpath = newachievementindexpaths.first {             if let cell = collectionview.cellforitem(at: indexpath) as? trophycollectionviewcell {                 cell.animate(completion: {                     if self.newachievementindexpaths.count > 0 {                         self.newachievementindexpaths.removefirst()                     }                    self.perform(#selector(self.animatenewachievements), with: nil, afterdelay: 0.4)                 })             }         }     } 

as can see if cycle through index paths in newachievementindexpaths 1 one. works if interrupt process , switch tabs , return view controller animation not continue. calls collectionview.scrolltoitem(at: indexpath, at: .bottom, animated: true) doesn't animate , delegate isn't called. if scroll table finger , leave , return screen resume animation without issues.

any ideas happening here?

i think because collection view isn't programmatically scrolling when switch view controllers.

just try adding this:

override func viewwilldisappear(_ animated: bool) {     if let indexpath = newachievementindexpaths[1] {         collectionview.scrolltoitem(at: indexpath, at: .bottom, animated: true)     } } 

this way scroll item in viewdidappear when return view controller.