Detecting when a view controller is visible

1
votes

The Situation

Let's say that I have 2 view controllers inside a navigation controller. Inside the storyboard, they would look like:

NC -> VC1 -> VC2

NC: Navigation controller

VC1: View controller one

VC2: View controller 2

So VC1 is the root view controller of the navigation controller, and VC1 is connected to VC2 via a show segue.

What must happen:

VC1 must call a function every time it opens, (e.g necessaryFunction()). It currently calls this function in viewDidLoad(). The problem is that when the user presses the back button in VC2 (the one on the navigation bar) and VC1 is now showing, viewDidLoad() is not called. This means that necessaryFunction() will not be called either.

What I am looking for:

I am looking for a way to make sure that necessaryFunction() is called when returning to VC1 from VC2. I understand that this could be solved with a delegate, but that seems overly complicated for such a simple thing, surely there is another way to do this.

Maybe I could put necessaryFunction() into viewWillAppear(), but I don't think this would work.

By the way: I am coding in Swift.

EDIT: I have found this post: iOS how to detect programmatically when top view controller is popped?, but it is for objective-C, not swift.

1
iosswiftuinavigationcontrollerviewcontrolleruinavigationitem
"but I don't think this would work." You don't? What happened when you tried it? – matt
When I tried it, nothing changed from when necessaryFunction() was called inside viewDidLoad() – Zack
But viewWillAppear is called when you pop back to VC1, so why doesn't that solve the problem? If it doesn't, you must be describing the problem incorrectly. – matt
You are expressing two major misunderstandings. (1) View controllers are UIViewControllers, and thus, are not visible! (UIViews, on the other hand, are.) It sounds like you haven't grasped some basics of MVC. (2) The next - and larger - misunderstanding I'm seeing is about views, their controllers, and life cycles each have. Search on some of those terms - you should know that viewWillAppear() will be called while viewDidLoad() likely won't. – dfd
Possible duplicate of iOS how to detect programmatically when top view controller is popped? – Lal Krishna

1 Answers

2
votes

These four methods can be used in a view controller's appearance callbacks to determine if it is being

presented, dismissed, or added or removed as a child view controller. 

@available(iOS 5.0, *)
open var isBeingPresented: Bool { get }

@available(iOS 5.0, *)
open var isBeingDismissed: Bool { get }


@available(iOS 5.0, *)
open var isMovingToParentViewController: Bool { get }

@available(iOS 5.0, *)
open var isMovingFromParentViewController: Bool { get }

For example, a view controller can

check if it is disappearing because it was dismissed or popped by asking itself in its viewWillDisappear:

method by checking the expression 

([self isBeingDismissed] || [self isMovingFromParentViewController]).

for e.g , you can call the method as

 override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    if (self.isMovingFromParentViewController())  {
        // we're already on the navigation stack
        // another controller must have been popped off
    }


}