In SwiftUI, how do you perform a code before the interface orientation change?

Question

I need to execute a code before the device's interface starts rotating but I can't seem to find any viable solution.

I came across UIDevice.orientationDidChangeNotification but couldn't find anything like that to be notified that an orientation change will start.

I tried creating an UIViewControllerRepresentable with a custom UIViewController that overrides func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator), but that function doesn't get invoked at all (I didn't forget to propagate it using super).

Any idea?

EDIT: Needs support for iOS 15.0+

EDIT 2: This is needed in a SwiftUI app (so not UIKit)

Answer 1

If you create a regular UIViewController that post's a Notification you can onReceive the notification after you subscribe.

import SwiftUI

struct TransitionNotificationView: View {
    typealias SizeData = TransitionVM.SizeData
    var body: some View {
        Text("Hello, World!")
            .subscribeToTransition() //subscribe
            .onReceive(NotificationCenter.default.publisher(for: .willTransitionSize), perform: { notification in
                guard let object = notification.object as? SizeData else {return}

                print("received size Transision \(object.size.debugDescription)")
            })
    }
}
#Preview {
    TransitionNotificationView()
}

The code behind this is pretty standard.

extension View {
    func subscribeToTransition() -> some View {
        modifier(TransitionVM())
    }
}
struct TransitionVM: ViewModifier {
    typealias TraitData = Transition_UI.TraitData
    typealias SizeData = Transition_UI.SizeData
    func body(content: Content) -> some View {
        content.background(Transition_UI().frame(width: 0, height: 0))
    }
    
    struct Transition_UI: UIViewControllerRepresentable {
        typealias TraitData = (newCollection: UITraitCollection, coordinator: UIViewControllerTransitionCoordinator)
        typealias SizeData = (size: CGSize, coordinator: UIViewControllerTransitionCoordinator)
        func makeUIViewController(context: Context) -> some UIViewController {
            TransitionVC(nibName: nil, bundle: nil)
        }
        func updateUIViewController(_ uiViewController: UIViewControllerType, context: Context) {
            
        }
        class TransitionVC: UIViewController {
            ///UIKit calls this method before changing the size of a presented view controller’s view.
            override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
                print(#function)
                super.viewWillTransition(to: size, with: coordinator)
                NotificationCenter.default.post(name: .willTransitionSize, object: (size, coordinator))
                
            }
            ///UIKit calls this method before changing the current object’s **traits**
            override func willTransition(to newCollection: UITraitCollection, with coordinator: UIViewControllerTransitionCoordinator) {
                super.willTransition(to: newCollection, with: coordinator)
                print(#function)
                NotificationCenter.default.post(name: .willTransitionTrait, object: (newCollection, coordinator))
            }
        }
    }
}

extension Notification.Name {

    static let willTransitionTrait = Notification.Name("willTransitionTrait")
    static let willTransitionSize = Notification.Name("willTransitionSize")

}