Adds viewDidAppear callback to SwiftUI

OnDidAppearSwiftUI

struct ViewControllerLifecycleHandler: UIViewControllerRepresentable {
    func makeCoordinator() -> ViewControllerLifecycleHandler.Coordinator {
        Coordinator(onDidAppear: onDidAppear)
    }

    let onDidAppear: () -> Void
    
    func makeUIViewController(context: UIViewControllerRepresentableContext<ViewControllerLifecycleHandler>) -> UIViewController {
        context.coordinator
    }

    func updateUIViewController(_ uiViewController: UIViewController, context: UIViewControllerRepresentableContext<ViewControllerLifecycleHandler>) {
    }

    typealias UIViewControllerType = UIViewController

    class Coordinator: UIViewController {
        let onDidAppear: (() -> Void)?

        init(onDidAppear: (() -> Void)?) {
            self.onDidAppear = onDidAppear

            super.init(nibName: nil, bundle: nil)
        }

        required init?(coder: NSCoder) {
            fatalError("init(coder:) has not been implemented")
        }
        
        override func viewDidAppear(_ animated: Bool) {
            super.viewDidAppear(animated)
           (onDidAppear ?? {})()
        }
    }
}

struct DidAppearModifier: ViewModifier {
    let onDidAppearCallback: (() -> Void)?

    func body(content: Content) -> some View {
        content
            .background(ViewControllerLifecycleHandler(onDidAppear: onDidAppearCallback ?? {}))
    }
}

extension View {
    func onDidAppear(_ perform: @escaping () -> Void) -> some View {
        self.modifier(DidAppearModifier(onDidAppearCallback: perform))
    }
}

Had an issue where it wasn't recognizing the ViewModifier as valid, changed some stuff around and now it works:

struct ViewControllerLifecycleHandler: UIViewControllerRepresentable {
    func makeCoordinator() -> ViewControllerLifecycleHandler.Coordinator {
        Coordinator(onDidAppear: onDidAppear)
    }

    let onDidAppear: () -> Void
    
    func makeUIViewController(context: UIViewControllerRepresentableContext<ViewControllerLifecycleHandler>) -> UIViewController {
        context.coordinator
    }

    func updateUIViewController(_ uiViewController: UIViewController, context: UIViewControllerRepresentableContext<ViewControllerLifecycleHandler>) {
    }

    typealias UIViewControllerType = UIViewController

    class Coordinator: UIViewController {
        let onDidAppear: (() -> Void)?

        init(onDidAppear: (() -> Void)?) {
            self.onDidAppear = onDidAppear

            super.init(nibName: nil, bundle: nil)
        }

        required init?(coder: NSCoder) {
            fatalError("init(coder:) has not been implemented")
        }
        
        override func viewDidAppear(_ animated: Bool) {
            super.viewDidAppear(animated)
           (onDidAppear ?? {})()
        }
    }
}

extension View {
    func onDidAppear(_ perform: @escaping () -> Void) -> some View {
        return background(ViewControllerLifecycleHandler(onDidAppear: perform))
    }
}

Hope this helps anyone else trapped on older versions of iOS!