This Swift code is a SwiftUI extension for the View protocol. It adds a new method `onChangeVisibility(perform:)` that allows you to perform an action when the visibility of the view changes.

BecomingVisible.swift

import SwiftUI

extension View {
    func onChangeVisibility(perform action: @escaping (Bool) -> Void) -> some View {
        modifier(BecomingVisible(action: action))
    }
}

private struct BecomingVisible: ViewModifier {
    @State var action: ((Bool) -> Void)

    func body(content: Content) -> some View {
        content.overlay {
            GeometryReader { proxy in
                Color.clear
                    .preference(
                        key: VisibleKey.self,
                        value: UIScreen.main.bounds.intersects(proxy.frame(in: .global))
                    )
                    .onPreferenceChange(VisibleKey.self) { isVisible in
                        action(isVisible)
                    }
            }
        }
    }

    struct VisibleKey: PreferenceKey {
        static var defaultValue: Bool = false
        static func reduce(value: inout Bool, nextValue: () -> Bool) { }
    }
}

// Simple sample
struct ContentView: View {
    var body: some View {
        Text("Hi there!")
            .font(.largeTitle)
            .padding()
            .onChangeVisibility { isVisible in
                print("Text is now \(isVisible ? "visible" : "hidden")")
            }
    }
}