View Modifier that will fetch a SwiftUI view's safe area inset and expose it via the preferences system.

SafeAreaReader.swift

import SwiftUI

public extension View {
    func onSafeAreaChange(onChange: @escaping (EdgeInsets) -> Void) -> some View {
        modifier(SafeAreaReader(onChange: onChange))
    }
}

internal struct SafeAreaReader: ViewModifier {
    var onChange: (EdgeInsets) -> Void
    
    func body(content: Content) -> some View {
        content
            .background(
                GeometryReader { proxy in
                    Color.clear
                        .preference(key: SafeAreaPreferenceKey.self, value: proxy.safeAreaInsets)
                }
                // For some reason, preference changes are lost after leaving the background modifier.
                // Maybe the background modifier contains its own separate view hierarchy?
                .onPreferenceChange(SafeAreaPreferenceKey.self, perform: onChange)
            )
    }
}

internal struct SafeAreaPreferenceKey: PreferenceKey {
    public static var defaultValue: EdgeInsets = .init(top: 0, leading: 0, bottom: 0, trailing: 0)
    public static func reduce(value: inout EdgeInsets, nextValue: () -> EdgeInsets) {
        value = nextValue()
    }
}