OnGeometryChange.swift

import SwiftUI

public extension View {
    func onGeometryChange<T>(
        for type: T.Type,
        of transform: @escaping (GeometryProxy) -> T,
        action: @escaping (_ newValue: T) -> Void
    ) -> some View where T: Equatable {
        modifier(GeometryModifier(transform: transform, action: action))
    }
}

private struct GeometryModifier<T: Equatable>: ViewModifier {
    var transform: (GeometryProxy) -> T
    var action: (T) -> Void

    func body(content: Content) -> some View {
        content
            .background {
                GeometryReader { proxy in
                    Color.clear
                        .preference(key: GeometryPreferenceKey.self, value: .init(proxy: proxy))
                }
            }
            .onPreferenceChange(GeometryPreferenceKey.self) { geo in
                if let geo {
                    action(transform(geo.proxy))
                }
            }
    }
}

private struct GeometryPreferenceKey: PreferenceKey {
    nonisolated(unsafe) static let defaultValue: Geometry? = nil
    static func reduce(value: inout Geometry?, nextValue: () -> Geometry?) {
        value = nextValue()
    }
}

private struct Geometry: Equatable {
    var proxy: GeometryProxy
    static func == (lhs: Geometry, rhs: Geometry) -> Bool {
        (lhs.proxy.size, lhs.proxy.safeAreaInsets)
            == (rhs.proxy.size, rhs.proxy.safeAreaInsets)
    }
}