ReadPositionModifier.swift

import SwiftUI

struct ReadPositionModifier<Key: PreferenceKey>: ViewModifier where Key.Value == CGRect {
    let parentCoordinateSpaceName: String

    func body(content: Content) -> some View {
        content
            .background {
                GeometryReader { proxy in
                    Color.clear
                        .preference(key: Key.self, value: proxy.frame(in: .named(parentCoordinateSpaceName)))
                }
            }
    }
}

extension View {
    /// Read view position from a parent coordinate space.
    ///
    /// Define a `PreferenceKey` which its value is a `CGRect`. In the parent view, call two methods below,
    /// to determine the parent coordinate space, and receive value changes:
    ///
    ///     .coordinateSpace(name: parentCoordinateSpaceName)
    ///     .onPreferenceChange(CustomPreferenceKey.self, perform: onPositionChange)
    ///
    /// - Parameter key: the PreferenceKey to receive value changes, which its value is a CGRect
    /// - Parameter parentCoordinateSpaceName: name of parent coordinate space
    func readPosition<Key: PreferenceKey>(
        key: Key.Type,
        in parentCoordinateSpaceName: String
    ) -> some View where Key.Value == CGRect {
        self.modifier(ReadPositionModifier<Key>(parentCoordinateSpaceName: parentCoordinateSpaceName))
    }
}