A SwiftUI ViewModifier that reads a view's size

SizeReader.swift

import SwiftUI

public extension View {
    /// Exposes the size of the specified view
    /// - Parameter onChange: This function will be called when the size of a view changes.
    /// - Returns: some View
    func onSizeChange(onChange: @escaping (CGSize) -> Void) -> some View {
        modifier(SizeReader(onChange: onChange))
    }
}

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

internal struct SizePreferenceKey: PreferenceKey {
    public static var defaultValue: CGSize = .zero
    public static func reduce(value: inout CGSize, nextValue: () -> CGSize) {
        value = nextValue()
    }
}