SwiftUI - Have a child view report its size to a parent.

ViewSize.swift

import SwiftUI

struct ViewSize: ViewModifier {
    struct ViewSizePreferenceKey: PreferenceKey {
        typealias Value = CGSize
        static var defaultValue: Value = .zero

        static func reduce(value _: inout Value, nextValue: () -> Value) {
            _ = nextValue()
        }
    }
    
    func body(content: Content) -> some View {
        content
            .background(
                GeometryReader { proxy in
                    Color.clear
                        .preference(key: ViewSizePreferenceKey.self, value: proxy.size)
                }
            )
    }
}

extension View {
    func viewSize(size: @escaping (CGSize) -> ()) -> some View {
        modifier(ViewSize())
            .onPreferenceChange(ViewSize.ViewSizePreferenceKey.self) { preferences in
                size(preferences)
            }
    }
}

struct ViewSize_Previews: PreviewProvider {
    struct ViewSizePreviewView: View {
        @State private var size: CGSize = .zero
        @State private var width: CGFloat = 100
        @State private var height: CGFloat = 100
        
        var body: some View {
            VStack {
                Spacer()
                
                Text("width: \(size.width), height: \(size.height)")
                    .frame(width: width, height: height)
                    .background(Color.red.opacity(0.3))
                    .viewSize { size in
                        self.size = size
                    }
                
                Spacer()
                
                Slider(value: $width, in: 100...300) {
                    EmptyView()
                }
                .padding(.horizontal)
                
                Slider(value: $height, in: 100...300) {
                    EmptyView()
                }
                .padding(.horizontal)
            }
        }
    }
    
    static var previews: some View {
        ViewSizePreviewView()
    }
}