ParentChild.swift

struct ChildSize: PreferenceKey {
    typealias Value = CGSize
    static var defaultValue: Value {
        return .zero
    }
    
    static func reduce(value: inout Value, nextValue: () -> Value) {
        value = nextValue()
    }
}

struct Parent: View {
    @State var childReqSize: CGSize = .zero
    var body: some View {
        
        Child().frame(width:childReqSize.width > 0 ? childReqSize.width : nil, height: childReqSize.height).onPreferenceChange(ChildSize.self) { size in
            print(size)
            self.childReqSize = size
        }
    }
}
struct Child: View {
    var body: some View {
        GeometryReader { proxy in
            return Rectangle().preference(key: ChildSize.self, value: self.requiredSizeBasedOnOfferedSize(proxy: proxy))
        }
    }
    
    func requiredSizeBasedOnOfferedSize(proxy: GeometryProxy) -> CGSize {
        //  calc size. For instance, we wanted height to be half of width offered by parent...
        return CGSize(width: proxy.size.width, height: proxy.size.width / 2)
    }
}