Use a string key to reference a property in a class or struct; Get the name of a property as a string

DynamicMemberLookupExample.swift

//  Demonstrates Swift-based reflection concept like Key-Value coding in Objective-C (but slower)
//   and use of @dynamicMemberLookup.
//
//  Uses Mirror to reference a property in a class or struct by string key name; 
//  Uses @dynamicMemberLookup to get the name of a property as a string, to reverse the reflection.
//  
//  DynamicMemberLookupExample.swift
//
//  Copyright Andrew Kingdom 2023-05-17.
//  MIT License
//
import SwiftUI

@main
struct DynamicMemberLookupExample: App {

    @State var result = MemberLookup().b  // .a or .b work but .c does not

    var body: some Scene {
        WindowGroup() {  // normally references ContentView definition
          
            Text("\(result)")
                 .padding()
        }
    }
}

@dynamicMemberLookup
public struct MemberLookup {
    fileprivate var a = "Hi"
    fileprivate var b = "There"
    public subscript(dynamicMember member: String) -> Any? {
        let value = Mirror.lookup(key: member, in: self)
        print(".lookup(\"\(member)\" -> \(value ?? "??")")
        return value
    }
}

public extension Mirror {
    static func lookup(key:String, `in`:Any) -> Any? {
        let mirror = Mirror(reflecting:`in`)
        let children = mirror.children
        if let firstIndex = children.firstIndex(where: {$0.label == key}) {
            let value = children[firstIndex].value
            return value
        }
        return nil  // default value is nil
    }
}