Proxy and ProxyBuilder in Swift 5.1 using @dynamicMemberLookup

Proxy.swift

import Foundation

// MARK: - Proxy

public protocol ProxyProtocol {
  associatedtype ProxyType
  /// The wrapped proxied object.
  var proxiedObject: ProxyType { get }
}

public extension ProxyProtocol {
  /// Use `@dynamicMemberLookup` keypath subscript to forward the value of the proxied object.
  subscript<T>(dynamicMember keyPath: KeyPath<ProxyType, T>) -> T {
    return self.proxiedObject[keyPath: keyPath]
  }
}

@dynamicMemberLookup
public struct Proxy<T>: ProxyProtocol {
  public let proxiedObject: T

  /// Constructs a new proxy for the object passed as argument.
  init(of object: T) {
    self.proxiedObject = object
  }
}

// MARK: - ProxyBuilder

public protocol ProxyBuilderProtocol {
  associatedtype ObjectType
  /// The initial instance that is going to be used by the builder.
  var createInstanceClosure: () -> ObjectType { get }
  /// All of the `set` commands that will performed by this builder.
  var keypathSetValueDictionary: [AnyKeyPath: (inout ObjectType) -> Void] { get set }
  /// All of the values currently set.
  var keypathGetValueDictionary: [AnyKeyPath: Any] { get set }
}

public extension ProxyBuilderProtocol {
  /// Use `@dynamicMemberLookup` keypath subscript to store the object configuration and postpone
  /// the object construction.
  subscript<T>(dynamicMember keyPath: WritableKeyPath<ObjectType, T>) -> T? {
    get {
      return keypathGetValueDictionary[keyPath] as? T
    }
    set {
      guard let newValue = newValue else {
        keypathGetValueDictionary.removeValue(forKey: keyPath)
        keypathSetValueDictionary.removeValue(forKey: keyPath)
        return
      }
      keypathSetValueDictionary[keyPath] = { object in
        object[keyPath: keyPath] = newValue
      }
    }
  }

  /// Build the target object.
  func build() -> ObjectType {
    var obj = createInstanceClosure()
    for (_, setValueClosure) in keypathSetValueDictionary {
      setValueClosure(&obj)
    }
    return obj
  }
}

@dynamicMemberLookup
public class ProxyBuilder<T>: ProxyBuilderProtocol {
  public let createInstanceClosure: () -> T
  public var keypathSetValueDictionary: [AnyKeyPath: (inout T) -> Void] = [:]
  public var keypathGetValueDictionary: [AnyKeyPath: Any] = [:]

  init(createInstanceClosure: @escaping () -> T) {
    self.createInstanceClosure = createInstanceClosure
  }
}

ProxyDemo.swift

import Foundation

struct Foo {
  var bar = "bar"
  var baz = 42
}

// Creates an immutable (read-only) proxy of a given object.
var proxyFoo = Proxy(of: Foo())
proxyFoo.baz

// Create a proxy-based builder that postpone the object creation to 
// the invocation of `build()`.
var proxyBuilderFoo = ProxyBuilder(createInstanceClosure: { Foo() })
proxyBuilderFoo.baz = 1337
proxyBuilderFoo.bar = "New"
let newFoo = proxyBuilderFoo.build()