Delegate.swift

//: Playground - noun: a place where people can play

import UIKit

@propertyWrapper struct Delegate<T, Root> {
    private var keyPath: ReferenceWritableKeyPath<Root, T>
    private var delegate: Root
    
    init(wrappedValue: T, _ keyPath: ReferenceWritableKeyPath<Root, T>, to delegate: Root) {
        self.keyPath = keyPath
        self.delegate = delegate
    }
    
    var wrappedValue: T {
        get { delegate[keyPath: keyPath] }
        set { delegate[keyPath: keyPath] = newValue }
    }
}

class Inner {
    var a: Int?
    
    init() { }
}

struct Foo {
    private var inner: Inner = Inner()
    @Delegate(\Inner.a, to: inner) lazy var a: Int? = nil // impossible 😢
}

var f = Foo()
print(f.a)
f.a = 10
print(f.a)