Proxy for reference types, like SwiftUI.Binding

OptionalProxy.swift

import Foundation

@propertyWrapper
public struct OptionalProxy<Value> where Value: OptionalProtocol {
    public var wrappedValue: Value {
        get { get() }
        set { set(newValue) }
    }

    public let get: () -> Value
    public let set: (Value) -> Void

    public init(get: @escaping () -> Value,
                set: @escaping (Value) -> Void) {
        self.get = get
        self.set = set
    }

    public init<Root: AnyObject>(
        object: Root,
        keyPath: ReferenceWritableKeyPath<Root, Value>
    ) {
        self.get = { [weak object] in
            guard let object else { return nil }
            return object[keyPath: keyPath]
        }
        self.set = { [weak object] value in
            guard let object else { return }
            object[keyPath: keyPath] = value
        }
    }
}

public protocol OptionalProtocol: ExpressibleByNilLiteral {
    associatedtype OptionalType
}

extension Optional: OptionalProtocol {
    public typealias OptionalType = Wrapped
}