Mocking.swift

import Foundation

struct Maker {
    var code: () -> Any
    func make<A: Any>() -> A? {
        return code() as? A
    }
}

extension Maker {
    static var table: [String: Maker] = [:]

    static func register<A: Any>(code: @escaping () -> A) {
        Maker.table[String(describing: A.self)] = Maker(code: code)
    }
}

Maker.register {
    "<random_string>"
}

// Mocking
@dynamicMemberLookup
public struct Mock<T> {

    var value: T

    subscript<V>(dynamicMember keyPath: KeyPath<T, V>) -> V {

        if let maker = Maker.table[String(describing: V.self)],
            let value: V = maker.make() {
            return value
        }
        return value[keyPath: keyPath]
    }
}