Tusker/MyPlayground.playground/Contents.swift

import UIKit

class Client {
    func test<A>(_ thing: A) {
        if var thing = thing as? ClientModel {
            thing.client = self
        } else if var arr = thing as? [ClientModel] {
            arr.client = self
        }
//        } else if let arr = thing as? Array<Any> {
//            for el in arr {
//                if var el = el as? ClientModel {
//                    el.client = self
//                }
//            }
//        }
    }
}

protocol ClientModel {
    var client: Client! { get set }
}

class Something: ClientModel {
    var client: Client!
}

extension Array: ClientModel where Element: ClientModel {
    var client: Client! {
        get {
            return first?.client
        }
        set {
            for var el in self {
                el.client = newValue
            }
        }
    }
}
//extension Array: ClientModel where Element == ClientModel {
//    var client: Client! {
//        get {
//            return first?.client
//        }
//        set {
//            for var el in self {
//                el.client = newValue
//            }
//        }
//    }
//}

var array = [Something(), Something()]

let client = Client()
client.test(array)
array[0].client
array[1].client