Swift has no access modifier "Protected". Below is the closest thing to protected. You can call this semi-protected.

almostProtected.swift

import UIKit

// discourage directly setting property m by using private(set). Anyone can still set the value of the property via setValue(_ :forKey:)

public class Abe: NSObject {
    public private(set) var m: [String] = ["abc"]
}

public class Billy: Abe {
    override init() {
        super.init()
        setValue(["abc", "def", "ghi"], forKey: "m")
    }
}


Abe().m // ["abc"]
Billy().m // ["abc", "def", "ghi"]