access-control.swift

//////////////////////
// File 1

public class Base {

    public struct Overridable { // Do not store
        private init() {}
    }
    
    public struct Protected { // OK to store
        private init() {}
    }
    
    public struct Secrets {
        var message = "Treasure!"
    }
    
    private let secrets : Secrets = Secrets()
    
    public func getSecrets(key: Protected)->Secrets {
        return secrets
    }
    
    public func layout(key: Overridable) {
    }
    
    public func updateLayout() {
        layout(Overridable())
    }
    
    public init(_ proof: (Protected)->()) {
        proof(Protected())
    }
}

//////////////////////
// File 2

class Derived : Base {
    private var key : Protected? = nil
    
    init() {
        var key : Protected?
        super.init({ k in key = k })
        self.key = key!
    }
    
    func callSecrets() {
        print(getSecrets(key!).message)
    }
    
    override func layout(key: Overridable) {
        super.layout(key) // Can call the base class
    }
}

var derived = Derived()
derived.callSecrets()

Swift doesn't provide "protected" as an access level, but you can emulate it. And you can distinguish between methods that are supposed to be called and those that are only supposed to be overridden.

Write up at https://www.callionica.com/developer/#swift-protected