Generating auto-incrementing Instance IDs in Swift using Protocols and protocol-extensions only - swift

Question

Goal
To create an "AutoIDable" protocol with the following behaviour.
Every instance of a class conforming to this protocol will get an auto-generated "id" property of String type.
The code should generate id strings in the format <prefix><Instance-count-starting-from-1> (Eg: E-1, E-2, ...E-<n> and so on for 1st , 2nd ... nth Instance of the conforming class.
The protocol & protocol extensions should do ALL of the required work to generate the id strings. The conforming class will only have to subscribe to the protocol and nothing more.
Current status:
I have achieved Goal-1 & Goal-2 with the following implementation:
protocol Identifiable {
var id: String { get }
}
protocol AutoIDable: Identifiable{
static var _instanceCount: Int { get set }
}
class AutoID: AutoIDable {
init(idPrefix: String) {
setAutoID(prefix: idPrefix)
}
internal static var _instanceCount: Int = 0
var id: String = ""
func setAutoID(prefix: String = ""){
Self._instanceCount += 1
self.id = "\(prefix)\(Self._instanceCount)"
}
}
class Employee: AutoID {
init(){
super.init(idPrefix: "E-")
}
}
let e1 = Employee()
let e2 = Employee()
let e3 = Employee()
print(e1.id)
print(e2.id)
print(e3.id)
print(e1.id)
The output from running the above code:
E-1
E-2
E-3
E-1
Todo:
To achieve Goal-3, I need to eliminate the AutoID superclass and implement the same functionality using protocol extensions.
I ran into trouble because:
Protocol extensions do not allow static stored properties. I do know how to work around this limitation without using a superclass.
I do not know how to inject code into all the initialisers the creator of the Employee class might create. Again, I could not think of a workaround without using a superclass.
I would be grateful if you can point me in the right direction.
PS: New to Swift programming. If you’ve suggestions for implementing the code in a more “swifty” way, please do let me know. :-)

Answer 1

Since you want to use protocols, you can't have a stored property in the protocol. So, you'll need some place to store the incrementing ID value, if not the IDs themselves.
Not sure if it violates your requirements of using only protocols, because it would require a type for storage, but at least it won't require conforming classes to have a superclass.
So, let's say we build such a class that holds all the IDs and keeps the incrementing counter:
class AutoIncrementId {
static private var inc: Int = 0
static private var ids: [ObjectIdentifier: String] = [:]
static func getId(_ objectId: ObjectIdentifier, prefix: String) -> String {
if let id = ids[objectId] { return id }
else {
inc += 1
let id = "\(prefix)\(inc)"
ids[objectId] = id
return id
}
}
}
Then the protocol requirement could be:
protocol AutoIdentifiable {
static var prefix: String { get }
var id: String { get }
}
So, a class would need to define its prefix. But we could define a default implementation for id:
extension AutoIdentifiable where Self: AnyObject {
var id: String {
AutoIncrementId.getId(ObjectIdentifier(self), prefix: Self.prefix)
}
}
The usage would be:
class Employee: AutoIdentifiable {
static let prefix = "E-"
}
let e1 = Employee()
let e2 = Employee()
let e3 = Employee()
print(e1.id) // E-1
print(e2.id) // E-2
print(e3.id) // E-3
print(e1.id) // E-1