Implementing Swift protocol with a constrained type parameter - swift

Question

I have a couple of Swift protocols that describe a general interface that I'm trying to implement in multiple ways:
protocol Identifiable
{
var identifier:String { get }
}
protocol ItemWithReference
{
var resolveReference<T:Identifiable>(callback:(T) -> ())
}
Now I want to implement the ItemWithReference protocol using CloudKit as the back end (this will eventually work with an alternate back-end as well, at which time I expect to provide an alternative implementation of the ItemWithReference protocol.
In my CloudKit implementation, I have something like this:
class CloudKitIdentifiable : Identifiable
{
...
}
class CloudKitItemWithReference : ItemWithReference
{
func resolveReference<T:Identifiable>(callback:(T) -> ())
{
// In this implementation, I want to only proceed if `T` is a CloudKitIdentifiable subtype
// But not sure how to enforce that
}
}
What I would like to do is to constrain T to be a CloudKitIdentifiable rather than just a simple Identifiable. I can't do that directly in the resolveReference declaration because then the function wouldn't conform to the ItemWithReference protocol. So instead, I am hoping to confirm that T is indeed a CloudKitIdentifiable and then invoke it's initializer to create a new instance of the class being resolved.
Is there any way in Swift to use T's metatype T.Type and determine if it is a subtype of another type? Furthermore, is there any way to invoke a required initializer that has been declared on that subtype?

Answer 1

try:
class CloudKitIdentifiable : Identifiable {
var identifier:String = ...
required init() {}
// you need `required`.
}
class CloudKitItemWithReference : ItemWithReference {
func resolveReference<T:Identifiable>(callback:(T) -> ()) {
if T.self is CloudKitIdentifiable.Type {
// do work..
let obj = (T.self as CloudKitIdentifiable.Type)()
callback(obj as T)
}
}
}
OR:
class CloudKitItemWithReference : ItemWithReference {
func resolveReference<T:Identifiable>(callback:(T) -> ()) {
if let CKT = T.self as? CloudKitIdentifiable.Type {
// do work..
let obj = CKT()
callback(obj as T)
}
}
}
But, In this case, you have to call resolveReference like this:
let ref = CloudKitItemWithReference()
ref.resolveReference { (obj: CloudKitIdentifiable) -> () in
// ^^^^^^^^^^^^^^^^^^^^ explicit type is necessary.
println(obj.identifier)
return
}
Rathar than that, I would recommend to use Associated Type:
protocol Identifiable {
var identifier:String { get }
}
protocol ItemWithReference {
typealias Item: Identifiable // <-- HERE is associated type
func resolveReference(callback:(Item) -> ())
}
class CloudKitIdentifiable : Identifiable {
var identifier:String
init(identifier: String) {
self.identifier = identifier
}
}
class CloudKitItemWithReference : ItemWithReference {
// `Item` associated type can be inferred from
// the parameter type of `resolveReference()`
//
// typealias Item = CloudKitIdentifiable
func resolveReference(callback:(CloudKitIdentifiable) -> ()) {
let obj = CloudKitIdentifiable(identifier: "test")
callback(obj)
}
}
let ref = CloudKitItemWithReference()
ref.resolveReference { obj in
println(obj.identifier)
return
}