> On Sep 28, 2016, at 5:43 PM, Eric Conner via swift-users
> <swift-users@swift.org> wrote:
>
> Hello all!
>
> I hope it's not frowned upon to repost a question here from Stack Overflow,
> but I have not found help there. In any case, the email gets some nice
> formatting out of it :-D.
>
> I am trying to create a subclass with a generic type that can be passed into
> a completion block, but am getting the following error:
>
> Cannot convert value of type '(ModelListResponse) -> ()' to expected argument
> type '(APIResponse) -> ()'
> Parent class:
>
> import ObjectMapper
>
> class APIResponse: Mappable {
> var status: String!
> var message: String?
>
> required init?(map: Map) {}
>
> func mapping(map: Map) {
> status <- map["status"]
> message <- map["message"]
> }
> }
> Subclass:
>
> import ObjectMapper
>
> class ModelListResponse<T: Mappable>: APIResponse {
> var data: ModelList<T>?
>
> required init?(map: Map) {
> super.init(map: map)
> }
>
> override func mapping(map: Map) {
> super.mapping(map: map)
> data <- map["data"]
> }
> }
> Then I have a function definition:
>
> func _GET(path: String, parameters: Parameters, completion:@escaping
> (APIResponse) -> ())
> And I'm trying to call it like so:
>
> // error is on this line
> self._GET(path: "/rest/posts", parameters: parameters) { (response:
> ModelListResponse<Post>) in
> // ...
> }
> But here I get the error about how ModelListResponse cannot be converted.
> Isn't a ModelListResponse an APIResponse? I don't understand why this does
> not work.
>
>
Below is a simplified version of what you’re attempting to do, which will
hopefully shed some light.
In your example, you’ve got a function, _GET() which expects to be passed a
function that can take an APIReponse (which it could then invoke methods on).
You’re instead passing in a function that can take a ModelListResponse<Post>,
and that function would expect to be able to invoke methods on a
ModelListResponse<Post>, but the caller (_GET()), might pass in an APIRespons
that is not a ModelListResponse<Post>.
Mark
class A {}
class B : A {
func gotcha() {}
}
func apply(_ f: (A) -> (), _ a: A) { f(a) }
func processB(_ b: B) -> () { b.gotcha() }
apply(processB, A()) // cannot convert value of type '(B) -> ()' to expected
argument type '(A) -> ()'
> I managed to get around the issue by using a generic. Though I'm not sure why
> this works and the above code does not. Here is my working declaration:
>
> func _GET<T: Mappable>(path: String, parameters: Parameters,
> completion:@escaping (T) -> ()) { }
> Thanks!
>
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