在 Swift 5 中转换为泛型类型

Question

I'd like to write a generic function, that fetches some kind of data locally, using the Core Data framework. I wrote the function like this:

func fetch<T:NSManagedObject>(type: T.Type = T.self, completion: @escaping (([T]?) -> ())) {
    do {
        let data = try context.fetch(T.fetchRequest()) as [T]
        completion(data)
    } catch {
        completion(nil)
    }
}

However, the complier tells me this error "Cannot convert value of type '[Any]' to type '[T]' in coercion, arguments to generic parameter 'Element' ('Any' and 'T') are expected to be equal"

Is there the possibility to do this kind of cast?

Answer 1

Try the following code:

func fetch<T:NSManagedObject>(type: T.Type = T.self, completion: @escaping (([T]?) -> ())) {
    do {
        let data = try context.fetch(T.fetchRequest()) as? [T]
        completion(data)
    } catch {
        completion(nil)
    }
}

(As Sweeper has pointed out fetch returns an array, so you have to cast the result to [T] and not just T )