NSManagedObjectContext+Deferred.swift

private extension NSManagedObjectContext {

    func executeAsync<T: NSFetchRequestResult>(request: NSFetchRequest, resultsOf _: T.Type = T.self) -> Future<[T]> {
        let d = Deferred<[T]>()

        let afr = NSAsynchronousFetchRequest(fetchRequest: request) { results in
            d.fill(results.finalResult.map({ $0 as! [T] }) ?? [])
        }

        do {
            try performAndWait {
                try executeRequest(afr)
            }
        } catch {
            d.fill([])
        }

        return Future(d)
    }

}