How do I resolve: Unable to Cast due to 'Unknown Context'? - swift

Question

Scenario:
A function takes a generic parameter (CovidResource) to grab data from a server which returns a data in a particular format depending on the value of the function's parameter, CovidResource.
The intent is to use one (1) function to grab data from assorted endpoints in a format/endpoint.
The function returns a struct AppleSubRegions (in this example, one of any possible data types via generic value) in CovidResource format.
let url = URL(string: "https://disease.sh/v3/covid-19/apple/countries/CANADA")!
countryListViewModel.getList(urlDataModel: CovidResource<AppleSubRegions>(url: url))
struct AppleSubRegions: Codable {
let country, subregion: String
let data: [AppleDatum]
}
import Combine
import UIKit
protocol URLResource {
associatedtype DataModel: Decodable
var url: URL? { get }
}
struct CovidResource<T: Decodable>: URLResource {
typealias DataModel = T
var url = URL(string: "https://disease.sh/v3/covid-19/apple/countries/Canada")
}
// =====================================================================================================
class CountryRegionListModel: ObservableObject {
#Published var countryRegionList: [String] = []
// Data Persistence:
var cancellables: Set<AnyCancellable> = []
// ---------------------------------------------------------------------------
func getList<Resource>(urlDataModel: Resource) where Resource: URLResource {
let remoteDataPublisher = URLSession.shared.dataTaskPublisher(for: urlDataModel.url!)
.map(\.data)
.handleEvents(receiveOutput: { data in
print(String(data: data, encoding: .utf8)!)
})
.receive(on: DispatchQueue.main)
.decode(type: Resource.DataModel.self, decoder: JSONDecoder())
//.print("getList: ")
remoteDataPublisher
.eraseToAnyPublisher()
.sink(receiveCompletion: { completion in
switch completion {
case .finished:
print("Publisher Finished")
case let .failure(anError):
Swift.print("\nReceived error: ", anError)
}
}, receiveValue: { someValue in
print("\n\n ---- Model: \(someValue)")
DataSource.shared.countryName = (someValue as! AppleSubRegions).country
}).store(in: &cancellables)
}
}
You can see the datatype returned is 'AppleSubRegions':
(Notice the 'unknown context...)?
(lldb) po type(of: someValue)
Covid19.TabNavView.(unknown context at $10bb96490).(unknown context at $10bb96504).AppleSubRegions
I can see the country, 'Canada' via the debugger:
---- Model: AppleSubRegions(country: "Canada", subregions: ["Alberta", "Calgary", "Edmonton", "British Columbia", "Vancouver", "Manitoba", "New Brunswick", "Newfoundland and Labrador", "Northwest Territories", "Halifax", "Nova Scotia", "Ontario", "Ottawa", "Toronto", "Prince Edward Island", "Montreal", "Quebec", "Saskatchewan", "All", "Yukon Territory"])
(lldb) po someValue.country
"Canada"
I tried repeating what I did via the debugger in code:
... but as you can see, the DataModel isn't aware of 'country':
Yet I can't access its members.
So I tried to cast 'someValue' to AppleSubRegions type:
Which crashed the compile.
Question: How do I convert the CovidResource into AppleSubRegions proper (Swift data item)?
Suggestions tried:
1. Qualifying closure with
...receiveValue { someValue: AppleSubRegions in
But I got a compiler syntax error:
Adding an additional constraint:
But this causes another compiler error:

Answer 1

This should work if you further constrain the type to AppleSubRegions:
func getList<R: URLResource>(urlDataModel: R) where R.DataModel == AppleSubRegions {
let remoteDataPublisher = Just(json)
.receive(on: DispatchQueue.main)
.decode(type: R.DataModel.self, decoder: JSONDecoder())
remoteDataPublisher
.sink(
receiveCompletion: {...},
receiveValue: { someValue in
print(someValue.country)
}).store(in: &c)
}