类型“MovieSearchContainer.Type”不能符合“Decodable”； 只有结构/枚举/类类型可以符合协议

Question

I wrote a generic function, but I get an error when using this function. What is the cause of this error? I added Codable to my models. I could not find the cause of this problem.

Error

Type 'MovieSearchContainer.Type' cannot conform to 'Decodable'; only struct/enum/class types can conform to protocols

Model

struct MovieSearchContainer: Codable {
    var page: Int
    var results: [MovieSearch]
}

struct MovieSearch: Codable {
    var id: Int
    var title: String
}

MovieService

class MovieBaseService {
    func fetchNewspaper<T: Codable>(stringUrl: String, model: T, completion: @escaping (T) -> ()) {
        if let url = URL(string:stringUrl) {
            let session = URLSession(configuration: .default)
            var request = URLRequest(url: url)
            request.httpMethod = "GET"
            
            request.addValue("application/json", forHTTPHeaderField: "Content-Type")
            request.addValue("application/json", forHTTPHeaderField: "Accept")
            
            let task = session.dataTask(with: request as URLRequest) { (data, response, error) in
                if error == nil {
                    let decoder = JSONDecoder()
                    if let movieData = data {
                        do {
                            let results = try decoder.decode(T.self, from: movieData)
                            completion(results)
                        } catch {
                            print(error)
                        }
                    }
                }
            }
            task.resume()
        }
    }
}

SearchMovieVM

class SearchMovieVM: ObservableObject {
    var searchMovieService = MovieBaseService()
    @Published var searchedMovie: [MovieSearch] = []
    @Published var query: String = ""
    @Published var page: Int = 1
    
    func getSearchingMovie() {
        self.searchMovieService.fetchNewspaper(stringUrl: "https://api.themoviedb.org/3/search/movie?api_key=594b8eb4999a8b44ad5136ee3ed1ebdb&language=tr-TR&query=\(query)&page=1&include_adult=false", model: MovieSearchContainer) { (result) in
            
        }
    }
}

Answer 1

The generic T can be inferred by the compiler with the completion closure in your case

func fetchNewspaper<T: Codable>(stringUrl: String, completion: @escaping (T) -> ())

service.fetchNewspaper(stringUrl: url) { (result: MovieSearchContainer) in
    // ...
}

unless you want to explicitly passing on the model type in which case

func fetchNewspaper<T: Codable>(stringUrl: String, model: T.Type, completion: @escaping (T) -> ())

service.fetchNewspaper(stringUrl: url, model: MovieSearchContainer.self) { result in
    // ...
}

For more information on Swift Generics, refer to the official docs