CODEWITHSUNDEEP
jsonswiftgenericscodableswift4.2
Frakcool
Frakcool

Reputation: 11163

Decode a JSON array using generics and Decodable

I'm trying to make an application in order to learn how to use generics in Swift. So I decided to use Studio Ghibli's API

For this, I do the following:

  1. Create a request to the API
  2. Get the response
  3. Parse the JSON

As in their API, all the information comes with a baseURL equal to https://ghibliapi.herokuapp.com and all the endpoints are baseURL/foo and return an array of whatever info we want. So, for this particular case, I'm trying to use /films and /people.

So, in my head, it would be better to have a generic decoding function that does that and returns an array of the corresponding class that we want to decode.

private func decode<T: Decodable>(json: Data, as clazz: T) {
    do {
        let decoder = JSONDecoder()
        let films = try decoder.decode([T].self, from: json)
    } catch {
        print("An error occurred while parsing JSON")
    }
}

And I call it as:

self.decode(json: safeData, as: Films.self) //Error thrown here

My Films class is as follows:

import Foundation
import CoreData

@objc(Films)
public class Films: NSManagedObject, Decodable {
    enum CodingKeys: String, CodingKey {
        case id
        case title
        case film_description = "description"
        case director
        case producer
        case release_date
        case rt_score
    }

    required public init(from decoder: Decoder) throws {
        guard let context = CodingUserInfoKey(rawValue: "context"),
            let managedObjectContext = decoder.userInfo[context] as? NSManagedObjectContext,
            let entity = NSEntityDescription.entity(forEntityName: "Films", in: managedObjectContext) else {
                fatalError("Failed to decode Films!")
        }
        super.init(entity: entity, insertInto: nil)

        let container = try decoder.container(keyedBy: CodingKeys.self)
        do {
            id = try container.decodeIfPresent(String.self, forKey: .id)
            title = try container.decodeIfPresent(String.self, forKey: .title)
            film_description = try container.decodeIfPresent(String.self, forKey: .film_description)
            director = try container.decodeIfPresent(String.self, forKey: .director)
            producer = try container.decodeIfPresent(String.self, forKey: .producer)
            release_date = try container.decodeIfPresent(String.self, forKey: .release_date)
            rt_score = try container.decodeIfPresent(String.self, forKey: .rt_score)
        } catch let error {
            print(error)
            throw error
        }
    }
}

But I keep getting an error:

Argument type 'Films.Type' does not conform to expected type 'Decodable'

I have read in the Swift Forums these topics:

And some have said that those solutions have worked for them, but it's the same concept as mine and I still can't figure out how to solve it.

What would be the correct way to do it?

Upvotes: 0

Views: 569

Answers (1)

Frakcool
Frakcool

Reputation: 11163

The solution was quite simple:

This function:

private func decode<T: Decodable>(json: Data, as clazz: T) {

Became:

private func decode<T: Decodable>(json: Data, as clazz: T.Type) {

And that's it! It's all that was needed to be done

Upvotes: 2

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