JSON decoding in Swift. The data couldn’t be read because it isn’t in the correct format

Question

I am having trouble decoding my JSON. It says: "The data couldn’t be read because it isn’t in the correct format." I can't put my finger on it what's wrong. Would you mind to take a pick?

Endpoint: https://images-api.nasa.gov/search?q=apollo%2011&media_type=video

JSON Sample:

{ "collection": { "version": "1.0", "href": "https://images-api.nasa.gov/search?q=apollo%2011&media_type=video", "items": [ { "href": "https://images-assets.nasa.gov/video/Apollo 11 Overview/collection.json", "links": [ { "href": "https://images-assets.nasa.gov/video/Apollo 11 Overview/Apollo 11 Overview~thumb.jpg", "render": "image", "rel": "preview" }, { "href": "https://images-assets.nasa.gov/video/Apollo 11 Overview/Apollo 11 Overview.srt", "rel": "captions" } ], "data": [ { "description": "Video highlights from the historic first manned landing on the moon, during the Apollo 11 mission in July 1969.", "date_created": "2013-05-15T00:00:00Z", "media_type": "video", "keywords": [ "Apollo 11", "Moon" ], "nasa_id": "Apollo 11 Overview", "center": "HQ", "title": "Apollo 11 Overview" } ] },

My model:

struct NasaCollection: Codable { var collection: Collection } // MARK: - Collection struct Collection: Codable { let version: String let href: String let items: [Item] } // MARK: - Item struct Item: Codable { let href: String let links: [ItemLink] let data: Datum } // MARK: - ItemLink struct ItemLink: Codable { let href: String let render: Render? } // MARK: - Datum struct Datum: Codable { let datumDescription: String let dateCreated: Date let keywords: [String] let nasaID: String let title: String let location, description508, photographer, secondaryCreator: String? let album: [String]? } enum Render: String, Codable { case image = "image" } // MARK: - CollectionLink struct CollectionLink: Codable { let prompt, rel: String let href: String } // MARK: - Metadata struct Metadata: Codable { let totalHits: Int enum CodingKeys: String, CodingKey { case totalHits = "total_hits" } }

Decding:

func getVideos(completed: @escaping (Result<[Item], Error>)-> Void) { let endpoint = "https://images-api.nasa.gov/search?q=apollo%2011&media_type=video" guard let url = URL(string: endpoint) else { return } let task = URLSession.shared.dataTask(with: url) { (data, response, error) in if let _ = error { completed(.failure(error?.localizedDescription as! Error)) } guard let response = response as? HTTPURLResponse, response.statusCode == 200 else { completed(.failure(error?.localizedDescription as! Error)) return } guard let data = data else { completed(.failure(error?.localizedDescription as! Error)) return } do { let decoder = JSONDecoder() decoder.dataDecodingStrategy = .base64 let videos = try decoder.decode([Item].self, from: data) completed(.success(videos)) } catch { print(error.localizedDescription) } } task.resume() }

I think my model is correct and it "should" be decoded. I tried to decode the very root of JSON and still getting the same error.

Don't print(error.localizedDescription). Just print(error). You get a way better error message. — Sweeper
– Sweeper, Commented Oct 9, 2020 at 0:58

gcharita · Accepted Answer · 2020-10-12 17:27:11Z

First you need to change data type from Datum to array [Datum], in Item, as @emrcftci mentioned in his answer:

struct Item: Codable { let href: String let links: [ItemLink] let data: [Datum] }

Then you need to change nasaID property to nasaId and dateCreated type from Date to String, in Datum:

struct Datum: Codable { let description: String let dateCreated: String let keywords: [String] let nasaId: String let title: String let location, description508, photographer, secondaryCreator: String? let album: [String]? }

And finally when you decoding, pass .convertFromSnakeCase to keyDecodingStrategy property of JSONDecoder and use NasaCollection.self as type when calling decode(_:from:) function, instead of [Item].self:

let decoder = JSONDecoder() decoder.keyDecodingStrategy = .convertFromSnakeCase do { let decoded = try decoder.decode(NasaCollection.self, from: data) print(decoded) } catch { print(error) }

emrcftci · Accepted Answer · 2020-10-09 06:18:00Z

Item's data should be an array of Datum -> [Datum]

As you can see the response ->

{ "collection": { "version": "1.0", "href": "https://images-api.nasa.gov/search?q=apollo%2011&media_type=video", "items": [ { "href": "https://images-assets.nasa.gov/video/Apollo 11 Overview/collection.json", "links": [ { "href": "https://images-assets.nasa.gov/video/Apollo 11 Overview/Apollo 11 Overview~thumb.jpg", "render": "image", "rel": "preview" }, { "href": "https://images-assets.nasa.gov/video/Apollo 11 Overview/Apollo 11 Overview.srt", "rel": "captions" } ], "data": [ // <------ Data has an array of an object(Datum). { "description": "Video highlights from the historic first manned landing on the moon, during the Apollo 11 mission in July 1969.", "date_created": "2013-05-15T00:00:00Z", "media_type": "video", "keywords": [ "Apollo 11", "Moon" ], "nasa_id": "Apollo 11 Overview", "center": "HQ", "title": "Apollo 11 Overview" } ] },

You should set data as an array [Datum]

struct Item: Codable { let href: String let links: [ItemLink] let data: [Datum] }

And you should update Datum object as following

{ "description": "Video highlights from the historic first manned landing on the moon, during the Apollo 11 mission in July 1969.", "date_created": "2013-05-15T00:00:00Z", "media_type": "video", "keywords": [ "Apollo 11", "Moon" ], "nasa_id": "Apollo 11 Overview", "center": "HQ", "title": "Apollo 11 Overview" }

struct Datum: Codable { let description: String let dateCreated: Date let keywords: [String] let nasaID: String let title: String let location, description508, photographer, secondaryCreator: String? let album: [String]? enum CodingKeys: String, CodingKey { case dateCreated = "date_created" case nasaID = "nasa_id" } }

BONUS

You are trying to parse NasaCollection instead of [Item]. Because of that in getVideos(completed:) function your do block should be like this ->

do { let decoder = JSONDecoder() decoder.dataDecodingStrategy = .base64 // You should try to decode `NasaCollection`!!! let videos = try decoder.decode(NasaCollection.self, from: data) completed(.success(videos)) }

It says : "Swift.DecodingError.Context(codingPath: [], debugDescription: "Expected to decode Array<Any> but found a dictionary instead.", underlyingError: nil)) " It still didn't fix the problem...

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