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How to download JSON from the internet and decode it into any Codable type

Paul Hudson @twostraws November 28th 2021

Updated for Xcode 14.2

Fetching JSON from the network and using Codable to convert it into native Swift objects is probably the most common task for any Swift developer, usually followed by displaying that data in a List or UITableView depending on whether they are using SwiftUI or UIKit.

Well, using Swift’s concurrency features we can write a small but beautiful extension for URLSession that makes such work just a single line of code – you just tell iOS what data type to expect and the URL to fetch, and it will do the rest. To add some extra flexibility, we can also provide options to customize decoding strategies for keys, data, and dates, providing sensible defaults for each one to keep our call sites clear for the most common usages.

Here’s how it’s done:

extension URLSession {
    func decode<T: Decodable>(
        _ type: T.Type = T.self,
        from url: URL,
        keyDecodingStrategy: JSONDecoder.KeyDecodingStrategy = .useDefaultKeys,
        dataDecodingStrategy: JSONDecoder.DataDecodingStrategy = .deferredToData,
        dateDecodingStrategy: JSONDecoder.DateDecodingStrategy = .deferredToDate
    ) async throws  -> T {
        let (data, _) = try await data(from: url)

        let decoder = JSONDecoder()
        decoder.keyDecodingStrategy = keyDecodingStrategy
        decoder.dataDecodingStrategy = dataDecodingStrategy
        decoder.dateDecodingStrategy = dateDecodingStrategy

        let decoded = try decoder.decode(T.self, from: data)
        return decoded
    }
}

That does several things:

It’s an extension on URLSession, so you can go ahead and create your own custom session with a unique configuration if needed.
It uses generics, so that it will work with anything that conforms to the Decodable protocol – that’s half of Codable, so if you use Codable it will work there too.
It uses T.self for the default data type, so if Swift can infer your type then you don’t need to repeat yourself.
It allows all errors to propane to your call site, so you can handle networking and/or decoding errors as needed.

To use the extension in your own code, first define a type you want to work with, then go ahead and call decode() in whichever way you need:

extension URLSession {
    func decode<T: Decodable>(
        _ type: T.Type = T.self,
        from url: URL,
        keyDecodingStrategy: JSONDecoder.KeyDecodingStrategy = .useDefaultKeys,
        dataDecodingStrategy: JSONDecoder.DataDecodingStrategy = .deferredToData,
        dateDecodingStrategy: JSONDecoder.DateDecodingStrategy = .deferredToDate
    ) async throws  -> T {
        let (data, _) = try await data(from: url)

        let decoder = JSONDecoder()
        decoder.keyDecodingStrategy = keyDecodingStrategy
        decoder.dataDecodingStrategy = dataDecodingStrategy
        decoder.dateDecodingStrategy = dateDecodingStrategy

        let decoded = try decoder.decode(T.self, from: data)
        return decoded
    }
}

struct User: Codable {
    let id: UUID
    let name: String
    let age: Int
}

struct Message: Codable {
    let id: Int
    let user: String
    let text: String
}

do {
    // Fetch and decode a specific type
    let url1 = URL(string: "https://hws.dev/user-24601.json")!
    let user = try await URLSession.shared.decode(User.self, from: url1)
    print("Downloaded \(user.name)")

    // Infer the type because Swift has a type annotation
    let url2 = URL(string: "https://hws.dev/inbox.json")!
    let messages: [Message] = try await URLSession.shared.decode(from: url2)
    print("Downloaded \(messages.count) messages")

    // Create a custom URLSession and decode a Double array from that
    let config = URLSessionConfiguration.default
    config.requestCachePolicy = .reloadIgnoringLocalAndRemoteCacheData
    let session = URLSession(configuration: config)

    let url3 = URL(string: "https://hws.dev/readings.json")!
    let readings = try await session.decode([Double].self, from: url3)
    print("Downloaded \(readings.count) readings")
} catch {
    print("Download error: \(error.localizedDescription)")
}

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