How to use Codable to load and save custom data types

Swift version: 5.6

Paul Hudson @twostraws May 28th 2019

Swift 4 introduced a new way to load and save data, replacing the old NSCoding protocol with something that’s more flexible, safer, and easier to write: Codable.

Unless you want a custom implementation, just making your data type conform to Codable is all it takes to allow it to be saved to property list XML or JSON.

For example, here’s a custom struct that conforms to Codable, along with a few instances of it:

struct Language: Codable {
    var name: String
    var version: Int
}

let swift = Language(name: "Swift", version: 4)
let php = Language(name: "PHP", version: 7)
let perl = Language(name: "Perl", version: 6)

You can see I've marked the Language struct as conforming to the Codable protocol – there’s no need to add custom loading and saving code like we had with NSCoding.

With that one tiny conformance, we can convert it to a Data representation of JSON like this:

let encoder = JSONEncoder()
if let encoded = try? encoder.encode(swift) {
    // save `encoded` somewhere
}

Swift will automatically encode all properties inside your data type – you don't need to do anything.

To prove that everything is working well, we can try converting that Data object into a string so we can print it out, then decode it back into a new Language instance that we can read from:

if let encoded = try? encoder.encode(swift) {
    if let json = String(data: encoded, encoding: .utf8) {
        print(json)
    }

    let decoder = JSONDecoder()
    if let decoded = try? decoder.decode(Language.self, from: encoded) {
        print(decoded.name)
    }
}

Notice how decoding doesn't require a typecast – you provide the data type name as its first parameter, so Swift infers the return type from there.

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