How to convert JSON into Swift objects using Codable

Swift version: 5.1

Paul Hudson @twostraws May 28th 2019

Swift’s Codable protocol makes it easy to convert JSON to native Swift structs and classes – just design data types that hold the same keys and values as your JSON, then use JSONDecoder to convert.

Here’s some example JSON we can work with:

let jsonString = """
[
    {
        "name": "Taylor Swift",
        "age": 26
    },
    {
        "name": "Justin Bieber",
        "age": 25
    }
]
"""

let jsonData = Data(jsonString.utf8)

That stores two people in an array, each with a name and an age.

We need to make a matching Swift struct that can hold those fields. The only requirement Codable has is that all the properties inside the struct also conform to Codable – in our case that’s a string and an integer, so we’re all set.

Start by adding this type:

struct Person: Codable {
    var name: String
    var age: Int
}

Now we can go ahead and decide the JSON data into an array of that Person struct. This is a throwing operation, so you need to use try. Here’s some example code:

let decoder = JSONDecoder()

do {
    let people = try decoder.decode([Person].self, from: jsonData)
    print(people)
} catch {
    print(error.localizedDescription)
}

That will result in people storing the two items from the JSON, except now they are parsed into Swift types so we can refer to them in a type-safe way.