How to parse JSON using JSONSerialization

Swift version: 5.6

Paul Hudson @twostraws May 28th 2019

If you want to parse JSON by hand rather than using Codable, iOS has a built-in alternative called JSONSerialization and it can convert a JSON string into a collection of dictionaries, arrays, strings and numbers in just a few lines of code.

In the example below, I create a dummy piece of JSON that contains three names in an array cunningly called “names”. This then gets sent to JSONSerialization (by converting it into a Data object, which is how JSONSerialization likes to receive its content), and I conditionally pull out and print the names array:

let str = "{\"names\": [\"Bob\", \"Tim\", \"Tina\"]}"
let data = Data(str.utf8)

do {
    // make sure this JSON is in the format we expect
    if let json = try JSONSerialization.jsonObject(with: data, options: []) as? [String: Any] {
        // try to read out a string array
        if let names = json["names"] as? [String] {
            print(names)
        }
    }
} catch let error as NSError {
    print("Failed to load: \(error.localizedDescription)")
}

There are a couple of things that might confuse you there. First, because parsing JSON will fail if the JSON isn't valid, you need to use try/catch and have some sort of error handling. Second, you need to typecast my example JSON to be a dictionary of type [String: Any] so that you can start working with your JSON values. Third, you don't know for sure that any values exist inside the JSON, so you need to conditionally check for and unwrap the names value.