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How to parse JSON using JSONSerialization

Paul Hudson       @twostraws

The built-in iOS way of parsing JSON is 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 = str.data(using: String.Encoding.utf8, allowLossyConversion: false)!

do {
    let json = try JSONSerialization.jsonObject(with: data, options: []) as! [String: AnyObject]
    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 force typecast the JSON to be a dictionary of type [String: AnyObject] 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.

Available from iOS 5.0 – see Hacking with Swift tutorial 7

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