How to parse JSON with Swift 2

I’ve written about how to parse JSON in Swift before, but someone asked recently about how to do it with Swift 2 and I realized that everything changed from Swift 1.2 to Swift 2.0. So how do you parse JSON with Swift 2?

We’ll look at how to parse a JSON array that we could use to display a list of items in a table view. More spefically, we’ll use the following JSON (copied and tweaked from David Owens II) to display a list of blog titles in our app.

{
    "blogs": [
        {
            "needspassword": true,
            "id": 73,
            "url": "http://remote.bloxus.com/",
            "name": "Bloxus test"
        },
        {
            "needspassword": false,
            "id": 74,
            "url": "http://flickrtest1.userland.com/",
            "name": "Manila Test"
        }
    ],
    "stat": "ok"
}

Assuming we’ve made a request and received the JSON above, we just need to ask NSJSONSerialization to give us a JSON object, and then pull out the “blogs” and “name” keys. First, take a look at the full code, then we’ll break it down and explain it in smaller pieces:

var names = [String]()

do {
    let json = try NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments)

    if let blogs = json["blogs"] as? [[String: AnyObject]] {
        for blog in blogs {
            if let name = blog["name"] as? String {
                names.append(name)
            }
        }
    }
} catch {
    print("error serializing JSON: \(error)")
}

print(names) // ["Bloxus test", "Manila Test"]

Let’s break down what’s actually happening here.

First of all, everything is surrounded in a do/catch block since JSONObjectWithData:options: may throw an error. If you’re familiar with Objective-C, you’ll notice that we don’t pass in an NSError pointer here – instead, JSONObjectWithData is marked with throws, meaning it may throw an error. The error that we catch (which is called error by default) replaces the NSError pointer from Objective-C.

Let’s look at the next line (with a bit of commentary added):

if let blogs = json["blogs"] as? [[String: AnyObject]] {
    // if we get "blogs" from the JSON
    // AND we can cast it to an array of dictionaries
    // then this code will execute
}

Here, we’re attempting to take "blogs" from the JSON, cast it to an array of dictionaries ([[String: AnyObject]]), and assign it to a constant called blogs. If all of that is successful, our if block will execute. (Aside: this if let syntax is called optional binding, and you need to understand it if you’re writing Swift. You can read about it in The Swift Programming Language.)

Moving on:

for blog in blogs {
    if let name = blog["name"] as? String {
        names.append(name)
    }
}

Here, we’re iterating through our array of blogs, and we know that blog is a dictionary of type [String: AnyObject] based on the type of our blogs array. Inside the for loop, we’re doing optional binding again to grab the name of the blog as a String, then appending it to our names array.

Conclusion

I hope this helps you understand how to parse JSON in Swift 2. It’s relatively simple once you understand optional binding and the do/try/catch syntax in Swift 2. So many people seem to want grab a third-party library to parse JSON, but for most use cases, I see very little benefit to using a third-party library to do something that’s so simple. You don’t need a third-party library for everything.

You can get the playground on GitHub.