JSON Parsing 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.

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{     "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:

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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, JSONObjectWithDatais 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):

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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:

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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.

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