Reducing clutter in your JSON parser through proper typing

This article is an excerpt from Parsing JSON in Swift.

When you’re casting types as you parse your JSON, be as specific as you can to reduce clutter. This is especially important when working with arrays, which generally contain just one type. Let’s look at how we might parse the following JSON (from the GitHub Search Repositories API):

{
  "total_count": 78457,
  "incomplete_results": false,
  "items": [
    {
      "id": 22458259,
      "name": "Alamofire",
      "description": "Elegant HTTP Networking in Swift"
    }
  ]
}

For now, let’s assume we have a parseRepository method with the following signature:

func parseRepository(json: [String: AnyObject]) -> Repository?

This should take a JSON dictionary that represents a repository and convert it into our Repository model object.

Now, if we want to get the repository out of the items array and transform it into our model object, we might try something like this:

if let items = json["items"] as? [AnyObject],
    item = items[0] as? [String: AnyObject] {
        parseRepository(item)
}

We start by casting our items to an array containing elements of type AnyObject. Then, since our parseRepository method requires a dictionary ([String: AnyObject]), we’re forced to cast item – the first element of our array – to a [String: AnyObject]. This extra line is unnecessary clutter – it’s an extra cast that we don’t need. And it’s all because we started by casting items to [AnyObject]. (Note that casting to NSArray causes exactly the same issue.)

Instead, we can just cast items to the type we expect, which is more specific than an array of objects – it’s an array of dictionaries.

if let items = json["items"] as? [[String: AnyObject]] {
    parseRepository(items[0])
}

This allows us to call parseRepository without an additional cast, since accessing any element of the items array will yield a dictionary.

But now the cast to [[String: AnyObject]] is hard to read due to the all the square brackets. Is it an array or a dictionary? It’s hard to tell at first glance. Can we get rid of a pair of square brackets and make it clear that we’re working with an array of dictionaries and not just a dictionary?

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