Working with JSON – Part 2

In the last post we went through a little sneak peak of how easy it is to work with JSON since Swift 4. Now let’s keep it up and see how to deal with custom keys and custom objects.

Custom keys

APIs often use snake case convention for naming keys (this_is_snake_case)… or even kebab case (this-is-kebab-case). Snake case is not a very Swifty thing because it doesn’t match the naming guidelines. Swift loves camel case (thisIsCamelCase). So if you get something like this:

{
    "product_id": 1423,
    "name": "Hockey Mask",
    "color-name": "white",
    "price-key-to-piss-the-developer-off": 39.90
}

it’s not a good practice to mirror those names so you should better create custom keys. Like this:

struct Product: Codable {
    let productId: Int
    let name: String
    let colorName: String?
    let price: Float
    
    enum CodingKeys: String, CodingKey {
          case productId = "product_id"
          case name
          case colorName = "color-name"
          case price = "price-key-to-piss-the-developer-off"
    }
}
  • Lines 2 to 5. We set our object property names as they best fit our needs.
  • Line 7. The keyword here is CodingKeys, which is a Coding Key enumeration. It connects each property to a value in the encoded format.
  • Lines 8 to 11. For each property we define which key to use in the encoded format.

Here’s the output:

Product(productId: 1423, name: "Hockey Mask", colorName: Optional("white"), price: 39.9)

Custom objects

Let’s say we have this JSON:

{
    "productId": 1423,
    "name": "Hockey Mask",
    "color": "white",
    "price": {
        "price": 39.90,
        "currency": "USD",
        "formatted": "39.90 USD"
    }
}

It’s a very common scenario and, fortunately, the Codable protocol makes our lives very easy: we just need to create an object Price which conforms to the Codable protocol as well and then use it as the type for our “price” property:

struct Price: Codable {
    let price: Float
    let currency: String
    let formatted: String
}   

struct Product: Codable {
    let productId: Int
    let name: String
    let color: String?
    let price: Price?
}

Here’s the output:

Product(productId: 1423, name: "Hockey Mask", color: Optional("white"), price: Optional(__lldb_expr_3.Price(price: 39.9, currency: "USD", formatted: "39.90 USD")))

You can use custom keys in custom objects as well, as explained above.

What’s next?

In the next post, we’ll talk about top level entities and arrays.

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