Working with JSON Data in Swift

Swift and JSON don’t play nicely together. Swift wants everything to have a type; JSON, on the other hand, is a blob of data from which types can only be inferred.

How can we write good Swift when working with JSON?

JSONValue

The JSONValue enum is a Swift container around JSON values. It can represent any of the primitive JSON types:

• String
• Number (Double or Integer)
• Boolean
• Null
• Object
• Array

To implement JSONValue , we’ll create six enum cases, corresponding to the six possible JSON types listed above.

Next up, we’re going to implement decoding. This allows us to transform a blob of JSON data into usable Swift data.

Notice that for Dictionaries and Arrays, the nested type is JSONValue . This is because the decoder doesn’t know what type the Dictionary or Array contains; and, in fact, they could contain multiple types as JSON doesn’t enforce homogeneity.

Test Cases for JSONValue can be found at this gist.

JSONPrimitive

JSONPrimitive is a “pseudo-type” that is adopted by the Swift types which correspond to primitive JSON types.

To handle null , we extend Optional to adopt JSONPrimitive. We also extend JSONValue itself so it can act as a JSONPrimitive.

Finally, to extract the wrapped value, we add a property value to the JSONValue enum, which returns an Optional JSONPrimitive? . This return value represents the raw value and can be passed around instead of Any .