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Parsing JSON with Swift 5

jaumevn profile image Jaume Viñas Navas ・2 min read

There are great libraries out there that help you parse JSON data, but it is great to see how Swift 5 implements a fully-native solution to make your data types encodable and decodable for compatibility with JSON.

There are three new protocols in the Swift standard library that define an standardized approach to data encoding and decoding. These new protocols are Encodable, Decodable and Codable

Encoding and Decoding

The easiest way to encode or decode a type is to use standard properties that are already Codable, which can be types like String, Int, Double, Data, Date and URL.

Let’s consider a User structure that stores the name, the username and the phone number of a user.

We can make it automatically decodable and encodable by conforming to the Codable protocol, which is a type alias of Encodable and Decodable.

We can also use custom types that also conform to the Codable protocol, and use built-in types such as Array, Dictionary and Optional whenever they contain codable types.

Let’s add to the previous example a list of devices that the user owns. It is represented by an Array of Device. The Device stores the name and the manufacturer of a device.

We can now deserialize a JSON document into our User instance, and serialize that instance back into a JSON document.

Encode or Decode Exclusively

In some cases, you may not need to decode and encode the same type. For example, you may only need to read data and decode them into an instance, or you just need to make API calls and there is no need to decode the response containing the same type.

You can achieve that by conforming Encodable and Decodable protocols. The example below shows how the User structure only encode data:

Custom Key Names

It is often the case that we parse JSON documents with properties that doesn’t match our property names. In that case, we need to define custom key names in order to parse these properties into our instances.

In order to define the custom list of key names we need to define an enumeration named CodingKeys which conforms to the CodingKeys protocol. The names of the enumeration cases should match the names you’ve given to the corresponding properties in your type.

Let’s update the property phoneNumber of our User structure, which is defined in the JSON document using snake-case style. The enumeration CodingKeys provide the phoneNumber alternative key specified by a String as the raw-value type for the CodingKeys enumeration.


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