codable

Codable is a protocol in the Swift programming language that enables easy encoding and decoding of values to and from external representations such as JSON or plist. It provides a unified approach to data interchange, allowing developers to convert in-memory values to a portable format and back again.

In Swift, Codable is a type alias for the combination of two protocols: Encodable and Decodable. A type that conforms to Codable can be both encoded and decoded. The feature was introduced with Swift 4 to standardize serialization patterns and reduce boilerplate for common data models.

Automatic synthesis is a core aspect of Codable. If all stored properties of a type are themselves Codable, the compiler can automatically generate the required encoding and decoding logic. This works for many struct and class definitions, making it easy to adopt Codable for simple data models without writing boilerplate code.

For cases where defaults are not sufficient or the external representation uses different keys, developers can customize conformance. A CodingKeys enum, conforming to CodingKey, maps property names to the keys used in the encoded data. When more control is needed, a type can implement its own encode(to:) and init(from:) methods to manually perform the encoding and decoding steps.

Typical usage involves the JSONEncoder and JSONDecoder classes. An instance of JSONEncoder can encode a Codable value into Data, while JSONDecoder can decode Data back into the corresponding Swift type. Encoder and decoder instances can be configured with strategies for dates, data, and key decoding to handle variations in formats.

Limitations include that complex enums with associated values often require manual conformance, and properties that are not themselves Codable cannot be automatically synthesized. Despite these caveats, Codable remains a central tool for data serialization in Swift.