Structs can’t be directly stored in UserDefaults because UserDefaults does not know how to serialize it. As UserDefaults is backed by plist files, struct needs to be converted representation supported by it. The core idea boils down to the question, how to convert struct into Dictionary.
Converting struct to Dictionary
The most straight-forward way would be to manually create dictionary by adding all the properties one by one. Depending on the struct, it might get pretty long and also requires maintenance when changing the struct. Therefore, let’s take a look on a way of converting struct into JSON representation instead. This can be achieved by conforming to Encodable and using JSONEncoder and JSONSerialization. In the same way, we can convert Dictionary back to struct with JSONSerialization, JSONDecoder and conforming to Decodable. When conforming struct to Encodable and Decodable, Swift compiler will take care of generating default implementations for methods in those protocols. JSONEncoder and JSONDecoder use those methods for converting struct to Data and back. It should be noted that we could just store data in user defaults. But as data is not human-readable, let’s convert it to Dictionary instead.
Adding DictionaryConvertible and DictionaryDecodable
This implementation can be made a bit more usable by using protocols and protocol extensions for providing default implementations. This makes it extremely easy to adopt this to any objects.
Using Swift’s Codable together with JSONEncoder, JSONDecoder and JSONSerialization we can skip writing code for converting data into different types and instead, providing a concise implementation for turning structs into dictionaries. We only talked about structs but this approach can be applied to classes as well.
StoringStructInUserDefaults (Xcode 10.2.1, Swift 5.0)