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Puasonych Nov 18 2020 at 11:17

Confusing extensions in Swift

4 min

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Development for iOS*Swift*Development for MacOS*Development for Linux*

This post is a little bit the information aggregator. If you find a mistake, you could write to me about it I really appreciate that. Have a nice read.

Example with JSONDecoder

What would happen if we run the following piece of code?

struct Test<T>: Codable where T: Codable {
    enum CodingKeys: String, CodingKey {
        case value
    }
    
    let value: T
    let info: String
}

extension Test {
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.value = try container.decode(T.self, forKey: .value)
        self.info = "Default init(from decoder:)"
    }
}

extension Test where T == String {
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.value = try container.decode(T.self, forKey: .value)
        self.info = "Custom init(from decoder:)"
    }
}

let data = #"{"value":"Hello, World!"}"#.data(using: .utf8)!
let object = try? JSONDecoder().decode(Test<String>.self, from: data)
print(object.debugDescription)

Try thinking for 5 seconds about the result.

The result

Optional(
    Test<String>(
        value: "Hello, World!", 
        info: "Default init(from decoder:)"
    )
)