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Confusing extensions in Swift
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.
Optional(
Test<String>(
value: "Hello, World!",
info: "Default init(from decoder:)"
)
)