290320_part_of_official_apple_code.swift

open func decode<T : Decodable>(_ type: T.Type, from data: Data) throws -> T {
    let topLevel: Any
    do {
        topLevel = try JSONSerialization.jsonObject(with: data)
    } catch {
        throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: [], debugDescription: "The given data was not valid JSON.", underlyingError: error))
    }

    let decoder = __JSONDecoder(referencing: topLevel, options: self.options)
    guard let value = try decoder.unbox(topLevel, as: type) else {
        throw DecodingError.valueNotFound(type, DecodingError.Context(codingPath: [], debugDescription: "The given data did not contain a top-level value."))
    }

    return value
}

// MARK: - Concrete Value Representations

private extension __JSONDecoder {
    
    ...

    func unbox<T : Decodable>(_ value: Any, as type: T.Type) throws -> T? {
        return try unbox_(value, as: type) as? T
    }

    func unbox_(_ value: Any, as type: Decodable.Type) throws -> Any? {
        ... {
            return try type.init(from: self)
        }
    }
}