Cannot invoke 'encode' with an argument list of type 'NSAttributedString'

Question

How can I encode a NSAttributedString and store it in disk using UserDefaults?

First I tried setting the Codable protocol:

class CoachProgram: Codable {
    var name: String
    var details: NSAttributedString
}

but the compiler yields Type 'Coach' does not conform to protocol 'Encodable'. So, I started to implement the encode(to:) method:

public func encode(to encoder: Encoder) throws {
    var container = encoder.unkeyedContainer()
    try container.encode(details)
                  ^~~~~~ "error: cannot invoke 'encode' with an argument list of type '(NSAttributedString)'"
}

but no success.

My idea is to use the CoachProgram like:

let archiver = NSKeyedArchiver()
do {
    try archiver.encodeEncodable(coachProgram, forKey: NSKeyedArchiveRootObjectKey)
}
catch {
    print(error)
}
UserDefaults.standard.set(archiver.encodedData, forKey: "CoachProgram")

Answer 1

First problem: I was trying to use a Unkeyed Container and that doesn't makes sense because I definitely need a Keyed Container because I have 2 attributes (name and details).

So, I needed to implement some CodingKey's and use decoder.container(keyedBy:) method.

Second problem: after some experiments, I noticed I could turn a NSAttributedString into Data by simple using the NSKeyedArchiver! The Data is Codable so I can encode and decode it.

So, the final solution I got:

class CoachProgram: Codable {
    var name: String
    var details: NSAttributedString

    enum CodingKeys: String, CodingKey {
        case name
        case details
    }

    init(name: String, details: NSAttributedString) {
        self.name = name
        self.details = details
    }

    required public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        if let name = try container.decodeIfPresent(String.self, forKey: .name) {
            self.name = name
        }
        if let data = try container.decodeIfPresent(Data.self, forKey: .details) {
            self.details = NSKeyedUnarchiver.unarchiveObject(with: data) as? NSAttributedString ?? NSAttributedString()
        }
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(name, forKey: .name)
        try container.encode(NSKeyedArchiver.archivedData(withRootObject: details), forKey: .details)
    }
}

In action:

let coachProgram = CoachProgram(name: "John Doe", details: NSAttributedString())

let archiver = NSKeyedArchiver()
do {
    try archiver.encodeEncodable(coachProgram, forKey: NSKeyedArchiveRootObjectKey)
}
catch {
    print(error)
}
UserDefaults.standard.set(archiver.encodedData, forKey: "CoachProgram")