Why `NSKeyedUnarchiver` is unable to decode for `Date` - ios

Question

Recently, I was dealing with persistence on iOS with NSKeyedArchiver and NSKeyedUnarchiver. But I got some problem.
Here is the code:
import Foundation
class Model {
static let instance = Model()
static let DocumentsDirectory = FileManager().urls(for: .documentDirectory, in: .userDomainMask).first!
static let TestArchiveURL = DocumentsDirectory.appendingPathComponent("t")
static func getInstance() -> Model {
return instance
}
var str = "String"
var date = Date()
func save() {
let coder = NSKeyedArchiver(requiringSecureCoding: false)
coder.encode(str, forKey: ModelPropertyKey.str)
coder.encode(date, forKey: ModelPropertyKey.date)
do {
try coder.encodedData.write(to: Model.TestArchiveURL)
} catch {
print("error write to file: " + error.localizedDescription)
}
do {
let data = try Data(contentsOf: Model.TestArchiveURL)
let decoder = try NSKeyedUnarchiver(forReadingFrom: data)
let strGot = decoder.decodeObject(forKey: ModelPropertyKey.str) as? String
let dateGot = decoder.decodeObject(forKey: ModelPropertyKey.date) as? Date
print("\(String(describing: strGot)) \(String(describing: dateGot))")
} catch {
print("Error read from file")
}
}
}
I encode data with encode(:forKey:) and gain persistence with encodedData.write(to:). Then I try to read the data from disk with NSKeyedUnarchiver(forReadingFrom:) and decodeObject(forKey:). But this strategy works for String and fails for Date. The print output:
Optional("String") nil
I wonder why this happened.

Answer 1

You need to specify the object type you are trying to decode and since you are decoding Date object you need to understand that NSKeyedArchiver and NSKeyedUnarchiver will bridge your Date Swift object to NSDate Objective-C object for compatibility between swift and obj-c because NSKeyedArchiver uses NSCoding and Date does not conform to NSCoding
To do that change your decoding to be like this
let dateGot = decoder.decodeObject(of: NSDate.self, forKey: ModelPropertyKey.date)! as Date
However I would suggest that you use the swift Codable protocol with JSONEncoder and JSONDecoder they have a really good dateDecodingStrategy which allows for specifying your custom date formatter when encoding and decoding and they also support decoding Date objects directly without bridging to obj-c NSDate that's good when your trying to send your object as a json payload or decoding it from a network call
here is an example
let encoder = JSONEncoder()
let formatter = DateFormatter()
formatter.dateStyle = .full
formatter.timeStyle = .short
encoder.dateEncodingStrategy = .formatted(formatter)
let data = try? encoder.encode(Date())
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .formatted(formatter)
let newDate = try? decoder.decode(Date.self, from: data!)