Why is retrieval from NSUserDefaults failing for my custom object? - swift

Question

I have a class PredicPair which inherits from NSCoding and NSObject as such:
class PredicPair: NSObject, NSCoding {
var weight : Float
var prediction : String
init(weight: Float, prediction: String) {
self.weight = weight
self.prediction = prediction
super.init()
}
func encode(with aCoder: NSCoder) {
aCoder.encode(weight, forKey: "weight")
aCoder.encode(prediction, forKey: "prediction")
}
required convenience init(coder aDecoder: NSCoder) {
let unarchivedWeight = aDecoder.decodeObject(forKey: "weight") as! Float
let unarchivedPrediction = aDecoder.decodeObject(forKey: "prediction") as! String
self.init(weight: unarchivedWeight, prediction: unarchivedPrediction)
}
class func saveToUserDefaults(pairs: [PredicPair]) {
let dataBlob = NSKeyedArchiver.archivedData(withRootObject: pairs)
UserDefaults.standard.set(dataBlob, forKey: "test")
UserDefaults.standard.synchronize()
}
class func loadFromUserDefaults() -> [PredicPair]? {
guard let decodedNSDataBlob = UserDefaults.standard.object(forKey: "test") as? NSData,
let loadedFromUserDefault = NSKeyedUnarchiver.unarchiveObject(with: decodedNSDataBlob as Data) as? [PredicPair]
else {
return nil
}
return loadedFromUserDefault
}
}
I am trying to store an array of the class in UserDefaults and retrieving it, but the latter always returns nil. Any reason why?
let predicPair1 = PredicPair(weight: 0, prediction: "there")
let predicPair2 = PredicPair(weight: 1, prediction: "hello")
let array = [predicPair1, predicPair2]
PredicPair.saveToUserDefaults(pairs: array)
if let retreivedArray = PredicPair.loadFromUserDefaults() {
print("loaded \(retreivedArray.count) players from NSUserDefaults")
} else {
print("failed")
}
I've also tried saving to a file using NSKeyedArchiver but retrieval fails as well.