unable to decode saved info in Swift

Question

I am trying to save a very simple object using NSKeyedArchiver in swift, and I see that it is saving correctly, with values, but whenever it tries to decode the saved data it fails. I am new to swift and I have tried googling and os_log (ing) everything, but I can't see where the problem lies. Here is the Object I am trying to save...

class TrackedAmount: NSObject, NSCoding {
  var cups: Int
  var fraction: Float?

  struct PropertyKey {
    static let cups = "cups"
    static let fraction = "fraction"
  }
  init?(cups: Int, fraction: Float?) {
    os_log("Running init? function", log: .default, type: .debug)
    if(cups < 0) {
      os_log("Cups less than 0. Returning nil", log: .default, type: .debug)
      return nil
    }
    self.cups = cups
    self.fraction = fraction
  }

  required convenience init? coder aDecoder: NSCoder) {
    os_log("Running required convenience init? function", log: .default, type: .debug)
    guard let cups = aDecoder.decodeObject(forKey: PropertyKey.cups) as? Int else {
      os_log("Unable to decode cups", log: .default, type: .debug)
      return nil
    }
    guard let fraction = aDecoder.decodeObject(forKey: PropertyKey.fraction) as? Float else {
      os_log("Unable to decode fraction", log: .default, type: .debug)
      return nil
    }
    self.init(cups: cups, fraction: fraction)
  }

  func encode(with aCoder: NSCoder) {
    aCoder.encode(cups, forKey: PropertyKey.cups)
    aCoder.encode(fraction, forKey: PropertyKey.fraction)
  }

Then in my ViewController I am trying to get and save them like so,...

 private func saveAmount() {
    trackedToday = TrackedAmount(cups: selectedCups, fraction: selectedFraction)
    print("data.... cups: " + String(trackedToday!.cups) + " fractions: " + String(trackedToday!.fraction!))
    let fullPath = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0].appendingPathComponent("dailytracked")
    do {
      let data = try NSKeyedArchiver.archivedData(withRootObject: trackedToday!, requiringSecureCoding: false)
      try data.write(to: fullPath)
    } catch {
      os_log("Unable to save tracking amount", log: .default, type: .debug)
    }
  }

  private func loadDailyTracked() -> TrackedAmount? {
    let fullPath = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0].appendingPathComponent("dailytracked")
    if let nsData = NSData(contentsOf: fullPath) {
      do {
        let data = Data(referencing: nsData)
        if let loadedData = try NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(data) as? TrackedAmount {
          os_log("it finally worked", log: .default, type: .debug)
          return loadedData
        }
      } catch {
        os_log("Couldn't read from file", log: .default, type: .debug)
        return nil
      }
    return nil
  }

It is saving correctly but whenever I try to run the loadDailyTracked() method I get the following in the output section in xcode...

Running required convenience init? function

Unable to decode cups

I can't seem to figure out why it can't decode the cups, I know the data is being saved since none of the logs show any failure except for when it is trying to read from the saved data. I even logged the information before saving and it is showing the correct data. Anyone have any ideas? I am really new to IOS and Swift and I just can't figure this out. Thank you.

*Side note, if you need more code or information I am happy to provide it, I was just trying to keep the post from being too big if it didn't need to be.

Answer 1

In terms of NSCoder scalar types like Int and Float are not objects and the dedicated methods don't return optionals

required convenience init? coder aDecoder: NSCoder) {
    os_log("Running required convenience init? function", log: .default, type: .debug)
    let cups = aDecoder.decodeInteger(forKey: PropertyKey.cups)
    let fraction = aDecoder.decodeFloat(forKey: PropertyKey.fraction)
    self.init(cups: cups, fraction: fraction)
}

NSCoding in Swift is pretty heavy. It's highly recommended to use the Codable protocol to serialize structs and classes.