swift: nscoding decodeObject with nil all the time

Question

I got the following codes on writing an object named Packet and send to the other side through Multipeer connectivity. However, I got the following error whenever it try to decode the encoded object.

  class Packet : NSObject, NSCoding {

  var tmp1: Double = 0
  var tmp2: Double = 0

  struct PropertyKey {
    static let tmp1Key = "tmp1Key"
    static let tmp2Key = "tmp2Key"
  }


  init(tmp1: Double, tmp2: Double) {
    self.tmp1 = tmp1
    self.tmp2 = tmp2
    super.init()
  }

  deinit {
  }

  required convenience init(coder aDecoder: NSCoder) {
    debugPrint("initcoder")
    let tmp1 = aDecoder.decodeObject(forKey: PropertyKey.tmp1Key) as! Double // crash here
    let tmp2 = aDecoder.decodeObject(forKey: PropertyKey.tmp2Key) as! Double
    self.init(tmp1: tmp1, tmp2: tmp2)
  }

  public func encode(with aCoder: NSCoder) {
    debugPrint("encodeCoder")
    aCoder.encode(tmp1, forKey: PropertyKey.tmp1Key)
    aCoder.encode(tmp2, forKey: PropertyKey.tmp2Key)
  }
}

The error I got is ---- Print out from me ---- "initcoder" fatal error: unexpectedly found nil while unwrapping an Optional value 2016-09-30 13:32:55.901189 Connection[323:33022] fatal error: unexpectedly found nil while unwrapping an Optional value

But when I construct the object, all the values are set. I contracted a Packet object with no problem.

---- Additional information ------ I used the following codes for encode and decode the data when send to the other side through multiplier connectivity.

 func dataForPacket(packet: Packet) -> Data {
    let data = NSMutableData()
    let archiver = NSKeyedArchiver(forWritingWith: data)
    archiver.encode(packet, forKey: "Packet")
    archiver.finishEncoding()
    debugPrint("dataForPacket \(data) \(packet)")
    return data as Data
  }

  func packetForData(_ data: Data) -> Packet {
    debugPrint("packetForData")
    let unarchiver = NSKeyedUnarchiver(forReadingWith: data)

    let packet: Packet = unarchiver.decodeObject(forKey: "Packet") as! Packet 
     // crash here (as this will call the init of the Packet class)
    debugPrint("packetForData \(packet)")
    return packet
  }

I wonder what can cause the error. Thanks.

Answer 1

Swift 3:

I was encoding a double value. Use decoder.containsValue to check if a value was encoded.

And use decodeDouble(forKey:). Not decodeObject(forKey:)

 var dateTime: TimeInterval = SOME_TIME_INTERVAL (Double)//Can be saved(encoded) previously, and not encoded as well.
 aCoder.encode(dateTime, forKey: "dateTime")
 if decoder.containsValue(forKey: "dateTime") {
            dateTime = decoder.decodeDouble(forKey: "dateTime")
        }

Answer 2

Swift 4.2:

When using decodeObject(forKey:) to decode primitive types, it returns nil. Because the overload of encode(_:forKey:) takes an Int directly and writes it NSNumber type, and you cannot decode it as an object.

decodeInteger(forKey:) reads it out as an Int properly and returns it. You can use decodeInteger(forKey:), Int(decodeInt32(forKey:)) or Int(decodeInt64(forKey:)) instead.

Answer 3

When encoding or decoding primitive types such as Double and Bool you must use the designated methods decodeDouble:forKey and decodeBool:forKey respectively otherwise they will fail to decode and return nil.