swift: nscoding decodeObject with nil all the time : <p>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.</p>

<pre><code>  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)
  }
}
</code></pre>

<p>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</p>

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

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

<pre><code> func dataForPacket(packet: Packet) -&gt; 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) -&gt; 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
  }
</code></pre>

<p>I wonder what can cause the error. Thanks.</p>
