Swift Struct to NSData

StructTest.swift

protocol Constructible {
    init()
}

struct devicePacketHeader{
    var head:Byte = 1;
    var checksum:Byte = 2;
    var deviceID:Byte = 3;
    var command:Byte = 4;
    var len:Int16 = 5;
}

struct devicePacketAskDeviceID : Constructible{
    var revVersion:Int = 1
    
//    init() {
//        revVersion = 1
//    }
}

struct devicePacketSetting:Constructible{
    var revVersion:UInt16 = 6
    var subVersion:UInt8 = 7
    var mainVersion:UInt8 = 8
}

struct devicePacketRadios:Constructible{
    var  Aileron:Float = 10;
    var  Elevator:Float = 11;
    var  Throttle:Float = 12;
    var  Rudder:Float = 13;
    var  Gear_Mode:Float = 14;
    var  Pitch_Return_Home:Float = 15;
    var  Aux1_Smart:Float = 16;
    var  Aux2:Float = 17;
}

struct devicePacketSingleParameter:Constructible{
    var index:Float = 20
    var value:Float = 21
}

struct devicePacketAck:Constructible{
    var ack:Byte = 30
}

struct devicePacketNil:Constructible{
//    init
}

struct devicePacket <T:Constructible> {
    var deviceHeader:devicePacketHeader = devicePacketHeader();
    var deviceContent: T = T()
}

var packet0 = devicePacketHeader()
var data0 = NSData(bytes: &packet0, length: sizeof(devicePacketHeader))
print("data0 : \(data0)\n") //data0 : <01020304 0500>

var packet1 = devicePacket<devicePacketNil>()
var data1 = NSData(bytes: &packet1, length: sizeof(devicePacket<devicePacketNil>))
//var data1 = NSData(bytes: &packet1, length: sizeof(devicePacketNil) + sizeof(devicePacketHeader))
print("data1 : \(data1)\n") //data1 : <01020304 0500>

var packet2 = devicePacket(deviceHeader: devicePacketHeader(), deviceContent: devicePacketSetting())
packet2.deviceContent.mainVersion = 25
var data2 = NSData(bytes: &packet2, length: sizeof(devicePacketSetting) + sizeof(devicePacketHeader))
print("data2 : \(data2)\n") //data2 : <01020304 05000600 0719>

var packet3 = devicePacket(deviceHeader: devicePacketHeader(), deviceContent: devicePacketRadios())
var data3 = NSData(bytes: &packet3, length: sizeof(devicePacket<devicePacketRadios>))
print("data3 : \(data3)\n") //data3 : <01020304 05000000 00002041 00003041 00004041 00005041 00006041 00007041 00008041 00008841>
//0500 + 0000  X

var packet4 = devicePacket(deviceHeader: devicePacketHeader(), deviceContent: devicePacketSingleParameter())
var data4 = NSData(bytes: &packet4, length: sizeof(devicePacket<devicePacketSingleParameter>))
print("data4 : \(data4)\n") //data4 : <01020304 05000000 0000a041 0000a841>
//0500 + 0000  X

var packet5 = devicePacket(deviceHeader: devicePacketHeader(), deviceContent: devicePacketAck())
var data5 = NSData(bytes: &packet5, length: sizeof(devicePacket<devicePacketAck>))
print("data5 : \(data5)\n") //data5 : <01020304 05001e>