Swift structs to NSData and back

Question

I have a struct containing a struct and an NSObject that I want to serialize into an NSData object:

struct Packet {   var name: String   var index: Int   var numberOfPackets: Int   var data: NSData }  var thePacket = Packet(name: name, index: i, numberOfPackets: numberOfPackets, data: packetData) 

How do I best serialize the Packet into an NSData, and how do I best deserialize it?

Using

var bufferData = NSData(bytes: & thePacket, length: sizeof(Packet)) 

of only gives me the pointers of name and data. I was exploring NSKeyedArchiver, but then I'd have to make Packet an object, and I'd prefer to keep it a struct.

Cheers

Nik

Answer 1

Not really getting any feedback, this is the solution I ended up with:

1. Make encode() and decode() functions for my struct
2. Change Int to Int64 so the Int has the same size on 32-bit and 64-bit platforms
3. Have an intermediate struct (ArchivedPacket) that has no String or Data, but only Int64

Here is my code, I would be very grateful for your feedback, especially if there are less cumbersome ways to do this:

public struct Packet {     var name: String     var index: Int64     var numberOfPackets: Int64     var data: NSData      struct ArchivedPacket {         var index : Int64         var numberOfPackets : Int64         var nameLength : Int64         var dataLength : Int64     }      func archive() -> NSData {          var archivedPacket = ArchivedPacket(index: Int64(self.index), numberOfPackets: Int64(self.numberOfPackets), nameLength: Int64(self.name.lengthOfBytesUsingEncoding(NSUTF8StringEncoding)), dataLength: Int64(self.data.length))          var metadata = NSData(             bytes: &archivedPacket,             length: sizeof(ArchivedPacket)         )          let archivedData = NSMutableData(data: metadata)         archivedData.appendData(name.dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)!)         archivedData.appendData(data)          return archivedData     }      func unarchive(data: NSData!) -> Packet {         var archivedPacket = ArchivedPacket(index: 0, numberOfPackets: 0, nameLength: 0, dataLength: 0)         let archivedStructLength = sizeof(ArchivedPacket)          let archivedData = data.subdataWithRange(NSMakeRange(0, archivedStructLength))         archivedData.getBytes(&archivedPacket)          let nameRange = NSMakeRange(archivedStructLength, Int(archivedPacket.nameLength))         let dataRange = NSMakeRange(archivedStructLength + Int(archivedPacket.nameLength), Int(archivedPacket.dataLength))          let nameData = data.subdataWithRange(nameRange)         let name = NSString(data: nameData, encoding: NSUTF8StringEncoding) as! String         let theData = data.subdataWithRange(dataRange)          let packet = Packet(name: name, index: archivedPacket.index, numberOfPackets: archivedPacket.numberOfPackets, data: theData)          return packet     } } 

Answer 2

Swift 5

If you are on Apple platforms, use Codable now. See documentation.

Swift 3

This is an unaltered copy-paste from a Playground in Xcode 8.2.1 that works. It is a bit simpler than other answers.

import Foundation  enum WhizzoKind {     case floom     case bzzz }  struct Whizzo {     let name: String     let num: Int     let kind:WhizzoKind      static func archive(w:Whizzo) -> Data {         var fw = w         return Data(bytes: &fw, count: MemoryLayout<Whizzo>.stride)     }      static func unarchive(d:Data) -> Whizzo {         guard d.count == MemoryLayout<Whizzo>.stride else {             fatalError("BOOM!")         }          var w:Whizzo?         d.withUnsafeBytes({(bytes: UnsafePointer<Whizzo>)->Void in             w = UnsafePointer<Whizzo>(bytes).pointee         })         return w!     } }  let thing = Whizzo(name:"Bob", num:77, kind:.bzzz) print("thing = \(thing)") let dataThing = Whizzo.archive(w: thing) let convertedThing = Whizzo.unarchive(d: dataThing) print("convertedThing = \(convertedThing)") 

Notes

I couldn't make archive and unarchive instance methods because Data.init(bytes:​count:​) is mutating on the bytes parameter? And self isn't mutable, so... This made no sense to me.

The WhizzoKind enum is in there because that is something I care about. It's not important for the example. Someone might be paranoid about enums like I am.

I had to cobble this answer together from 4 other SO question/answers:

• Getting data out of NSData with Swift
• Extract struct from NSData in Swift
• 'bytes' is unavailable: use withUnsafeBytes instead
• Unsafe bytes in Swift 3

And these docs: - http://swiftdoc.org/v3.1/type/UnsafePointer/

And meditating on the Swift closure syntax until I wanted to scream.

So thanks to those other SO askers/authors.

Update

So this will not work across devices. For example, sending from iPhone 7 to Apple Watch. Because the stride is different. The above example is 80 bytes on iPhone 7 Simulator but 40 bytes on Apple Watch Series 2 Simulator.

It looks like the approach (but not syntax) by @niklassaers is still the only one that will work. I'm going to leave this answer here because it might help others with all the new Swift 3 syntax and API changes surrounding this topic.

Our only real hope is this Swift proposal: https://github.com/apple/swift-evolution/blob/master/proposals/0166-swift-archival-serialization.md