Swift structs to NSData and back

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
    }
}

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

I used Jeff's example to create the following struct:

struct Series {
    
    var name: String?
    var season: String?
    var episode: String?
    
    init(name: String?, season: String?, episode: String?) {
        self.name = name
        self.season = season
        self.episode = episode
    }
    
    static func archive(w: Series) -> Data {
        var fw = w
        return Data(bytes: &fw, count: MemoryLayout<Series>.stride)
    }
    
    static func unarchive(d: Data) -> Series {
        guard d.count == MemoryLayout<Series>.stride else {
            fatalError("Error!")
        }
        
        var w: Series?
        d.withUnsafeBytes({(bytes: UnsafePointer<Series>) -> Void in
            w = UnsafePointer<Series>(bytes).pointee
        })
        return w!
    }
}

Like Dag mentioned the whole thing is a bit fragile. Sometimes the App crashes when the name contains whitespace or an underline/underscore, and sometimes it crashes just without reason. In all cases the name which is unarchived looks similar to this '4\200a\256'. Surprisingly this is not a problem in the case of season or episode (like in "Season 2"). Here the whitespace doesn't force the app to crash.

Maybe it's an alternative to encode the strings to utf8 but I'm not familiar enough with the archive/unarchive methods to adapt them for this case.

The easiest way for basic struct objects is PropertyListEncoder & PropertyListDecoder.

This is the sample code;

Swift 5

struct Packet: Codable {
   var name: String
   var index: Int
   var numberOfPackets: Int
   var data: Data
}

func getDataFromPacket(packet: Packet) -> Data?{
  do{
    let data = try PropertyListEncoder.init().encode(packet)
    return data
  }catch let error as NSError{
    print(error.localizedDescription)
  }
    return nil
}

func getPacketFromData(data: Data) -> Packet?{
    do{
      let packet = try PropertyListDecoder.init().decode(Packet.self, from: data)
      return packet
    }catch let error as NSError{
      print(error.localizedDescription)
    }
    
    return nil
}