Convert NSData bytes to custom object - ios

Question

I am starting a project to create an iOS app to communicate with a device over BLE. Being a new effort, I am trying to do this is Swift if possible. The interface uses GATT and an existing set of custom message structures. I get to a point where I have the data from BLE in an NSData object. I'd like to cast it or directly convert it to my message structure in a fairly generic way.
I know that I can extract the data by hand either directly from the byte array from the NSData object or using an NSInputStream. While that works, it could be a maintenance issue and the interface has a number of different messages in it.
Is there an easier ways to do this?
I'd be willing to create the message structures in Objective-C and do the casting there, but my knowledge of Objective-C is not much better than my knowledge of Swift.
Some sample code of what I've been playing in my playground is shown below. It all works as expected.
func getBytesFromNSData(data: NSData, start: Int) -> [UInt8] {
let count = data.length / sizeof(UInt8)
let remaining = count - start
let range = NSMakeRange(start, remaining )
var dataArray = [UInt8](count: remaining, repeatedValue: 0)
data.getBytes(&dataArray, range: range)
return dataArray
}
class TestObject {
var a: Byte
var b: Byte
init() {
a = 0x01
b = 0x02
}
init(data: NSData) {
let dataBytes = getBytesFromNSData(data, 0)
a = Byte(dataBytes[0])
b = Byte(dataBytes[1])
}
func populateFromStream(data: NSData) {
var stream = NSInputStream(data: data)
stream.open()
var bytesRead = stream.read(&a, maxLength: 1)
println("\(bytesRead)")
bytesRead = stream.read(&b, maxLength: 1)
println("\(bytesRead)")
}
func toArray() -> [Byte] {
var result = [Byte](count: 2, repeatedValue: 0)
result[0] = a
result[1] = b
return result
}
}
let test = TestObject()
let testArray = test.toArray()
let length = testArray.count
let testData = NSData(bytes: testArray, length: length)
println("\(testData)")
let testIn = [ Byte(0x0d), Byte(0x0e) ]
let testDataIn = NSData(bytes: testIn, length: testIn.count)
println("\(testDataIn)")
let testConstructor = TestObject(data: testDataIn)
var testObject = TestObject()
testObject.populateFromStream(testDataIn)

Answer 1

I found a method that is fairly generic that may work is some cases.
Create an Objective-C riding header
Create the data structure as an Objective-C structure
Import the header with the data structure into the bridging header
Assuming that you have a struct called Foo and an NSData object called rawData:
Use the following code to get an cast a pointer.
let structureSize = sizeof(Foo)
var myObject = UnsafeMutablePointer<Foo>.alloc(1).memory
rawData.getbytes(&myObject, length: structureSize)
This will not work in all instances and unfortunately does work in my particular case. The specific problems I have found are:
The Objective-C structure is word aligned. If your structure is not properly aligned to work boundaries, you may have a size that is incorrect. (something I ran into in my particular interface)
If you and dealing with a system that doesn't send the data in the same order you are expecting, this will not handle any byte order conversion, that would still need to be done and the structure would possibly need to be reordered to compensate. That work might negate any saving from this method.
This is the most concise method I have found if it happens to work with your particular message formats.