iOS Swift CFSocketCallBack - Could not read data - ios

Question

I am trying to read data from UDP socket. I am using Xcode 9.2 and Swift and I want to implement communication without external libraries. I can send the data (testing using Packet Sender), but the problem is when I send the packet back and try to read data. My socket callback function gets called and callback type matches data callback. If I try to load CFData from memory using the pointer, the operation doesn't crash. Then when I try to use a CFDataGetLength or similar function on that object, the app crashes.
func socketCallback(socket: CFSocket?, callbackType: CFSocketCallBackType, address: CFData?, data: UnsafeRawPointer?, info: UnsafeMutableRawPointer?) {
if callbackType == CFSocketCallBackType.dataCallBack {
print("dataCallback called.")
let cf = data!.load(as: CFData.self)
print(cf) // prints __NSCFData
print(CFDataGetLength(cf)) // crashes
}
}
The exception being thrown:
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[__NSCFData length]: unrecognized selector sent to class 0x10ca08348'
Same exception for other CFData functions, not just CFDataGetLength, for example
var bytes = [UInt8]()
print(CFDataGetBytes(cf, CFRange(location: 0, length: 3), &bytes))
I have tried to read some data from memory as unsigned bytes, at pointer (data) and with offset:
var array = [UInt8]()
for i in 0...55 {
let pointer = data! + i
array.append(pointer.load(as: UInt8.self))
}
print(array, separator: ", ", terminator: "\n")
It prints the following:
[72, 67, 103, 5, 1, 0, 0, 0, 132, 20, 0, 0, 1, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 97, 97, 97, 0, 0, 0, 0, 0]
I noticed that the number 3 is the length of the message, and bytes 48, 49 and 50 are the data (I have send "aaa", 97 is "a" in ascii).
Socket initialization code:
var socket = CFSocketCreate(nil, PF_INET, SOCK_DGRAM, IPPROTO_UDP, CFSocketCallBackType.dataCallBack.rawValue, socketCallback, nil)
let runLoopSource = CFSocketCreateRunLoopSource(nil, socket, 0)
CFRunLoopAddSource(CFRunLoopGetMain(), runLoopSource, CFRunLoopMode.defaultMode)
Most information that I found was for ObjC and I am not very familiar with ObjC.
What could I have done wrong?