Swift: In NWConnection class when does receiveMessage(completion:) gets called - ios

Question

I wanna have UDP connection with a wifi module as a server and an iOS as a client and send/receive data (obviously).First i wanted to use Sockets but then i realized Apple has introduced Network Framework; So i used NWConnection which is a class from Network Framework for my purpose and i was successful at sending the data to the device but unable to receive the response (which i am sure exists as i monitor devices I/O packets via serial port monitor).Here is a test version of the code where i use netcat as server to test the connection:
ViewController Class
import UIKit
import Network
class ViewController: UIViewController
{
var network: UDPNetwork!
override func viewDidLoad()
{
super.viewDidLoad()
}
#IBAction func button(_ sender: Any)
{
self.network = UDPNetwork(host: "192.168.200.4", port:"4210")!
self.network.connect()
}
}
UDPNetwork Class
import Foundation
import Network
class UDPNetwork {
var hostUDP: NWEndpoint.Host
var portUDP: NWEndpoint.Port
private var connection: NWConnection?
private var queue = DispatchQueue(label: "NetworkQuue", qos: .utility)
init?(host: String, port: String) {
guard !host.isEmpty, let portUDP = NWEndpoint.Port(port) else {
return nil
}
self.hostUDP = NWEndpoint.Host(host)
self.portUDP = portUDP
}
func connect()
{
connection = NWConnection(host: hostUDP, port: portUDP, using: .udp)
connection?.stateUpdateHandler =
{
(newState) in switch (newState)
{
case .ready:
//The connection is established and ready to send and recieve data.
print("ready")
self.sendPaket("Hello")
self.receive()
case .setup:
//The connection has been initialized but not started
print("setup")
case .cancelled:
//The connection has been cancelled
print("cancelled")
case .preparing:
//The connection in the process of being established
print("Preparing")
default:
//The connection has disconnected or encountered an error
print("waiting or failed")
}
}
connection?.start(queue: self.queue)
}
func sendPaket(_ packet:String)
{
let packetData = packet.data(using: .utf8)
self.connection?.send(content: packetData, completion: NWConnection.SendCompletion.contentProcessed(({ (error) in
if let err = error {
print("Sending error \(err)")
} else {
print("Sent successfully")
}
})))
}
func receive()
{
self.connection?.receiveMessage(completion:
{
(data, context, isComplete, error) in
print("Got it")
if let err = error {
print("Recieve error: \(err)")
}
if let rcvData = data,
let str = String(data:rcvData, encoding: .utf8) {
print("Received: \(str)")
}
self.receive()
})
}
}
Apple Documentation says:
receiveMessage(completion:)
Schedules a single receive completion handler for a complete message
completion :
A receive completion is invoked exactly once for a call to receive.
My Question is:
How can we call to receive ?
Assuming the receiveMessage(completion:) method is the Call to receive and also after receiving complete message calls the completion itself, What could be the problem if it doesn't get invoked?
Related code and more detail about my use case can be found here:
Swift: Receiving UDP packets from server on serial port monitor but not in ios app
With these assumptions :
We call to receive on the same connection that we send data to
UDP is used method for connection
Server is tested for swapping ports correctly, meaning it responds on the same IP,Port that data is sent to
My Setup:
13" MacBookPro Early 2015 with MacOS Catalina 10.15
Xcode Version 11.0 (11A420a)
Swift 5.1
target iOS 12+
And this is the result from Wire Shark: