How to keep BackgroundService task receiving mqtt messages without Console.ReadLine()-CodePudding

In my ASP.Net Core 6 application, a BackgroundService task called MqttClientService runs a MQTTNet client that handles incoming mqqt messages and responds with a message to indicate it was successful.

I have gotten the sample console app from the MQTTNet repo to work using Console.ReadLine(), however this feels like a hack for my use case. Is there a better way to keep the BackgroundService handling incoming messages without restarting constantly?

There is an example with Asp.Net Core and MQTTNet version 3, but it uses handles implemented by interfaces rather than async events that the library now uses: the MQTTNet's Upgrading Guide.

Any information will be appreciated, thank you.

MqttClientService.cs in Services/

using MQTTnet;
using MQTTnet.Client;
using System.Text;

namespace MqttClientAspNetCore.Services
{
    public class MqttClientService : BackgroundService
    {
        protected override async Task ExecuteAsync(CancellationToken stoppingToken)
        {
            while (!stoppingToken.IsCancellationRequested)
            {
                await Handle_Received_Application_Message();
            }
        }

        public static async Task Handle_Received_Application_Message()
        {

            var mqttFactory = new MqttFactory();

            using (var mqttClient = mqttFactory.CreateMqttClient())
            {
                var mqttClientOptions = new MqttClientOptionsBuilder()
                    .WithTcpServer("test.mosquitto.org")
                    .Build();

                // Setup message handling before connecting so that queued messages
                // are also handled properly. 
                mqttClient.ApplicationMessageReceivedAsync  = e =>
                {
                    Console.WriteLine("### RECEIVED APPLICATION MESSAGE ###");
                    Console.WriteLine($"  Payload = {Encoding.UTF8.GetString(e.ApplicationMessage.Payload)}");

                    // Publish successful message in response
                    var applicationMessage = new MqttApplicationMessageBuilder()
                        .WithTopic("keipalatest/1/resp")
                        .WithPayload("OK")
                        .Build();

                    mqttClient.PublishAsync(applicationMessage, CancellationToken.None);

                    Console.WriteLine("MQTT application message is published.");

                    return Task.CompletedTask;
                };

                await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);

                var mqttSubscribeOptions = mqttFactory.CreateSubscribeOptionsBuilder()
                    .WithTopicFilter(f =>
                    {
                        f.WithTopic("keipalatest/1/post");
                        f.WithAtLeastOnceQoS();
                    })
                    .Build();

                await mqttClient.SubscribeAsync(mqttSubscribeOptions, CancellationToken.None);

                Console.WriteLine("MQTT client subscribed to topic.");
                // The line below feels like a hack to keep background service from restarting
                Console.ReadLine();
            }
        }
    }
}

Program.cs

using MqttClientAspNetCore.Services;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddHostedService<MqttClientService>();

var app = builder.Build();

// To check if web server is still responsive
app.MapGet("/", () =>
{
    return "Hello World";
});


app.Run();

CodePudding user response：

If your service has nothing else useful to do, it can just wait for the CancellationToken to fire:

protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
  try
  {
    await Handle_Received_Application_Message(stoppingToken);
  }
  catch (OperationCanceledException) { }
}

public static async Task Handle_Received_Application_Message(CancellationToken cancellationToken)
{
  ...
  Console.WriteLine("MQTT client subscribed to topic.");
  await Task.Delay(Timeout.Infinite, cancellationToken);
}