Xamarin.Forms MQTT Publish upon button click

Question

I been working on a small peice of code in Xamarin Forms (Visual Studio - my first project) to send a mqtt message upon a button press. Ive written the code to publish the mqtt message to the test.mosquitto.org server within a C# console app and that works perfectly

using System;
using System.Net.Mqtt;
using System.Text;

namespace MqttTest.Client
{
class Program
{
const string topic = "test/test/button";

    static void Main (string[] args)
    {
        var config = new MqttConfiguration { Port = 1883 };
        var client = MqttClient.CreateAsync("test.mosquitto.org", config).Result;
        var clientId = "myClientID";
        string message = "test";


        client.ConnectAsync (new MqttClientCredentials (clientId)).Wait ();
        client.SubscribeAsync (topic, MqttQualityOfService.AtLeastOnce).Wait ();
        //Publishes "message" Var 
        client.PublishAsync(new MqttApplicationMessage(topic, Encoding.UTF8.GetBytes($"{message}")), MqttQualityOfService.AtLeastOnce).Wait();
    }
}

}

Ive now written a very simple xamarin cross platform app with 1 button, 1 button_clicked event and the code for the button clicked event which was written in the above console app.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;
using System.Net.Mqtt;

namespace App1
{
public partial class MainPage : ContentPage
{
public MainPage()
{
InitializeComponent();
}
const string topic = "test/test/button";

    private void Button_Clicked(object sender, EventArgs e)
    {
        var config = new MqttConfiguration { Port = 1883 };
        var client = MqttClient.CreateAsync("test.mosquitto.org", config).Result;
        var clientId = "clientIdhGHvpYY9uM";
        string message = "Hello";


        client.ConnectAsync(new MqttClientCredentials(clientId)).Wait();
        client.SubscribeAsync(topic, MqttQualityOfService.AtLeastOnce).Wait();
        client.PublishAsync(new MqttApplicationMessage(topic, Encoding.UTF8.GetBytes($"{message}")), MqttQualityOfService.AtLeastOnce).Wait();
    }
}

}

Upon testing the app in the emulator, it loads up OK but when I click the button, it freezes (I cannot re-click the button) the app and does not publish to the test mqtt server.

Can anyone offer any advice on how to get this working

thank you

Answer 1

I had the same issue. As pointed out by SushiHangover, the reason the button freezes is just that you are connecting on the main thread. An async connect method could look like this:

public async Task<bool> ConnectAsync()
{
    return await Task.Run(() =>
    {
        Log($"Connecting to {MqttSettings.Host}:{MqttSettings.Port}");

        try
        {
            var configuration = new MqttConfiguration();
            configuration.Port = SolaceSettings.Port;
            _mqttClient = MqttClient.CreateAsync(MqttSettings.Host, configuration).Result;
            _sessionState = _mqttClient.ConnectAsync(new MqttClientCredentials(MqttSettings.MqttClientId, MqttSettings.Username, MqttSettings.Password)).Result;

            Log("Connected.");
            return true;
        }
        catch (Exception ex)
        {
            Log($"Could not connect. {ex}");
            return false;
        }
    });
}

However that won't fix the root cause for not being able to connect. For me there were two main issues:

• Don't run in simulator, use a real device (at least if your Mac is behind a proxy / firewall).
• I specified a random client ID. After configuring a client ID on the broker and using the same in the client, it worked. Update: Obviously this is wrong and was not needed. The client ID entry is automatically generated when connecting with a new ID.