MQTT in MATLAB App

properties (Access = private)

MQTT = mqtt("tcp://test.mosquitto.org");

sub_topic = "/devices/test/Control";

Sub = subscribe(mqtt("tcp://test.mosquitto.org"),"/devices/KICTtest/Control");

first_sub = 1;

end

methods (Access = private)

function MQTT_Sub_Callback(app, topic, msg)

disp_msg = strcat("topic ",topic," : ",msg);

if(app.first_sub == 1)

app.MessageHistoryTextArea_2.Value = disp_msg;

app.first_sub = 0;

else

app.MessageHistoryTextArea_2.Value = [app.MessageHistoryTextArea_2.Value; disp_msg];

end

end

end

% Button pushed function: CREATEButton

function CREATEButtonPushed(app, event)

char Broker_Address;

Broker_Address = app.BrokerAddressEditField.Value;

app.MQTT = mqtt(Broker_Address));

end

% Button pushed function: SETButton_2

function SETButton_2Pushed(app, event)

app.sub_topic = app.TopicEditField_2.Value;

end

% Button pushed function: SubscribeButton

function SubscribeButtonPushed(app, event)

app.Sub = subscribe(app.MQTT,app.sub_topic, 'QoS', 2,'Callback', @MQTT_Sub_Callback);

end

end

I want to make an app that operates MQTT.

I already did publish message in MQTT.

But I didn't subscribe message in MQTT yet.

What is wrong?

Please Help me.

Answer 1

[Vinayak]

Hi GEONU,

While reviewing the code, I found out that the callback function is not being triggered, which could be due to incorrect syntax.

You should use "@(src,data) app.MQTT_Sub_Callback(src, data)" instead of just using "@MQTT_Sub_Callback".

Additionally, there's a syntax error involving an extra set of parentheses during the MQTT initialization in the 'CREATEButtonPushed' method.

Additionally, you will also need to make some adjustments to the Callback function as the data which it receives is different than what it expects. You may consider modifying the function along these lines:

function MQTT_Sub_Callback(app, ~, data) 
    topic = data.Topic; 
    msg = char(data.Message); 
    % Your previous code 
end 

You can refer to the documentation on MQTT and App designer for more information:

• https://www.mathworks.com/help/icomm/ug/get-started-with-mqtt.html
• https://www.mathworks.com/help/matlab/ref/appdesigner.html
• https://www.mathworks.com/help/matlab/creating_guis/write-callbacks-for-gui-in-app-designer.html