MQTT subscribe callback function not executed in AppDesigner

I am developing a graphic user interface on AppDesigner with the purpose of displaying the result of a classification algorithm using data coming via MQTT protocol. The “subscribe” function (related to the “mqtt” function) requires the use of a callback to perform tasks whenever data are received. This procedure works fine when applied outside AppDesigner, while within AppDesigner it seems like the callback function is never executed. An attempt to address this issue was using global variable (see code below) but it did not work.

function StartButtonPushed(app, event)

% Connection to MQTT

mqttAddress = 'tcp://10………';

mObj = mqtt(mqttAddress,'Port',1883);

mySub = subscribe(mObj,'sensors','callback',@appCallback);

function appCallback(~,~)

app.GoodLamp.Color = 'Green';

end

end

How can this issue be addressed?

Answer 1

[Sean de Wolski]

(This is a guess)

When the function exits, the mObj and mySub are destroyed. Set them to be properties of the app (add properties and then app.mObj = ...) so that they exist after the function call completes.

[Alberto Franzini]

Thank you very much, it works!

[Faraz Azmi]

Hello Mr. Sean,

you seem to know how mqtt works on matlab properly. I have a problem where it shows that there are no messages waiting to be read in the topic but i do have data published under it.

link to my question which contains the screenshots and dont have any answers yet

https://in.mathworks.com/matlabcentral/answers/524900-read-function-in-mqtt-says-there-are-no-messages-waiting-to-be-read