Callback Function of subscribe MQTT message

Hey guys,

I had some troubles to use Package “MQTT in Matlab” due to the missing Documentation, but managed to link to the subscribe() function a Callback Function to read the data of each received message.

Due to the reviews and disadvantages of this Package (Code is not viewable and editable / difficulties with high frequent messages / no real time capability over a long period), as well as the missing documentation what leads to not knowing how to use different MQTT messages (like unsubscribe and disconnect) I started to write my own MQTT Package. So far everything works regarding the MQTT messages Connect, Subscribe, Publish, Unsubscribe and Disconnect with QoS 0. I was able to check with the usage of "Wireshark" (visualizes the data transfer on different interfaces) and "MQTT.fx" (MQTT test client).

Never the less I am having troubles to make the callback function work, to read the actual data of the received messages. I receive the messages, but I didnt really figure out yet how to read them. I am transforming ROS messages on a Linux PC to MQTT messages to send them to a windows running PC with Matlab. I am really looking forward for your tipps. :)

Simplified Code:

%% % INITIALISATION %%%

%  Read the Communicaition Architecture / Parameters
m_communication_architecture;



%% % MAIN %%%

% Initial call to the server / broker
mqttCalling = connectToBroker(mqttCalling);

% Connect Message
connect = on_connect(connect, mqttCalling);

% Subscribe Messages with Callbacks
subscribe = on_subscribe(subscribe2, mqttCalling, 'Callback', @receive_Callback);


%% % NETWORK FUNCTIONS %%%

% TCP IP Connection - call the server / broker
function mqttCalling = connectToBroker(mqttCalling)
    % create Data Array of the TCP_IP_Connection
    mqttCalling.messageData = tcpip(mqttCalling.destinationAddress, mqttCalling.destinationPort);
    % Open TCP IP Connection to the Broker
    fopen(mqttCalling.messageData);
end



%% % MESSAGE FUNCTIONS %%%

% Connect Message - apply as new client at the server / broker
function connect = on_connect(connect, mqttCalling)
    mqttControlPacketType = 1;  % Message Type

    % Payload (Client Identifier, Will Properties, Will Topic, Will Payload, Username, Password)
    connect.payload = create_payload(mqttControlPacketType, connect.clientID);
    % Variable Header (Protocol Name, Protocol Version, Connect Flags, Keep Alive, Connect Properties)
    connect.variableHeader = create_variableHeader(mqttControlPacketType,...
        connect.protocolName, connect.protocolVersion,...
        connect.connectFlags, connect.keepAlive);       
    % Fixed Header
    connect.fixedHeader = create_fixedHeader(mqttControlPacketType,...
        connect.variableHeader, connect.payload);
    % Assembly to Data of Message
    connect.messageData = [connect.fixedHeader;...
        connect.variableHeader; connect.payload];
    % Send message
    fwrite(mqttCalling.messageData, connect.messageData);
    % Print into Console
    fprintf('CONNECT MESSAGE: Destination Address = %s; Destination Port = %i; \n',...
        mqttCalling.destinationAddress, mqttCalling.destinationPort);
    fprintf('                 Client ID = %s; Protocol Name = %s; Protocol Version = %i; Keep Alive Time = %i \n',...
        connect.clientID, connect.protocolName, connect.protocolVersion, connect.keepAlive);
    fprintf('                 CONNECT FLAGS: Clean Start = %i; Will Flag = %i; Will QoS = %i; Will Retain = %i; Password Flag = %i; Username Flag = %i \n',...
        connect.connectFlags.cleanStart, connect.connectFlags.willFlag, connect.connectFlags.willQos,...
        connect.connectFlags.willRetain, connect.connectFlags.passwordFlag, connect.connectFlags.usernameFlag);
    fprintf('                 Username = %s, Password = %s \n',...
        connect.username, connect.password);
    pause(0.1);
end


%% Subscribe Messages - subscribe topics from which messages should be received
function subscribe = on_subscribe(subscribe, mqttCalling)
    mqttControlPacketType = 8;  % Message Type

    % Payload
    subscribe.payload = create_payload(mqttControlPacketType, subscribe.topic, subscribe.options);
    % Variable Header
    subscribe.variableHeader = create_variableHeader(mqttControlPacketType, subscribe.subscriberId);
    % Fixed Header
    subscribe.fixedHeader = create_fixedHeader(mqttControlPacketType, subscribe.variableHeader, subscribe.payload);
    % Assembly to Data of Message
    subscribe.messageData = [subscribe.fixedHeader; subscribe.variableHeader; subscribe.payload];
    % Send message
    fwrite(mqttCalling.messageData, subscribe.messageData);
    % Print into Console
    fprintf('SUBSCRIBE MESSAGE: Description = %s \n',...
        subscribe.description);
    fprintf('                   Subscriber ID = %i; Topic = %s \n',...
        subscribe.subscriberId, subscribe.topic);
    fprintf('                   OPTIONS: QoS = %i; NL = %i; RAP = %i; Retain Handling = %i \n',...
        subscribe.options.qos, subscribe.options.nl, subscribe.options.rap, subscribe.options.retainHandling);
end



%% % CALLBACK FUNCTIONS %%%

%% Receive Message
function receive_callback(topic, msg)
    
    % Read Data
    fprintf('Received MQTT message: "%s": "%s"\n', topic, msg);
    receive2.topic = topic;
    receive2.msg = convertStringsToChars(msg);
end

[Jae Hong Lee]

Wow this has been of great help!

As of now I'm working on a project to display data from a sensor using MQTT. While there are other ways to display the data, due to matlab's simplicity and various ways to manipulate numbers and so on, I have been trying to graph the data in real time in Matlab. MQTT isn't still mainstream and more so if used with Matlab, so I have had no luck with this task.

Would there be any way to graph in real time while using the callback function, or any clues?

Thank you!