Connecting multiple topics- MQTT

Question

I'm configuring an ESP8266 using Arduino Uno.

I have several MQTT topics I wish it to subscribe, and I do not want to repeat code 3 or 4 times ( in Python I would have created a list - and make a for loop to subscribe all )

These are my topics:

//MQTT topic
const char* deviceName = "Sonoff1";
const char* deviceTopic = "HomePi/Switches/Sonoff1";
const char* msgTopic = "HomePi/Messages";
const char* groupTopic = "HomePi/All";

This is the function registering a single topic:

void reconnect() {
        // Loop until we're reconnected
        while (!client.connected()) {
                Serial.print("Attempting MQTT connection...");
                // Attempt to connect
                if (client.connect(deviceName,user, passw)) {
                        Serial.println("connected");
                        pub_msg("Power On");
                        client.subscribe(deviceTopic);
                } else {
                        Serial.print("failed, rc=");
                        Serial.print(client.state());
                        Serial.println(" try again in 5 seconds");
                        // Wait 5 seconds before retrying
                        delay(5000);
                }
        }
}

Appriciate you kind help,

Guy

make an array and use a for loop

Juraj
– Juraj ♦

2018-09-15 07:19:42 +00:00
Commented Sep 15, 2018 at 7:19 — Juraj
– Juraj ♦, Commented Sep 15, 2018 at 7:19

Rudy Albachten · Accepted Answer · 2022-05-31 16:00:21Z

You can make life easier if you arrange your topic names hierarchically. As an example suppose we have a multimedia controller that accepts various commmands:

mediaserver/set/volume
mediaserver/set/pause
mediaserver/set/audio/source
mediaserver/set/video/subtitles
etc...

You can then subscribe to "mediaserver/set/#"

For looping over the array of topics, you can take advantage of modern C++'s range-based for loop:

https://en.cppreference.com/w/cpp/language/range-for

Example code:

const char *topicArry[] = {
    deviceTopic, msgTopic, groupTopic,
    "HomePi/Dvir/Clock", "HomePi/Dvir/Alerts"
};
void reconnect() {
    // Loop until we're reconnected
    while (!client.connected()) {
        Serial.print("Attempting MQTT connection...");
        // Attempt to connect
        if (client.connect(deviceName,user, passw)) {
            Serial.println("connected");
            pub_msg("Power On");
            for (const char *topic : topicsArry) {
                client.subscribe(topic);
                char msg[50];
                sprintf(msg, "Subscribed to %s",topic);
                pub_msg(msg);
            }
        } else {
            Serial.print("failed, rc=");
            Serial.print(client.state());
            Serial.println(" try again in 5 seconds");
            // Wait 5 seconds before retrying
            delay(5000);
        }
    }
}

guyd · Accepted Answer · 2018-09-15 10:09:44Z

just for a closure:

added an array (thank to Juraj):const char* topicsArry[]={deviceTopic, msgTopic, groupTopic, "HomePi/Dvir/Clock", "HomePi/Dvir/Alerts"};

a for loop to over this that loop, and using sizeof ( which was not trivial for me ), led to :

void reconnect() {
        // Loop until we're reconnected
        while (!client.connected()) {
                Serial.print("Attempting MQTT connection...");
                // Attempt to connect
                if (client.connect(deviceName,user, passw)) {
                        Serial.println("connected");
                        pub_msg("Power On");
                        for (int i=0; i<=sizeof(topicsArry)/sizeof(char *); i++){
                          client.subscribe(topicsArry[i]);
                          char msg[50];
                          sprintf(msg, "Subscribed to %s",topicsArry[i]);
                          pub_msg(msg);
                        }
                } else {
                        Serial.print("failed, rc=");
                        Serial.print(client.state());
                        Serial.println(" try again in 5 seconds");
                        // Wait 5 seconds before retrying
                        delay(5000);
                }
        }
}

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