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

• Juraj: make an array and use a for loop

Answer 1

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);
 }
 }
}

Answer 2

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);
 }
 }
}