Reacting on incomung MQTT message

Projects Programming
1

Hi forum

I would like to react on a published MQTT message.
I have actually found guides for this but somehow they do not work for me.
In a nutshell I am subscribing to a topic with

void onMqttMessage(char* topic, char* payload, AsyncMqttClientMessageProperties properties, size_t len, size_t index, size_t total)

and then check for this topic with

if(strcmp(topic, MQTT_SUB) == 0 )

Unfortunately I am getting a compilation error: "'topic' was not declared in this scope", although for some people this seemed to work.

Any help would be welcome.

Here is the whole code in case needed:

#include <ESP8266WiFi.h>
#include <Ticker.h>
#include <AsyncMqttClient.h>

#define WIFI_SSID "ABC"
#define WIFI_PASSWORD "ABC"

#define MQTT_HOST IPAddress(xx, xx, xx, xx)
#define MQTT_PORT 1883
#define MQTT_SUB "ipcheck"

AsyncMqttClient mqttClient;
Ticker mqttReconnectTimer;

WiFiEventHandler wifiConnectHandler;
WiFiEventHandler wifiDisconnectHandler;
Ticker wifiReconnectTimer;

void connectToWifi() {
  Serial.println("Connecting to Wi-Fi...");
  WiFi.begin(WIFI_SSID, WIFI_PASSWORD);
}

void onWifiConnect(const WiFiEventStationModeGotIP& event) {
  Serial.println("Connected to Wi-Fi.");
  connectToMqtt();
}

void onWifiDisconnect(const WiFiEventStationModeDisconnected& event) {
  Serial.println("Disconnected from Wi-Fi.");
  mqttReconnectTimer.detach(); // ensure we don't reconnect to MQTT while reconnecting to Wi-Fi
  wifiReconnectTimer.once(2, connectToWifi);
}

void connectToMqtt() {
  Serial.println("Connecting to MQTT...");
  mqttClient.connect();
}

void onMqttConnect(bool sessionPresent) {
  Serial.println("Connected to MQTT.");
  Serial.print("Session present: ");
  Serial.println(sessionPresent);

  uint16_t packetIdSub = mqttClient.subscribe(MQTT_SUB, 2);
  Serial.print("Subscribing at QoS 2, packetId: ");
  Serial.println(packetIdSub);
}

void onMqttDisconnect(AsyncMqttClientDisconnectReason reason) {
  Serial.println("Disconnected from MQTT.");

  if (WiFi.isConnected()) {
    mqttReconnectTimer.once(2, connectToMqtt);
  }
}

void onMqttPublish(uint16_t packetId) {
  Serial.print("Publish acknowledged.");
  Serial.print("  packetId: ");
  Serial.println(packetId);
}

void onMqttMessage(char* topic, char* payload, AsyncMqttClientMessageProperties properties, size_t len, size_t index, size_t total) {
  Serial.println("Publish received.");
  Serial.print("  topic: ");
  Serial.println(topic);
  Serial.println("Payload");
  Serial.println(payload);
  Serial.print("  qos: ");
  Serial.println(properties.qos);
  Serial.print("  dup: ");
  Serial.println(properties.dup);
  Serial.print("  retain: ");
  Serial.println(properties.retain);
  Serial.print("  len: ");
  Serial.println(len);
  Serial.print("  index: ");
  Serial.println(index);
  Serial.print("  total: ");
  Serial.println(total);
}

void setup() {
  Serial.begin(115200);
  
  wifiConnectHandler = WiFi.onStationModeGotIP(onWifiConnect);
  wifiDisconnectHandler = WiFi.onStationModeDisconnected(onWifiDisconnect);

  mqttClient.onConnect(onMqttConnect);
  mqttClient.onDisconnect(onMqttDisconnect);
  mqttClient.onPublish(onMqttPublish); //Löschen?
  mqttClient.setServer(MQTT_HOST, MQTT_PORT);
  mqttClient.onMessage(onMqttMessage);
  
  connectToWifi();
}

void loop() {
if(strcmp(topic, MQTT_SUB) == 0 ) //match
{
Serial.println("Message received");
}
}
2

The compiler is not lying

In your sketch the topic variable is local to the onMqttMessage(0 function

void onMqttMessage(char* topic, char* payload, AsyncMqttClientMessageProperties properties, size_t len, size_t index, size_t total)

so it is out of scope in the loop() function

void loop()
{
    if (strcmp(topic, MQTT_SUB) == 0)  //match
    {
        Serial.println("Message received");
    }
}

Do the comparison in the onMqttMessage() function instead

3

Thanks a lot UKHeliBob for the explanation and solution :smiley:
Everything is working now.

4

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