## How-to: Adding a new MQTT subscription
This page describes the steps required to add a new MQTT subscription in the reference project. 

1. Add a new subscriber function block in the *PLC_PRG_MQTT* program variable declaration. Example below:
```
(* variables for MQTT subscribing *)
subscriber_FB_OUTPUT_SWITCH_MQTT:SD_MQTT.MQTTSubscribe;
subscriber_FB_OUTPUT_COVER_MQTT:SD_MQTT.MQTTSubscribe;	
subscriber_FB_VIRTUAL_MQTT:SD_MQTT.MQTTSubscribe;
```

2. Add a new callback function block in the *MqttVariables* global variables. Example below:
```
(* Shared Variables for MQTT subscribe communication *)
collector_FB_OUTPUT_SWITCH_MQTT		:SD_MQTT.CallbackCollector;		
collector_FB_OUTPUT_COVER_MQTT		:SD_MQTT.CallbackCollector;	
collector_FB_VIRTUAL_MQTT			:SD_MQTT.CallbackCollector;
```
These function blocks are created in a global variable list so they are accessible from all PRGs. This is necessary because function blocks require access to the callbackcollector for being able to register itself to a MQTT subscription.

3. Initialize the subscriber function block in the *MQTT_INIT* action in the *PLC_PRG_MQTT* program. Example below:
```
(* INIT MQTT SUBSCRIBE STUFF *)
subscriber_FB_OUTPUT_SWITCH_MQTT.SetMqttInOut(MQTT_IN_OUT:= ADR(MQTT_IN_OUT));	
subscriber_FB_OUTPUT_COVER_MQTT.SetMqttInOut(MQTT_IN_OUT:= ADR(MQTT_IN_OUT));
subscriber_FB_VIRTUAL_MQTT.SetMqttInOut(MQTT_IN_OUT:= ADR(MQTT_IN_OUT));
```
The *MQTT_IN_OUT* is the object that links the subscribers to the MQTT Client.

4. Call the subscriber function block in the *MQTT_SUBSCRIBE* action in the *PLC_PRG_MQTT* program. Example below:
```
subscriber_FB_OUTPUT_SWITCH_MQTT(
	Subscribe:= TRUE, 
	Topic:= ADR('Devices/PLC/House/In/DigitalOutputs/+'), 
	QoSSubscribe:= SD_MQTT.QoS.ExactlyOnce, 
	ExpectingString:= TRUE, 
	Callback:= MqttVariables.collector_FB_OUTPUT_SWITCH_MQTT,
);
	
subscriber_FB_OUTPUT_COVER_MQTT(
	Subscribe:= TRUE, 
	Topic:= ADR('Devices/PLC/House/In/Covers/+'), 
	QoSSubscribe:= SD_MQTT.QoS.ExactlyOnce, 
	ExpectingString:= TRUE, 
	Callback:= MqttVariables.collector_FB_OUTPUT_COVER_MQTT,
);
```

5. Register the required function blocks against the callback function block to trigger the callback method when a new message arrives on the subscribed topic. Example below:
```
pMqttCallbackCollector^.put(instance:= THIS^);
```
A more in depth implementation can be found in the *InitMqtt* method of every MQTT function block in the reference project.

6. Implement the callback method triggered by the callback function block. More details below:
    - Add the *IMPLEMENTS* statement:
        ```
        FUNCTION_BLOCK FB_VIRTUAL_STRING_MQTT IMPLEMENTS SD_MQTT.MQTT_SUBSCRIBE_CALLBACK
        ```
    - Add the *PublishReceived* callback method:
        ```
        METHOD PublishReceived : bool
        VAR_INPUT
	        (*Collection of received Data*)
	        Data	: SD_MQTT.CALLBACK_DATA;
        END_VAR
        ```
    - Add logic to process the received MQTT message: in this reference project a check is executed if the subscription that triggered the callback method matches the subscription configured in the *MqttInit* method. In that method the subscription topic is automatically concatenated with the function block name. For more in depth details study the *PublishReceived* and *MqttInit* methods of every MQTT function block in the reference project.