FB_IotMqtt5SubscribeProperties
The function block allows the definition of different properties, which can be set when creating a subscription. This allows additional properties to be defined for a subscription and transferred from the client to the message broker.
General description of the Subscribe Properties:
Property | Description |
|---|---|
No Local | When using MQTTv3, if a message is sent on the same topic that was also subscribed to, the sent message will be received again. By using this flag, you will not receive messages that have already been sent again. |
Retained Message Control | Retain messages still work as with MQTTv3, but this flag adds options that define what should happen when retain messages are received. |
Subscription Identifier | A numeric value used to identify a subscription. |
User Properties | User Properties are key/value pairs that can transport additional metadata. These are managed via the function block FB_IotMqtt5UserProperties. The meaning of the UserProperties is not part of the MQTT5 specification and therefore application specific. |
Syntax
Definition:
FUNCTION_BLOCK FB_IotMqtt5SubscribeProperties EXTENDS FB_IotMqtt5UserProperties
VAR_OUTPUT
bError : BOOL;
hrErrorCode : HRESULT;
END_VAR
Outputs
Name | Type | Description |
|---|---|---|
bError | BOOL | Becomes TRUE as soon as an error situation occurs. |
hrErrorCode | HRESULT | Returns an error code if the bError output is set. An explanation of the possible error codes can be found in the Appendix. |
Properties
Name | Type | Access | Description |
|---|---|---|---|
bNoLocal | BOOL | Get, Set | Specifies whether the NoLocal property should be set, see above. |
bRetainAsPublished | BOOL | Get, Set | Specifies whether the retain flag of a publisher should remain set when a message is received. |
nRetainHandling | BYTE | Get, Set | Specifies how to receive retain messages. 0: Retain messages are sent by the broker when the client subscribes. (Default behavior of MQTTv3) 1: Retain messages are sent by the broker only when the client subscribes if the subscription does not exist. 2: Retain messages are not sent by the broker when the client subscribes. |
nSubId | UDINT | Get, Set | Numeric value for optional identification of a subscription. |
pSubscribeProperties | POINTER TO MqttSubscribeProperties | Get | Pointer to an object of type MqttSubscribeProperties. When calling FB_IotMqtt5Client.Subscribe() it is possible to pass this directly. |
Methods
Name | Description |
|---|---|
SetSubscribeProperties | Allows you to set the Subscribe properties. |
Requirements
Development environment | Target platform | PLC libraries to include |
|---|---|---|
TwinCAT v3.1.4026.0 | IPC or CX (x86, x64, ARM) | Tc3_IotBase (>= v3.4.2.0) |