WillTopic Property

The topic that the server should publish the WillMessage to in the event of an ungraceful disconnection.

Syntax

 mqttsn.getWillTopic([callback])
 mqttsn.setWillTopic( willTopic, [callback])

Default Value

""

Callback

The 'callback' parameter specifies a function which will be called when the operation completes (or an error is encountered). If the 'callback' parameter is not specified, then the method will block and will not return until the operation completes (or an error is encountered).

The callback for the getWillTopic([callback]) method is defined as:

function(err, data){ }

'err' is the error that occurred. If there was no error, then 'err' is 'null'.

'data' is the value returned by the method.

The callback for the setWillTopic([callback]) method is defined as:

function(err){ }

'err' is the error that occurred. If there was no error, then 'err' is 'null'.

'err' has 2 properties which hold detailed information:

err.code
err.message

Remarks

This property may be set before calling Connect to specify the topic name that the server should publish the WillMessage on if the connection is closed ungracefully or lost.

MQTT Wills

The Will feature of MQTT allows a client to specify to the server a WillMessage to publish (as well as a WillTopic to publish it on) in the event that the server considers the client lost or ungracefully disconnected.

An "ungraceful disconnection" is any disconnection other than one triggered by calling Disconnect. If the client enters sleep mode, the server will not publish the Will. However, if the client is lost as a result of exceeding the sleep duration without sending a message, the server will publish the Will.

In addition to the WillTopic and WillMessage properties, the WillQOS setting may be used to specify the Will message's QoS level, and the WillRetain setting to set the Will message's Retain flag. Refer to those settings for more information.

If WillTopic is set to empty string (default) when Connect is called, the class will not send a Will to the server.

In MQTTSN, the WillTopic, WillMessage, WillQOS and WillRetain may all be updated using the UpdateWillMessage and UpdateWillTopic methods.

This property is not available at design time.

Data Type

String