StrMessage.sedona

** 
** Str message defines a string type MQTT topic data point. There are 2 use cases:
**  1. when put it within a Publisher object, it will publish it's payload to 
**     the MQTT broker configured by the Publisher object (publishing mode)
**  2. when put it within a Subscriber object, it will fetch data from MQTT broker
**     configured by the Subscriber object (subscription mode)
**
** Just like other type of Message object, StrMessage's parent must be Publisher or Subscriber, otherwise it will not work
** 
** By StrMessage, you can configure the following MQTT related things: 
**  1. MQTT topic 
**  2. MQTT QoS
**  3. publishInterval (how often to publish data, publishing mode)
**  4. updateInterval (how often to check data update, subscription mode)
** 
** The 'error' slot will show error message if something is wrong.
** 
class StrMessage extends Message
{
  @asStr property Buf(Message.strLen) payload;
  
  ** 
  ** return the payload string, 
  ** shouldn't be called by outside
  ** 
  protected override Str getPayload() { return payload.toStr() }
  
  ** 
  ** callback method to update local data,
  ** shouldn't be called by outside
  **
  protected override bool updateData(Obj handle) 
  {
    bool result = super.updateData(handle)
    if (!result)
      return result

    payload.copyFromStr(strBuf)
    changed(StrMessage.payload)
    return result
  }
}