Subscriber Use Cases

Working with subscribers. More...

Working with subscribers.

Setting up a subscriber

• Set up participant
• Create a DDSSubscriber

  DDS_SubscriberQos subscriber_qos;
  DDS_ReturnCode_t retcode;
  
  // MySubscriberListener is user defined and 
  // extends DDSSubscriberListener
  DDSSubscriberListener* subscriber_listener =
                                  new MySubscriberListener(); // or = NULL
  
  retcode = participant->get_default_subscriber_qos(subscriber_qos);
  
  if (retcode != DDS_RETCODE_OK) {
      // ... error 
  
  };
  
  DDSSubscriber* subscriber = 
                  participant->create_subscriber(subscriber_qos, 
                                                 subscriber_listener,
                                                 DDS_STATUS_MASK_ALL);
  if (subscriber == NULL) {
      // ... error 
  
  };

Set up subscriber to access received data

• Set up subscriber
• Set up to handle the DDS_DATA_ON_READERS_STATUS status, in one or both of the following two ways.
• Enable DDS_DATA_ON_READERS_STATUS for the DDSSubscriberListener associated with the subscriber
  • The processing to handle the status change is done in the DDSSubscriberListener::on_data_on_readers() method of the attached listener.
  • Typical processing will access the received data, either in arbitrary order or in a coherent and ordered manner.
• Enable DDS_DATA_ON_READERS_STATUS for the DDSStatusCondition associated with the subscriber
  • The processing to handle the status change is done when the subscriber's attached status condition is triggered and the DDS_DATA_ON_READERS_STATUS status on the subscriber is changed.
  • Typical processing will access the received data, either in an arbitrary order or in a coherent and ordered manner.

Access received data via a subscriber

• Ensure subscriber is set up to access received data
• Get the list of readers that have data samples available:

  DDSDataReaderSeq reader_seq;  // holder for list/set of readers
  DDS_SampleStateMask     sample_state_mask = DDS_NOT_READ_SAMPLE_STATE;
  DDS_ViewStateMask       view_state_mask = DDS_ANY_VIEW_STATE;
  DDS_InstanceStateMask   instance_state_mask = DDS_ANY_INSTANCE_STATE;
  
  
  DDS_ReturnCode_t retcode;
  
  // get_datareadersX is not supported yet.
  retcode = subscriber->get_datareaders(reader_seq, 
                                   sample_state_mask,
                                   view_state_mask,
                                   instance_state_mask);
  
  if (retcode != DDS_RETCODE_OK) {
      // ... check for cause of failure
  } 

• Upon successfully getting the list of readers with data, process the data readers to either:
  • Read the data in each reader, OR
  • Take the data in each reader
  If the intent is to access the data coherently or in order, the list of data readers must be processed in the order returned:

  for(int i = 0; i < reader_seq.length(); ++i) {
    
    TDataReader* reader = reader_seq[i];
  
    // Take the data from reader, 
    //            OR
    // Read the data from reader
  }

• Alternatively, call DDSSubscriber::notify_datareaders to invoke the DDSDataReaderListener for each of the data readers.

  retcode = subscriber->notify_datareaders(); 
  
  if (retcode != DDS_RETCODE_OK) {
      // ... check for cause of failure
  } 

Access received data coherently and/or in order

To access the received data coherently and/or in an ordered manner, according to the settings of the DDS_PresentationQosPolicy attached to a DDSSubscriber:

• Ensure subscriber is set up to access received data
• Indicate that data will be accessed via the subscriber:

  retcode = subscriber->begin_access();
  
  if (retcode != DDS_RETCODE_OK) {
      // ... check for cause of failure
  } 

• Access received data via the subscriber, making sure that the data readers are processed in the order returned.
• Indicate that the data access via the subscriber is done:

  retcode = subscriber->end_access();
  
  if (retcode != DDS_RETCODE_OK) {
      // ... check for cause of failure
  } 

Tearing down a subscriber

• Delete DDSSubscriber:

  DDS_ReturnCode_t retcode;
  
  retcode = participant->delete_subscriber(subscriber);
  
  if (retcode != DDS_RETCODE_OK) {
      // ... check for cause of failure
  }