DataReader Use Cases

Working with data readers. More...

Working with data readers.

Setting up a data reader

• Set up subscriber
• Set up a topic
• Create a data reader, FooDataReader, of user data type Foo:

  DataReaderQos reader_qos = new DataReaderQos();
  
  // MyReaderListener is user defined and 
  // implements DataReaderListener
  DataReaderListener reader_listener
      = new MyReaderListener(); // or = null
  
  subscriber.get_default_datareader_qos(reader_qos);
  
  FooDataReader reader = null;
  try {
      reader = (FooDataReader) subscriber.create_datareader(topic,
                                                            reader_qos,
                                                            reader_listener,
                                                      StatusKind.STATUS_MASK_ALL);
  } catch (RETCODE_ERROR err) {
      // respond to exception
  }

Managing instances

• Given a data reader

  FooDataReader reader = ...;

• Getting an instance "key" value of user data type Foo

  Foo data = new Foo();     // user data of type Foo
  // ...
  try {
      reader.get_key_value(data, instance_handle);
  } catch (RETCODE_ERROR err) {
      // ... check for cause of failure
  }    

Set up reader to access received data

• Set up data reader
• Set up to handle the DATA_AVAILABLE_STATUS status, in one or both of the following two ways.
• Enable DATA_AVAILABLE_STATUS for the DataReaderListener associated with the data reader
  • The processing to handle the status change is done in the com.rti.dds.subscription.DataReaderListener.on_data_available method of the attached listener.
  • Typical processing will access the received data.
• Enable DATA_AVAILABLE_STATUS for the StatusCondition associated with the data reader
  • The processing to handle the status change is done when the data reader's attached status condition is triggered and the DATA_AVAILABLE_STATUS status on the data reader is changed.
  • Typical processing will access the received data.

Access received data via a reader

• Ensure reader is set up to access received data
• Access the received data, by either:
  • Taking the received data in the reader, OR
  • Reading the received data in the reader

Taking data

• Ensure reader is set up to access received data
• Take samples of user data type Foo. The samples are removed from the Service. The caller is responsible for deallocating the buffers.

  FooSeq        data_seq = new FooSeq();        // holder for sequence of user data type Foo
  SampleInfoSeq info_seq = new SampleInfoSeq(); // holder for sequence of DDS_SampleInfo
  int           max_samples;
  int           sample_state_mask = SampleStateMask.ANY_SAMPLE_STATE;
  int           view_state_mask = ViewStateMask.ANY_VIEW_STATE;
  int           instance_state_mask = InstanceStateMask.ANY_INSTANCE_STATE;
  
  try {  
      reader.take(data_seq, info_seq, 
                  max_samples,
                  sample_state_mask, 
                  view_state_mask,
                  instance_state_mask); 
  } catch (RETCODE_ERROR) {
      // ... check for cause of failure
  }

• Use the received data

  // Use the received data samples 'data_seq' and associated 
  // information 'info_seq'
  for (int i = 0; i < data_seq.size(); ++i) {
      // use... data_seq.get(i) ...
      // use... info_seq.get(i) ...
  }

• Return the data samples and the information buffers back to the middleware. IMPORTANT: Once this call returns, you must not retain any pointers to any part of any sample or sample info object.

  reader.return_loan(data_seq, info_seq); 

Reading data

• Ensure reader is set up to access received data
• Read samples of user data type Foo. The samples are not removed from the Service. It remains responsible for deallocating the buffers.

  FooSeq        data_seq = new FooSeq();        // holder for sequence of user data type Foo
  SampleInfoSeq info_seq = new SampleInfoSeq)_; // holder for sequence of DDS_SampleInfo
  int           max_samples;
  int           sample_state_mask = SampleStateMask.ANY_SAMPLE_STATE;
  int           view_state_mask = ViewStateMask.ANY_VIEW_STATE;
  int           instance_state_mask = InstanceStateMask.ANY_INSTANCE_STATE;
  
  try {  
      reader.read(data_seq, info_seq, 
                  max_samples,
                  sample_state_mask, 
                  view_state_mask,
                  instance_state_mask); 
  } catch (RETCODE_ERROR) {
      // ... check for cause of failure
  }

• Use the received data

  // Use the received data samples 'data_seq' and associated 
  // information 'info_seq'
  for (int i = 0; i < data_seq.size(); ++i) {
      // use... data_seq.get(i) ...
      // use... info_seq.get(i) ...
  }

• Return the data samples and the information buffers back to the middleware

  reader.return_loan(data_seq, info_seq); 

Tearing down a data reader

• Delete DDSDataReader:

  try {
      subscriber.delete_datareader(reader);
  } catch (RETCODE_ERROR err) {
      // ... check for cause of failure
  }