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();
  DataReaderListener reader_listener = null;
  
  try {
      subscriber.get_default_datareader_qos(reader_qos);
  } catch (DDS.Exception) {
      Console.WriteLine(
          "***Error: failed to get default datareader qos");
  }
  
  DataReader reader = subscriber.create_datareader(
      topic,
      reader_qos,
      reader_listener,
      StatusMask.STATUS_MASK_NONE);
  if (reader == null) {
          Console.WriteLine("***Error: failed to create reader");
  }

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 (DDS.Exception) {
      // ... 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 DDS::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 T. The samples are removed from the Service. The caller is responsible for deallocating the buffers.

  FooSeq data_seq = new FooSeq(); // sequence of user data type Foo
  SampleInfoSeq info_seq = new SampleInfoSeq(); // sequence of SampleInfo
  int max_samples = ResourceLimitsQosPolicy.LENGTH_UNLIMITED;
  SampleStateKind sample_state_mask =
      SampleStateKind.ANY_SAMPLE_STATE;
  ViewStateKind view_state_mask =
      ViewStateKind.ANY_VIEW_STATE;
  InstanceStateKind instance_state_mask =
      InstanceStateKind.ANY_INSTANCE_STATE;
  
  try {
      reader.take(data_seq, info_seq, 
          max_samples,
          sample_state_mask, 
          view_state_mask, 
          instance_state_mask); 
  } catch (Retcode_NoData) {
      return;
  } catch (DDS.Exception) {
      Console.WriteLine(
          "***Error: failed to access data from the reader");
  }

• Use the received data

  // Use the received data samples 'data_seq' and associated
  // meta-information 'info_seq'
  for (int i = 0; i < data_seq.length; ++i) {
      SampleInfo info = info_seq.get_at(i);
      if (!info.valid_data) {
          continue;
      }
      Foo data = data_seq.get_at(i);
  
      Console.WriteLine(
          "  Data = (%d, %d)   sample_state = %d",
          data.x, data.y, info.sample_state);
  }

• 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.

  try {
      reader.return_loan(data_seq, info_seq);
  } catch (DDS.Exception) {
      Console.WriteLine("***Error: failed to return loan");
  }

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(); // sequence of user data type Foo
  SampleInfoSeq info_seq = new SampleInfoSeq(); // sequence of SampleInfo
  int max_samples = ResourceLimitsQosPolicy.LENGTH_UNLIMITED;
  SampleStateKind sample_state_mask =
      SampleStateKind.ANY_SAMPLE_STATE;
  ViewStateKind view_state_mask =
      ViewStateKind.ANY_VIEW_STATE;
  InstanceStateKind instance_state_mask =
      InstanceStateKind.ANY_INSTANCE_STATE;
  
  try {
      reader.read(data_seq, info_seq, 
          max_samples,
          sample_state_mask, 
          view_state_mask, 
          instance_state_mask); 
  } catch (Retcode_NoData) {
      return;
  } catch (DDS.Exception) {
      Console.WriteLine(
          "***Error: failed to access data from the reader");
  }

• Use the received data

  // Use the received data samples 'data_seq' and associated
  // meta-information 'info_seq'
  for (int i = 0; i < data_seq.length; ++i) {
      SampleInfo info = info_seq.get_at(i);
      if (!info.valid_data) {
          continue;
      }
      Foo data = data_seq.get_at(i);
  
      Console.WriteLine(
          "  Data = (%d, %d)   sample_state = %d",
          data.x, data.y, info.sample_state);
  }

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

  try {
      reader.return_loan(data_seq, info_seq);
  } catch (DDS.Exception) {
      Console.WriteLine("***Error: failed to return loan");
  }

Tearing down a data reader

• Delete DataReader:

  try {
      subscriber.delete_datareader(ref reader);
  } catch (DDS.Exception) {
      Console.WriteLine("***Error: failed to delete reader");
  }