org.omg.dds.topic

Interface MultiTopic<TYPE>

Type Parameters:

TYPE - The concrete type of the data that will be published and/ or subscribed by the readers and writers that use this topic description.

All Superinterfaces:

AutoCloseable, Closeable, DDSObject, TopicDescription<TYPE>

public interface MultiTopic<TYPE>
extends TopicDescription<TYPE>

MultiTopic is a specialization of TopicDescription that allows subscriptions to combine/filter/rearrange data coming from several Topics. MultiTopic allows a more sophisticated subscription that can select and combine data received from multiple topics into a single resulting type (specified by the inherited typeName). The data will then be filtered (selection) and possibly rearranged (aggregation/projection) according to a subscriptionExpression with parameters expressionParameters.

• The subscriptionExpression is a string that identifies the selection and rearrangement of data from the associated topics. It is similar to an SQL clause where the SELECT part provides the fields to be kept, the FROM part provides the names of the topics that are searched for those fields, and the WHERE clause gives the content filter. The Topics combined may have different types but they are restricted in that the type of the fields used for the NATURAL JOIN operation must be the same.
The expressionParameters attribute is a sequence of strings that give values to the "parameters" (i.e., "%n" tokens) in the subscriptionExpression. The number of supplied parameters must fit with the requested values in the subscriptionExpression (i.e., the number of "%n" tokens).
• DataReader entities associated with a MultiTopic are alerted of data modifications by the usual Listener or Condition mechanisms whenever modifications occur to the data associated with any of the topics relevant to the MultiTopic.
• DataReader entities associated with a MultiTopic access instances that are "constructed" at the DataReader side from the instances written by multiple DataWriter entities. The MultiTopic access instance will begin to exist as soon as all the constituting Topic instances are in existence. The ViewState and InstanceState are computed from the corresponding states of the constituting instances:
  • The viewState of the MultiTopic instance is NEW if at least one of the constituting instances has viewState = NEW, otherwise it will be NOT_NEW.
  • The instanceState of the MultiTopic instance is "ALIVE" if the instanceStates of all the constituting Topic instances are ALIVE. It is "NOT_ALIVE_DISPOSED" if at least one of the constituting Topic instances is NOT_ALIVE_DISPOSED. Otherwise, it is NOT_ALIVE_NO_WRITERS.

Method Summary

Methods

Modifier and Type	Method and Description
List<String>	getExpressionParameters() This operation returns the expression parameters associated with the MultiTopic, that is, the parameters specified on the last successful call to setExpressionParameters(List), or if setExpressionParameters(List) was never called, the parameters specified when the MultiTopic was created.
String	getSubscriptionExpression()
void	setExpressionParameters(List<String> expressionParameters) This operation changes the expression parameters associated with the MultiTopic.

Methods inherited from interface org.omg.dds.topic.TopicDescription

cast, close, getName, getParent, getTypeName, getTypeSupport

Methods inherited from interface org.omg.dds.core.DDSObject

getEnvironment

Method Detail

getSubscriptionExpression

String getSubscriptionExpression()

Returns:

the subscription expression associated with the MultiTopic, that is, the expression specified when the MultiTopic was created.

getExpressionParameters

List<String> getExpressionParameters()

This operation returns the expression parameters associated with the MultiTopic, that is, the parameters specified on the last successful call to setExpressionParameters(List), or if setExpressionParameters(List) was never called, the parameters specified when the MultiTopic was created.

Returns:

an unmodifiable list.

See Also:

setExpressionParameters(List)

setExpressionParameters

void setExpressionParameters(List<String> expressionParameters)

This operation changes the expression parameters associated with the MultiTopic.

See Also:

getExpressionParameters()