DDS_Listener Struct Reference

<<interface>> Abstract base class for all Listener interfaces. More...

#include <dds_c_infrastructure.h>

Data Fields
void *	listener_data
	A place for listener implementors to keep a pointer to data that may be needed by their listener.

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Detailed Description

<<interface>> Abstract base class for all Listener interfaces.

Entity:

DDS_Entity

QoS:

QoS Policies

Status:

Status Kinds

All the supported kinds of concrete DDS_Listener interfaces (one per concrete DDS_Entity type) derive from this root and add functions whose prototype depends on the concrete Listener.

Listeners provide a way for RTI Connext DDS Micro to asynchronously alert the application when there are relevant status changes.

Almost every application will have to implement listener interfaces.

Each dedicated listener presents a list of operations that correspond to the relevant communication status changes to which an application may respond.

The same DDS_Listener instance may be shared among multiple entities if you so desire. Consequently, the provided parameter contains a reference to the concerned DDS_Entity.

Access to Plain Communication Status

The general mapping between the plain communication statuses (see Status Kinds) and the listeners' operations is as follows:

• For each communication status, there is a corresponding operation whose name is on_<communication_status>(), which takes a parameter of type <communication_status> as listed in Status Kinds.

• on_<communication_status> is available on the relevant DDS_Entity as well as those that embed it, as expressed in the following figure:

"Hierarchical listener processing. The most specific relevant enabled listener is called."

• When the application attaches a listener on an entity, it must set a mask. The mask indicates to RTI Connext DDS Micro which operations are enabled within the listener (cf. operation DDS_Entity set_listener() ).

• When a plain communication status changes, RTI Connext DDS Micro triggers the most specific relevant listener operation that is enabled. In case the most specific relevant listener operation corresponds to an application-installed 'nil' listener the operation will be considered handled by a NO-OP operation that does not reset the communication status.

This behavior allows the application to set a default behavior (e.g., in the listener associated with the DDS_DomainParticipant) and to set dedicated behaviors only where needed.

Access to Read Communication Status

The two statuses related to data arrival are treated slightly differently. Since they constitute the core purpose of the Data Distribution Service, there is no need to provide a default mechanism (as is done for the plain communication statuses above).

The rule is as follows. Each time the read communication status changes:

• First, RTI Connext DDS Micro tries to trigger the DDS_SubscriberListener::on_data_on_readers with a parameter of the related DDS_Subscriber;

• If this does not succeed (there is no listener or the operation is not enabled), RTI Connext DDS Micro tries to trigger DDS_DataReaderListener::on_data_available on all the related DDS_DataReaderListener objects, with a parameter of the related DDS_DataReader.

The rationale is that either the application is interested in relations among data arrivals and it must use the first option or it wants to treat each DDS_DataReader independently and it may choose the second option (and then get the data by calling FooDataReader_read_next_sample or FooDataReader_take_next_sample on the related DDS_DataReader).

Note that if DDS_SubscriberListener::on_data_on_readers is called, RTI Connext DDS Micro will not try to call DDS_DataReaderListener::on_data_available.

The operations that are allowed in DDS_Listener callbacks depend the DDS_Entity to which the DDS_Listener is attached – or in the case of a DDS_DataWriter of DDS_DataReader listener, on the parent DDS_Publisher or DDS_Subscriber.

The following operations are not allowed:

• Within any listener callback, deleting the entity to which the DDS_Listener is attached
• Within a DDS_Topic listener callback, any operations on any subscribers, readers, publishers or writers

An attempt to call a disallowed function from within a callback will result in DDS_RETCODE_ILLEGAL_OPERATION.

The following are not allowed:

• Within any listener callback, creating any entity
• Within any listener callback, deleting any entity
• Within any listener callback, enabling any entity
• Within any listener callback, setting the QoS of any entities
• Within a DDS_DataReader or DDS_Subscriber listener callback, invoking any operation on any other DDS_Subscriber or on any DDS_DataReader belonging to another DDS_Subscriber.
• Within a DDS_DataReader or DDS_Subscriber listener callback, invoking any operation on any DDS_Publisher (or on any DDS_DataWriter belonging to such a DDS_Publisher).
• Within a DDS_DataWriter of DDS_Publisher listener callback, invoking any operation on another Publisher or on a DDS_DataWriter belonging to another DDS_Publisher.
• Within a DDS_DataWriter of DDS_Publisher listener callback, invoking any operation on any DDS_Subscriber or DDS_DataReader.

An attempt to call a disallowed function from within a callback will result in DDS_RETCODE_ILLEGAL_OPERATION.

See also:

Status Kinds

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Field Documentation

void* DDS_Listener::listener_data

A place for listener implementors to keep a pointer to data that may be needed by their listener.