WaitSet with Status Condition

Concept

Conditions and WaitSets provide a way for Connext to notify your application that data is available. While a Listener is used to provide a callback for asynchronous access, Conditions and WaitSets provide synchronous data access. This means that you use a WaitSet to block an application thread until data becomes available. This is safer than using a listener because you do not have to worry about blocking Connext's threads.

A WaitSet allows an application to wait until one or more attached Conditions becomes true (or until a timeout expires).

Briefly, your application can create a WaitSet, attach one or more Conditions to it, then call the WaitSet's wait() operation. The wait() blocks until one or more of the WaitSet's attached Conditions becomes TRUE.

A Condition has a trigger_value that can be TRUE or FALSE. You can retrieve the current value by calling the Condition's only operation, get_trigger_value().

There are three kinds of Conditions. This is a simple example that shows how to receive notifications about data becoming available using a StatusCondition. This is the most efficient way to use WaitSets to be notified that you have new data.

StatusConditions are created automatically by Connext, one for each Entity. A StatusCondition is triggered by Connext when there is a change to any of that Entity's enabled statuses.

If you would like more information about the different types of conditions you can use, look at the "waitsets" example.

A WaitSet can be associated with more than one Entity (including multiple DomainParticipants). It can be used to wait on Conditions associated with different DomainParticipants. A WaitSet can only be in use by one application thread at a time.

Example Description

This example shows how to use WaitSets to efficiently take data. To initialize the WaitSet, we define a set of status conditions on which to wait, and attach them to the WaitSet.

Note that a WaitSet is a "top-level" entity, not one created by a DDS entity. It is only related to a reader by the contained conditions, which are created via DataReaders. Also note that WaitSets block in the context of the application rather than DDS threads. Thus, there are fewer constraints on processing data than if we're working in an on_data_available callback, since DDS housekeeping threads continue to run.

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