Conditions and WaitSets

A WaitSet is a mechanism to block until one of a set of Conditions are triggered. There are several different types of conditions that are supported by the DDS API.

• StatusCondition
• ReadCondition
• QueryCondition

Additionally, the GuardCondition provides an application controlled Condition.

Using Conditions and WaitSets

Create a WaitSet and attach Conditions to it

dds::core::cond::WaitSet ws;
dds::sub::cond::ReadCondition  rc( reader, dds::sub::status::DataState::new_data() );
ws.attach_condition( rc );

Block on the WaitSet

dds::core::cond::WaitSet::ConditionSeq triggered;
ws.wait( triggered, dds::core::Duration( 10 ) );
// if none of the conditions trigger during the elapsed 10 seconds
// then it will throw a dds::core::TimeoutError

// If one or more of the conditions trigger before the timeout,
// then they will be added to the 'triggered' sequence.
for (const auto& c : triggered) {
   if ( c == rc ) {
      // handle the fact that the read condition triggered
   }
   else if ( ... ) {
   }
}

Instead of simply blocking, you can associate a functor a condition. The functor will be invoked when the WaitSet detects that the condition has triggered. The functor can be a lambda expression, or a functor object (that is, an object that overloads operator()").

// functor object
class ConditionTriggered {
public:
  ConditionTriggered() : triggered(false) {}
  bool triggered;
  void operator()() {
    triggered = true;
  }
};

ConditionTriggered trigger;
dds::sub::cond::ReadCondition  rc( reader, dds::sub::status::DataState::new_data(), trigger );
ws += rc; // alias for ws.attach_condition( rc );

// or, with a lambda function
dds::core::cond::GuardCondition gc( []() { std::cout << "GuardCondition triggered!" << std::endl; } );
ws += gc;

ws.dispatch( dds::core::Duration(10, 0) ); // wait for up to 10 seconds calling 'trigger()' the 'rc' condition is detected