`EventHandlerBase::get_number_of_ready_events()` does not compute number of ready events

Feature request

Feature description

The current implementation of EventHandlerBase::get_number_of_ready_events() always returns 1. This function was originally introduced in https://github.com/ros2/rclcpp/pull/695 and is only used in rclcpp/test_qos_event.

However, having an accurate implementation of this function could benefit other tests like test_publisher:run_event_handlers which is calling take_data() without checking if the event is ready (ie, not calling is_ready()) or if get_number_of_ready_events > 0.

Implementation considerations

To correctly compute the number of ready events, we would likely need to

• Add a method to rmw/event.h to retrieve the count of events for a given rmw_event_type_t.
• Add a methods to rcl/event.h to retrieve number of ready events of publishers and subscriptions resp. given an rcl_event_t handle. This method will in turn invoke the newly introduced rmw method above.

Then the implementation for EventHandlerBase::get_number_of_ready_events() can be updated to invoke the appropriate rcl methods added above.

If this approach sounds ok, I can work on the implementations.