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Where to check sub msg ready in ROS intra-process communication

I’m using intra-process communication in a ROS node, with SingleThreadedExecutor.
When using DDS communication, ROS check whether the subscription new message arrives sequently in


But when using intra-process communication, I noticed that the subscription new messages were not be checked as when using DDS communication, and the subscription callbacks were executed as waitable.

So I want to figure out the mechanism of waitable and where subscription new message is checked when using intra-process communication.

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Sorry, I will migrate this question there. Should I remove this topic?