Micro-ros reading operation

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I

In Microros Client, It is the function rclc_executor_spin_some() that go check if there is data available in the (custom)transport? I don't understand which is the flow to arrive to the Micro-XRCE-DDS function uxr_run_session_<...> for read the message just arrived.

I miss the RCL part in the flow:

RCLC: rclc_executor_spin_some() --> RCL: .???.. --> RMW: run_xrce_session() --> Micro-XRCE-DDS-Client: uxr_run_session() and recv_msg()

In other word I would like to understand how MicroRos listen incoming message. ( And who? It s the executor?)

Thank you so much in advance.