ros2 crystal tutorial gtest message and service testing

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Hi,

I would like to write unit tests to test my ROS2 crystal package based on what is communication. Example: Imagine the package is controlled via service commands and publishing its state in a topic. In the utest then I would like to send service commands and subscribe the state topic to validate if the correct changes appeared. In this problem, I have a lot of problems getting a proper synchronisation between sending commands, subscribing from topics, etc.

Is there any standard procedure for such kind of testing? Are there any tutorials/good example packages for such unit testing with ROS2 crystal?