Read data from ROS2 topic when I want to instead of callback

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What about wait_for_message(..)?

That exists in both ROS 1 and ROS 2.

Technically, it still creates a subscription and just waits for a single msg, so conceptually that's no different from creating your own subscription, but it's essentially what the OP asks for.

See #q270298 and #q292582 for previous Q&As about this.

@gvdhoorn That would be quite inefficient for repeat use; it still uses a standard subscribe callback, and each "read" would take much longer as the ros node now has to communicate with the master plus has to wait for the next message to arrive.

For everything more than incidental use I would not recommend using it no. But that's also not the intended use-case. It's intended for things like capturing a single message from a topic in a service or action callback, or for initialising a "current state" kind of thing.

message_filters/Cache would be something you'd use to implement a proper version of "save the message in a variable [..] and read this [..] any time you want to".