Subscribe to tf (e.g. tf callback)

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I'm playing around with the openni_skeletal tracker, which posts all of its position data only to /tf. Now I want to access that tf data like I would access a different sensor stream. I want to write a callback function that is only activated when there is new skeletal data to process. Is it possible?

I know that the traditional route for accessing tf is to write a listener that regularly polls tf, but I'd rather not have to write the synchronization code myself when the tf messages I'm interested in are published more or less at the same time.