Reconfiguring the flowgraph after processing N samples

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Hello Everyone,
I want to reconfigure my flowgraph after receiving N samples. I am
following this example:
https://www.gnuradio.org/doc/doxygen-3.7.4/index.html#reconfigure
Instead of using *sleep(0.1) *I want my flowgraph to lock after consumption
of N samples. Then I want to rearrange the flowgraph blocks. Do you have
any idea how to do this?
My guess is that I can use *Performance Counters *like *nproduced *for the
first block in my flowgraph and base on that, call *lock() *function on my
*top_block()*. What do you think?
Cheers
Marcin