USRP B210, how to switch the frequency correctly?

Question

Radio communication

Gnuradio has the ability to switch the USRP reception frequency in the following ways:
- via the "Ch0: Center Freq (Hz)" parameter of the UHD Source block
- by sending a pmt message to the command input of the UHD Source block
- from the scheme's Python code via the uhd_source.set_center_freq(_f0) , 0)

From the features of the circuit, I have to receive a vector of 2 ** 16 elements at each frequency, then immediately switch to the next frequency (+- 1 MHz)
The problem is that I do not know exactly the point in the data stream when the frequency switch occurred .
After receiving a command to set the USRP frequency, it thinks about 30k more samples, then gives out about 10-20k zeros, then a sharp surge in amplitude, and then there are samples from the new frequency.

How to track a specific data start location on a new frequency, or
How to stop USRP and collect data on your request?