[Discuss-gnuradio] Change frequency USRP sink/source block on the run

Hello everyone,

I have been working on tx/rx data using USRP B210. I am able to transmit a message and verify the correct content on the receiver side. The next step I want to do is: 

1. upon sending a specific message (frequency change), I want to change the frequency of the USRP sink (tx side) 

2. upon receiving a specific message (frequency change) I want to change the frequency of the USRP source (rx side)

My question is what will be the best way to do it? I know that I will need to pause the flow graph on tx/rx side, and reload the USRP blocks with the new frequency. But, I dont know when the message will arrive; I cannot do a timing triggering on the flow graph.

Thank you in advance.