Virtual interface in UVM

HI , 

I am wondering who is calling assign_vi virtual function while connecting interface of different members under Agent?

// Assign the virtual interfaces of the agent's children

  function void assign_vi(virtual interface ubus_if vif);

    monitor.vif = vif;

    if (is_active == UVM_ACTIVE) 

      driver.vif = vif;

  endfunction : assign_vi

thanks

joys