Callback in UVM !!!

Callbacks are empty virtual methods that are embedded in the user components at strategic  points to allow the user to make customization which allows better reuse.

In UVM this can be achieved in two ways. 

1. Simply add  empty virtual methods in the component  say in a driver  and invoke the virtual methods at appropriate locations , test writer or  the user extends the driver class implements the virtual methods and uses  set instance override to accomplish the functionality.
2. Create a class which extends form uvm_callback class and it  implements the virtual methods.  Use `uvm_do_callbacks() to place the virtual methods at strategic points in the component say a driver. Now the test writer or user extends the callback class and implements the virtual methods and registers or associate the extended callback class with the instance of the component say the driver using add to accomplish the functionality.