Detecting access of a particular OID, handler functions

When we create an MIB and load it, say by agentx support then is there any
way to find out when any OID created by  us is being accessed and also to
dynamically change the value of that OID depending on some condition. The
example at net-snmp site says that while registering the instance you can
specify a valid handler function instead of NULL.
netsnmp_register_int_instance("nstAgentSubagentObject",
                                  nstAgentSubagentObject_oid,
                                  OID_LENGTH(nstAgentSubagentObject_oid),
                                  &nstAgentSubagentObject, NULL);
What i cannot understand is that how do we declare this handler function,
how is it called and what are the tasks it is supposed to perform. Does the
handler function get called whenever somebody queries for that OID.

regards
Devvrat

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