This is one possible solution to exercise 5-5 in the book Erlang Programming by F. Cesarini and S. Thompson (O'Reilly, 2009). The exercise reads
Complete the coding of the phone FSM example, and then instrument it with logging using an event handler process. This should record enough information to enable billing for the use of the phone.
To the existing four states of the phone FSM (idle, ringing, dial, and connected), two more are added, namely, waiting (for other party to pick up our call) and disconnected (when other party hangs up while we are connected). A phone ringer FSM is also implemented that generates both the "ringing" and "dial tone" effects. The generic event manager and log handler codes from Chapter 5 required slight modifications before they would compile, and the log handler file mode was changed from write to append (supplied in a list).
The phone FSM includes client code for testing in the shell, which was done successfully with Erlang R16B (erts-5.10.1). The following is a transcript of a typical session.
1> phone_fsm:start("phone.log").
ok
2> phone_fsm:incoming(1234). % incoming call
ok
ringing
ringing
ringing
ringing
ringing
ringing
ringing
ringing
ringing
3> phone_fsm:off_hook(). % answer incoming call
ok
4> phone_fsm:other_on_hook(1234). % caller hangs up
ok
dial tone
dial tone
dial tone
dial tone
dial tone
dial tone
dial tone
dial tone
5> phone_fsm:on_hook(). % put receiver back on the hook
ok
6> phone_fsm:off_hook(). % pick up receiver to make a call
ok
dial tone
dial tone
dial tone
dial tone
dial tone
dial tone
dial tone
dial tone
7> phone_fsm:outgoing(3456). % wait for other end to answer
ok
8> phone_fsm:other_off_hook(3456). % other end answers
ok
9> phone_fsm:on_hook(). % hang up
ok
10> phone_fsm:stop().
ok
This is the corresponding contents of the log file phone.log.
1373,746312,180149,no_billing,1234,incoming_call
1373,746360,359205,stop_billing,1234,other_on_hook
1373,746410,565402,start_billing,3456,outgoing_call
1373,746432,651613,stop_billing,3456,on_hook
The no_billing action is associated with incoming calls, while start_billing is associated with outgoing calls. stop_billing at the end of either type of call.