In gen_event, what functional diff does it makes for gen_event:notify & gen_event:call

Ulf Wiger <>
Fri Feb 21 10:55:13 CET 2003


On Fri, 21 Feb 2003, Suresh S wrote:

>Hi Ulf,
>
>> The {Module,Id} construct is for the case that you
>> have lots
>> of instances of similar handlers, and it's not
>> practical, or
>> even desired, to implement a module for each
>> instance. Id
>> can be anything that helps you tell the different
>> handlers
>> apart in your application.
>
>I have one question left,
>If i have 'N' events in a callback module to be
>handled, i no need to have 'N' handlers ,
>
>i can manage all the events with one handler,
>if it is the case,

Yes, just like you can handle several different calls with
a gen_server, you can handle several different events in one
callback module. You simply do pattern matching in the
handle_event/2 function. Again, to exemplify with the EVA
application's eva_snmp_adaptation.erl:

handle_event({send_event, Event}, S) ->
    ...;
handle_event({send_alarm, Alarm}, S) ->
    ...;
handle_event({unregister_event, Name}, S) ->
    ...;
handle_event({unregister_alarm, Name}, S) ->
    ...;
handle_event(_, S) ->
    {ok, S}.

In this case, the same module handles 4 specific events, and
ignores all other events.

Multiple handlers would be used if you want to do several
unrelated things on the same event.


/Uffe
-- 
Ulf Wiger, Senior Specialist,
   / / /   Architecture & Design of Carrier-Class Software
  / / /    Strategic Product & System Management
 / / /     Ericsson AB, Connectivity and Control Nodes




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