Erlang gen_event don't work

Question

I try to write simple gen_event applcation in erlang.

My code:

-module(test).
-behavior(gen_event).

-export([notify/0]).

%% API
-export([start_link/0, add_handler/0, stop/0]).

%% gen_event callbacks
-export([init/1, handle_event/2, handle_call/2, 
     handle_info/2, terminate/2, code_change/3]).

-define(SERVER, ?MODULE). 

start_link() ->
    gen_event:start_link({local, ?SERVER}).

add_handler() ->
    gen_event:add_handler(?SERVER, ?MODULE, []).

stop() ->
    gen_event:stop(?MODULE).

init([]) ->
    %add_handler(),
    {ok, null}.

handle_event(_Event, State) ->
    {ok, State};
handle_event({test}, State) ->
    io:format("Test"),
    {ok, State}.

handle_call(_Request, State) ->
    Reply = ok,
    {ok, Reply, State}.

handle_info(_Info, State) ->
    {ok, State}.

terminate(_Reason, _State) ->
    ok.

code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

notify() ->
    gen_event:notify(?SERVER, {test}).

When i try notify i expect that function handle_event({test}, State) execute but nothing occurs. Why? What's wrong?

And where i can see simple example of gen_event?

Thank you.

If you just compile that source, the compiler points out the error quite sharply: "./test.erl:30: Warning: this clause cannot match because a previous clause at line 28 always matches" — Samuel Rivas, Jan 27 '11 at 22:14

score 2 · Accepted Answer · answered Jan 27 '11 at 18:13

Well, you have reversed your handle_event clauses. The first one will always match, so you won't reach the {test} clause.

As far as I know, the best online documentation is http://www.erlang.org/doc/design_principles/events.html. But I can recommend e.g. the book Erlang and OTP in action http://www.amazon.com/Erlang-OTP-Action-Martin-Logan/dp/1933988789

score 1 · Answer 2 · answered Jan 28 '11 at 13:11

Apart from swapping the clauses as @E Dominique mentioned have you actually added a handler using test:add_handler()? The call test:start_link() just starts the event manager, you need to tell it what to do by adding handlers. This even if the handler code is in the same module as the manager. This is generally not good practice even if alarm_handler which is part of sasl does this.

Doing this your code works writing "Test" when it gets an event.

Erlang gen_event don't work

2 Answers2