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Author image Marc A. Lehmann


AnyEvent - provide framework for multiple event loops

Event, Coro, Glib, Tk - various supported event loops


use AnyEvent;

   my $w = AnyEvent->io (fh => ..., poll => "[rw]+", cb => sub {
      my ($poll_got) = @_;

- only one io watcher per $fh and $poll type is allowed (i.e. on a socket you can have one r + one w or one rw watcher, not any more.

- AnyEvent will keep filehandles alive, so as long as the watcher exists, the filehandle exists.

   my $w = AnyEvent->timer (after => $seconds, cb => sub {

- io and time watchers get canceled whenever $w is destroyed, so keep a copy

- timers can only be used once and must be recreated for repeated operation

   my $w = AnyEvent->condvar; # kind of main loop replacement
   $w->wait; # enters main loop till $condvar gets ->broadcast
   $w->broadcast; # wake up current and all future wait's

- condvars are used to give blocking behaviour when neccessary. Create a condvar for any "request" or "event" your module might create, ->broadcast it when the event happens and provide a function that calls ->wait for it. See the examples below.


AnyEvent provides an identical interface to multiple event loops. This allows module authors to utilizy an event loop without forcing module users to use the same event loop (as only a single event loop can coexist peacefully at any one time).

The interface itself is vaguely similar but not identical to the Event module.

On the first call of any method, the module tries to detect the currently loaded event loop by probing wether any of the following modules is loaded: Coro::Event, Event, Glib, Tk. The first one found is used. If none is found, the module tries to load these modules in the order given. The first one that could be successfully loaded will be used. If still none could be found, it will issue an error.


The following program uses an io watcher to read data from stdin, a timer to display a message once per second, and a condvar to exit the program when the user enters quit:

   use AnyEvent;

   my $cv = AnyEvent->condvar;

   my $io_watcher = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
      warn "io event <$_[0]>\n";   # will always output <r>
      chomp (my $input = <STDIN>); # read a line
      warn "read: $input\n";       # output what has been read
      $cv->broadcast if $input =~ /^q/i; # quit program if /^q/i

   my $time_watcher; # can only be used once

   sub new_timer {
      $timer = AnyEvent->timer (after => 1, cb => sub {
         warn "timeout\n"; # print 'timeout' about every second
         &new_timer; # and restart the time

   new_timer; # create first timer

   $cv->wait; # wait until user enters /^q/i


Consider the Net::FCP module. It features (among others) the following API calls, which are to freenet what HTTP GET requests are to http:

   my $data = $fcp->client_get ($url); # blocks

   my $transaction = $fcp->txn_client_get ($url); # does not block
   $transaction->cb ( sub { ... } ); # set optional result callback
   my $data = $transaction->result; # possibly blocks

The client_get method works like LWP::Simple::get: it requests the given URL and waits till the data has arrived. It is defined to be:

   sub client_get { $_[0]->txn_client_get ($_[1])->result }

And in fact is automatically generated. This is the blocking API of Net::FCP, and it works as simple as in any other, similar, module.

More complicated is txn_client_get: It only creates a transaction (completion, result, ...) object and initiates the transaction.

   my $txn = bless { }, Net::FCP::Txn::;

It also creates a condition variable that is used to signal the completion of the request:

   $txn->{finished} = AnyAvent->condvar;

It then creates a socket in non-blocking mode.

   socket $txn->{fh}, ...;
   fcntl $txn->{fh}, F_SETFL, O_NONBLOCK;
   connect $txn->{fh}, ...
      and !$!{EWOULDBLOCK}
      and !$!{EINPROGRESS}
      and Carp::croak "unable to connect: $!\n";

Then it creates a write-watcher which gets called wehnever an error occurs or the connection succeeds:

   $txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'w', cb => sub { $txn->fh_ready_w });

And returns this transaction object. The fh_ready_w callback gets called as soon as the event loop detects that the socket is ready for writing.

The fh_ready_w method makes the socket blocking again, writes the request data and replaces the watcher by a read watcher (waiting for reply data). The actual code is more complicated, but that doesn't matter for this example:

   fcntl $txn->{fh}, F_SETFL, 0;
   syswrite $txn->{fh}, $txn->{request}
      or die "connection or write error";
   $txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'r', cb => sub { $txn->fh_ready_r });

Again, fh_ready_r waits till all data has arrived, and then stores the result and signals any possible waiters that the request ahs finished:

   sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};

   if (end-of-file or data complete) {
     $txn->{result} = $txn->{buf};

The result method, finally, just waits for the finished signal (if the request was already finished, it doesn't wait, of course, and returns the data:

   return $txn->{buf};

The actual code goes further and collects all errors (dies, exceptions) that occured during request processing. The result method detects wether an exception as thrown (it is stored inside the $txn object) and just throws the exception, which means connection errors and other problems get reported tot he code that tries to use the result, not in a random callback.

All of this enables the following usage styles:

1. Blocking:

   my $data = $fcp->client_get ($url);

2. Blocking, but parallelizing:

   my @datas = map $_->result,
                  map $fcp->txn_client_get ($_),

Both blocking examples work without the module user having to know anything about events.

3a. Event-based in a main program, using any support Event module:

   use Event;

   $fcp->txn_client_get ($url)->cb (sub {
      my $txn = shift;
      my $data = $txn->result;


3b. The module user could use AnyEvent, too:

   use AnyEvent;

   my $quit = AnyEvent->condvar;

   $fcp->txn_client_get ($url)->cb (sub {



Event modules: Coro::Event, Coro, Event, Glib::Event, Glib.

Implementations: AnyEvent::Impl::Coro, AnyEvent::Impl::Event, AnyEvent::Impl::Glib, AnyEvent::Impl::Tk.

Nontrivial usage example: Net::FCP.