NAME

       "IO::Async::Timer::Countdown" - event callback after a fixed delay

SYNOPSIS

        use IO::Async::Timer::Countdown;

        use IO::Async::Loop;
        my $loop = IO::Async::Loop->new;

        my $timer = IO::Async::Timer::Countdown->new(
           delay => 10,

           on_expire => sub {
              print "Sorry, your time's up\n";
              $loop->stop;
           },
        );

        $timer->start;

        $loop->add( $timer );

        $loop->run;

DESCRIPTION

       This subclass of IO::Async::Timer implements one-shot fixed delays.  The object implements
       a countdown timer, which invokes its callback after the given period from when it was
       started. After it has expired the Timer may be started again, when it will wait the same
       period then invoke the callback again. A timer that is currently running may be stopped or
       reset.

       For a "Timer" object that repeatedly runs a callback at regular intervals, see instead
       IO::Async::Timer::Periodic. For a "Timer" that invokes its callback at a fixed time in the
       future, see IO::Async::Timer::Absolute.

EVENTS

       The following events are invoked, either using subclass methods or CODE references in
       parameters:

   on_expire
       Invoked when the timer expires.

PARAMETERS

       The following named parameters may be passed to "new" or "configure":

   on_expire => CODE
       CODE reference for the "on_expire" event.

   delay => NUM
       The delay in seconds after starting the timer until it expires. Cannot be changed if the
       timer is running. A timer with a zero delay expires "immediately".

   remove_on_expire => BOOL
       Optional. If true, remove this timer object from its parent notifier or containing loop
       when it expires. Defaults to false.

       Once constructed, the timer object will need to be added to the "Loop" before it will
       work. It will also need to be started by the "start" method.

METHODS

   is_expired
          $expired = $timer->is_expired

       Returns true if the Timer has already expired.

   reset
          $timer->reset

       If the timer is running, restart the countdown period from now. If the timer is not
       running, this method has no effect.

EXAMPLES

   Watchdog Timer
       Because the "reset" method restarts a running countdown timer back to its full period, it
       can be used to implement a watchdog timer. This is a timer which will not expire provided
       the method is called at least as often as it is configured. If the method fails to be
       called, the timer will eventually expire and run its callback.

       For example, to expire an accepted connection after 30 seconds of inactivity:

        ...

        on_accept => sub {
           my ( $newclient ) = @_;

           my $watchdog = IO::Async::Timer::Countdown->new(
              delay => 30,

              on_expire => sub {
                 my $self = shift;

                 my $stream = $self->parent;
                 $stream->close;
              },
           );

           my $stream = IO::Async::Stream->new(
              handle => $newclient,

              on_read => sub {
                 my ( $self, $buffref, $eof ) = @_;
                 $watchdog->reset;

                 ...
              },

              on_closed => sub {
                 $watchdog->stop;
              },
           ) );

           $stream->add_child( $watchdog );
           $watchdog->start;

           $loop->add( $watchdog );
        }

       Rather than setting up a lexical variable to store the Stream so that the Timer's
       "on_expire" closure can call "close" on it, the parent/child relationship between the two
       Notifier objects is used. At the time the Timer "on_expire" closure is invoked, it will
       have been added as a child notifier of the Stream; this means the Timer's "parent" method
       will return the Stream Notifier. This enables it to call "close" without needing to
       capture a lexical variable, which would create a cyclic reference.

   Fixed-Delay Repeating Timer
       The "on_expire" event fires a fixed delay after the "start" method has begun the
       countdown. The "start" method can be invoked again at some point during the "on_expire"
       handling code, to create a timer that invokes its code regularly a fixed delay after the
       previous invocation has finished. This creates an arrangement similar to an
       IO::Async::Timer::Periodic, except that it will wait until the previous invocation has
       indicated it is finished, before starting the countdown for the next call.

        my $timer = IO::Async::Timer::Countdown->new(
           delay => 60,

           on_expire => sub {
              my $self = shift;

              start_some_operation(
                 on_complete => sub { $self->start },
              );
           },
        );

        $timer->start;
        $loop->add( $timer );

       This example invokes the "start_some_operation" function 60 seconds after the previous
       iteration has indicated it has finished.

AUTHOR

       Paul Evans <leonerd@leonerd.org.uk>