Author image Joel Berger
and 1 contributors


Mojo::IOLoop::Delay - (DISCOURAGED) Promises/A+ and flow-control helpers


  use Mojo::IOLoop::Delay;

  # Synchronize multiple non-blocking operations
  my $delay = Mojo::IOLoop::Delay->new;
  $delay->steps(sub { say 'BOOM!' });
  for my $i (1 .. 10) {
    my $end = $delay->begin;
    Mojo::IOLoop->timer($i => sub {
      say 10 - $i;

  # Sequentialize multiple non-blocking operations

    # First step (simple timer)
    sub ($delay) {
      Mojo::IOLoop->timer(2 => $delay->begin);
      say 'Second step in 2 seconds.';

    # Second step (concurrent timers)
    sub ($delay, @args) {
      Mojo::IOLoop->timer(1 => $delay->begin);
      Mojo::IOLoop->timer(3 => $delay->begin);
      say 'Third step in 3 seconds.';

    # Third step (the end)
    sub ($delay, @args) {
      say 'And done after 5 seconds total.';


Mojo::IOLoop::Delay adds flow-control helpers to Mojo::Promise, which can help you avoid deep nested closures that often result from continuation-passing style.

  use Mojo::IOLoop;

  # These deep nested closures are often referred to as "Callback Hell"
  Mojo::IOLoop->timer(3 => sub ($loop) {

    say '3 seconds';
    Mojo::IOLoop->timer(3 => sub ($loop) {

      say '6 seconds';
      Mojo::IOLoop->timer(3 => sub ($loop) {

        say '9 seconds';


The idea behind Mojo::IOLoop::Delay is to turn the nested closures above into a flat series of closures. In the example below, the call to "begin" creates a code reference that we can pass to "timer" in Mojo::IOLoop as a callback, and that leads to the next closure in the series when executed.

  use Mojo::IOLoop;
  use Mojo::IOLoop::Delay; # adds Mojo::IOLoop->delay

  # Instead of nested closures we now have a simple chain of steps
  my $delay = Mojo::IOLoop->delay(
    sub ($delay) { Mojo::IOLoop->timer(3 => $delay->begin) },
    sub ($delay) {
      say '3 seconds';
      Mojo::IOLoop->timer(3 => $delay->begin);
    sub ($delay) {
      say '6 seconds';
      Mojo::IOLoop->timer(3 => $delay->begin);
    sub ($delay) { say '9 seconds' }

Another positive side effect of this pattern is that we do not need to call "start" in Mojo::IOLoop and "stop" in Mojo::IOLoop manually, because we know exactly when our chain of "steps" has reached the end. So "wait" in Mojo::Promise can stop the event loop automatically if it had to be started at all in the first place.


This module has been extracted from Mojolicious and was removed from it at the 9.0 release. It is kept here for backwards compatibility purposes but there is no intention to maintain it further and it should be migrated away from as your earliest convenience.

Though there is no intention of removing it from CPAN in the future it should be treated as deprecated and the metadata will mark it as such. It will receive no no-security-related changes going forward.


As of Mojolicious 9.0, the package Mojo::IOLoop no longer provides a class constructor for delays. If you want to use Mojo::IOLoop->delay you must first load this class explicitly which will add it back. You can also use -MMojo::IOLoop::Delay at the command line to do so.


Mojo::IOLoop::Delay inherits all attributes from Mojo::Promise.


Mojo::IOLoop::Delay inherits all methods from Mojo::Promise and implements the following new ones.


  my $cb = $delay->begin;
  my $cb = $delay->begin($offset);
  my $cb = $delay->begin($offset, $len);

Indicate an active event by incrementing the event counter, the returned code reference can be used as a callback, and needs to be executed when the event has completed to decrement the event counter again. When all code references generated by this method have been executed and the event counter has reached zero, "steps" will continue.

  # Capture all arguments except for the first one (invocant)
  my $delay = Mojo::IOLoop->delay(sub ($delay, $err, $stream) { ... });
  Mojo::IOLoop->client({port => 3000} => $delay->begin);

Arguments passed to the returned code reference are spliced with the given offset and length, defaulting to an offset of 1 with no default length. The arguments are then combined in the same order "begin" was called, and passed together to the next step.

  # Capture all arguments
  my $delay = Mojo::IOLoop->delay(sub ($delay, $loop, $err, $stream) { ... });
  Mojo::IOLoop->client({port => 3000} => $delay->begin(0));

  # Capture only the second argument
  my $delay = Mojo::IOLoop->delay(sub ($delay, $err) { ... });
  Mojo::IOLoop->client({port => 3000} => $delay->begin(1, 1));

  # Capture and combine arguments
  my $delay = Mojo::IOLoop->delay(sub ($delay, $three_err, $three_stream, $four_err, $four_stream) { ... });
  Mojo::IOLoop->client({port => 3000} => $delay->begin);
  Mojo::IOLoop->client({port => 4000} => $delay->begin);


  $delay = $delay->pass;
  $delay = $delay->pass(@args);

Shortcut for passing values between "steps".

  # Longer version


  $delay = $delay->steps(sub {...}, sub {...});

Sequentialize multiple events, every time the event counter reaches zero a callback will run, the first one automatically runs during the next reactor tick unless it is delayed by incrementing the event counter. This chain will continue until there are no remaining callbacks, a callback does not increment the event counter or an exception gets thrown in a callback. Finishing the chain will also result in the promise being fulfilled, or if an exception got thrown it will be rejected.


Mojolicious, Mojolicious::Guides,


The "AUTHORS" in Mojolicious


While this module is no longer receiving non-security related maintenance, if you must contact someone about it please contact Joel Berger <> or as a last resort contact the Mojolicious Core Team.


Copyright (C) 2008-2021, Sebastian Riedel and others.

This program is free software, you can redistribute it and/or modify it under the terms of the Artistic License version 2.0.