The London Perl and Raku Workshop takes place on 26th Oct 2024. If your company depends on Perl, please consider sponsoring and/or attending.


Class::Trigger - Mixin to add / call inheritable triggers


  package Foo;
  use Class::Trigger;

  sub foo {
      my $self = shift;
      # some code ...
      # some code ...

  package main;
  Foo->add_trigger(before_foo => \&sub1);
  Foo->add_trigger(after_foo => \&sub2);

  my $foo = Foo->new;
  $foo->foo;                    # then sub1, sub2 called

  # triggers are inheritable
  package Bar;
  use base qw(Foo);

  Bar->add_trigger(before_foo => \&sub);

  # triggers can be object based
  $foo->add_trigger(after_foo => \&sub3);
  $foo->foo;                    # sub3 would appply only to this object


Class::Trigger is a mixin class to add / call triggers (or hooks) that get called at some points you specify.


By using this module, your class is capable of following two methods.

  Foo->add_trigger($triggerpoint => $sub);
  $foo->add_trigger($triggerpoint => $sub);

Adds triggers for trigger point. You can have any number of triggers for each point. Each coderef will be passed a the object reference, and return values will be ignored.

If add_trigger is called as object method, whole current trigger table will be copied onto the object and the new trigger added to that. (The object must be implemented as hash.)

  my $foo = Foo->new;

  # this trigger ($sub_foo) would apply only to $foo object
  $foo->add_trigger($triggerpoint => $sub_foo);

  # And not to another $bar object
  my $bar = Foo->new;

Any triggers added to the class after adding a trigger to an object will not be fired for the object because the object now has a private copy of the triggers.


Calls triggers for trigger point, which were added via add_trigger method. Each triggers will be passed a copy of the object. Triggers are invoked in the same order they were defined.


By default you can make any number of trigger points, but if you want to declare names of trigger points explicitly, you can do it via import.

  package Foo;
  use Class::Trigger qw(foo bar baz);

  package main;
  Foo->add_trigger(foo  => \&sub1); # okay
  Foo->add_trigger(hoge => \&sub2); # exception


Acknowledgement: Thanks to everyone at POOP mailing-list (


This module lets me add subs to be run before/after a specific subroutine is run. Yes?


You put various call_trigger() method in your class. Then your class users can call add_trigger() method to add subs to be run in points just you specify (exactly where you put call_trigger()).


Are you aware of the perl-aspects project and the Aspect module? Very similar to Class::Trigger by the look of it, but its not nearly as explicit. Its not necessary for foo() to actually say "triggers go *here*", you just add them.


Yep ;)

But the difference with Aspect would be that Class::Trigger is so simple that it's easy to learn, and doesn't require 5.6 or over.


How does this compare to Sub::Versive, or Hook::LexWrap?


Very similar. But the difference with Class::Trigger would be the explicitness of trigger points.

In addition, you can put hooks in any point, rather than pre or post of a method.


It looks interesting, but I just can't think of a practical example of its use...


(by Tony Bowden)

I originally added code like this to Class::DBI to cope with one particular case: auto-upkeep of full-text search indices.

So I added functionality in Class::DBI to be able to trigger an arbitary subroutine every time something happened - then it was a simple matter of setting up triggers on INSERT and UPDATE to reindex that row, and on DELETE to remove that index row.

See Class::DBI::mysql::FullTextSearch and its source code to see it in action.


Original idea by Tony Bowden <> in Class::DBI.

Code by Tatsuhiko Miyagawa <>.

Patches by Tim Buce <>.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.