NAME

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

SYNOPSIS

      package Foo;
      use Class::Trigger;
    
      sub foo {
          my $self = shift;
          $self->call_trigger('before_foo');
          # some code ...
          $self->call_trigger('middle_of_foo');
          # some code ...
          $self->call_trigger('after_foo');
      }
    
      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

DESCRIPTION

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

METHODS

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

    add_trigger

        Foo->add_trigger($triggerpoint => $sub);
        $foo->add_trigger($triggerpoint => $sub);
      
      
        Foo->add_trigger( name => $triggerpoint,
                          callback => sub {return undef},
                          abortable => 1); 
      
        # no further triggers will be called. Undef will be returned.

      Adds triggers for trigger point. You can have any number of triggers
      for each point. Each coderef will be passed a reference to the
      calling object, as well as arguments passed in via call_trigger.
      Return values will be captured in list context.

      If add_trigger is called with named parameters and the abortable
      parameter is passed a true value, a false return value from trigger
      code will stop processing of this trigger point and return a false
      value to the calling code.

      If add_trigger is called without the abortable flag, return values
      will be captured by call_trigger, but failures 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);
        $foo->foo;
      
        # And not to another $bar object
        my $bar = Foo->new;
        $bar->foo;

    call_trigger

        $foo->call_trigger($triggerpoint, @args);

      Calls triggers for trigger point, which were added via add_trigger
      method. Each triggers will be passed a copy of the object as the
      first argument. Remaining arguments passed to call_trigger will be
      passed on to each trigger. Triggers are invoked in the same order
      they were defined.

      If there are no abortable triggers or no abortable trigger point
      returns a false value, call_trigger will return the number of
      triggers processed.

      If an abortable trigger returns a false value, call trigger will stop
      execution of the trigger point and return undef.

    last_trigger_results

          my @results = @{ $foo->last_trigger_results };

      Returns a reference to an array of the return values of all triggers
      called for the last trigger point. Results are ordered in the same
      order the triggers were run.

TRIGGER POINTS

    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

FAQ

    Acknowledgement: Thanks to everyone at POOP mailing-list
    (http://poop.sourceforge.net/).

    Q.

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

    A.

      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()).

    Q.

      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.

    A.

      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.

    Q.

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

    A.

      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.

    Q.

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

    A.

      (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.

AUTHORS

    Original idea by Tony Bowden <tony@kasei.com> in Class::DBI.

    Code by Tatsuhiko Miyagawa <miyagawa@bulknews.net>.

    Jesse Vincent added a code to get return values from triggers and
    abortable flag.

LICENSE

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

SEE ALSO

    Class::DBI