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    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);

          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 run_trigger.

        If add_trigger is called with named_parameters and the "abortable"
        parameter is passed a true value, a false return values will stop
        processing of this trigger point. If the trigger is "abortable"
        return the return value from the last callback processed will be
        returned to calling code.

        If "add_trigger" is called without the "abortable" flag, 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;

          $foo->call_trigger($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.

    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

    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.

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

    Code by Tatsuhiko Miyagawa <>.

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