Toby Inkster

NAME

Sub::Trigger::Lock - a coderef for use in Moose triggers that will lock hashrefs and arrayrefs

SYNOPSIS

This module provides the antidote for:

   package Foo {
      use Moose;
      
      has bar => (is => 'ro', isa => 'ArrayRef');
   }
   
   my $foo = Foo->new( bar => [1,2,3] );
   push @{ $foo->bar }, 4;   # does not die!

All you need to do is:

   package Foo {
      use Moose;
      use Sub::Trigger::Lock;
      
      has bar => (is => 'ro', isa => 'ArrayRef', trigger => Lock);
   }

Or, a shortcut:

   package Foo {
      use Moose;
      use Sub::Trigger::Lock qw(RO);
      
      has bar => (is => RO, isa => 'ArrayRef');
   }

TL;DR

Force modifications of your arrayref/hashref attributes to be made via your documented API.

DESCRIPTION

This module provides two constants, Lock and RO. The first of these is the only one exported by default, and the key to understanding this module. This module also provides the utility functions lock and unlock, which are not exported by default.

Lock

Lock is a constant which evaluates to a coderef. The coderef takes two or more arguments. That is, Lock itself takes no arguments; it returns a coderef that takes arguments!

The first argument is supposed to be a blessed object, but it is actually completely ignored.

If the second argument is not a hashref or arrayref, it is also ignored. Everything is ignored! But if the second argument is a hashref or arrayref, it will be flagged as read-only.

This is a fairly shallow read-only flag. Attempts to add or remove keys from the hash, or change the value for any key will throw an exception. But if the value is reference to some other structure, that structure will be unaffected.

Similarly, attempts to push, pop, shift, or unshift a read-only array, or to change the value for any index will throw an exception. Buf if the values are references to other structures, these will also be unaffected.

Overall, the effect of Lock is that you can do something like this:

   package Person {
      use Moose;
      
      has name => (is => 'ro', writer => 'set_name');
   }
   
   package Band {
      use Moose;
      use Sub::Trigger::Lock;
      
      has members => (is => 'ro', trigger => Lock);
   }
   
   my $spice_girls = Band->new(
      members => [
         Person->new(name => 'Victoria Adams'),
         Person->new(name => 'Melanie Brown'),
         Person->new(name => 'Emma Bunton'),
         Person->new(name => 'Melanie Chisholm'),
         Person->new(name => 'Geri Halliwell'),
      ],
   );
   
   # This is OK, because deep changes work
   $spice_girls->members->[0]->set_name('Victoria Beckham');
   
   # This is not OK, because shallow changes throw!
   $spice_girls->members->[0] = Person->new(name => 'Johnny Cash');
RO

RO is a constant that evaluates to the list:

   'ro', 'trigger', Lock,
lock($ref)

A utility function for locking an arrayref or hashref in the same way that the Lock coderef would.

unlock($ref)

A utility function for unlocking an arrayref or hashref.

Note that this returns a guard object. You should store this object in a variable. Once the object is destroyed (e.g. because the variable has gone out of scope), $ref will be automatically locked again!

This allows you to temporarily unlock a hashref or arrayref in order to privately manipulate it:

   package Band {
      use Moose;
      use Sub::Trigger::Lock qw( Lock unlock );
      
      has members => (is => 'ro', trigger => Lock);
      
      sub add_members {
         my ($self, @members) = @_;
         my $guard = unlock( $self->members );
         push @{$self->members}, @members;
      }
   }

IMPLEMENTATION NOTES

This module uses the Perl internal Internals::SvREADONLY function for most of the heavy lifting. This is much, much faster than ties.

BUGS

Please report any bugs to http://rt.cpan.org/Dist/Display.html?Queue=Sub-Trigger-Lock.

SEE ALSO

Exporter::Tiny, Scope::Guard.

AUTHOR

Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

This software is copyright (c) 2014 by Toby Inkster.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.