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[gitmo/Moose.git] / lib / Moose / Cookbook / Snack / BUILD.pod

=pod

=head1 NAME

Moose::Cookbook::Snack::BUILD - Custom initialization methods for Moose objects

=head1 SYNOPSIS

    package Build::Demo;
    use Moose; 

    # once the object has been created, don't allow
    # changes to 'example_file'
    has 'example_file' => (is => 'ro', required => 1);

    sub BUILD {
        my $self = shift;
        # create the object only if the 'example_file' exists
        if (-e $self->example_file) {
            return $self;
        } 
        else {
            die('ERROR: file _' . $self->example_file . '_ does not exist');
        } 
    }

    package main;
    use Moose;
    
    # the name of this script, which works
    my $first_test = Build::Demo->new(example_file => $0); 
    # this should fail (unless there's a file named 'foo'
    # in the current directory)
    my $second_test = Build::Demo->new(example_file => 'foo');

=head1 DESCRIPTION

The C<BUILD()> method allows you to write your own initialization methods for
your Moose objects. 

=head2 Creating new objects in Perl and Moose

By convention, most objects in Perl are created by calling a C<new()> method
inside that object:

    package My::Perl::Class;

    sub new {
        # object blessing and initialization code goes here...
    } 

    package main;
    my $object = My::Perl::Class->new();

Moose is no different in this respect.  However, since Moose handles the
C<new()> method for you, how do you change the default behaivor of the
C<new()> method in Moose?  This is what the C<BUILD()> method was designed
for.

    package My::Moose::Class;

    sub BUILD {
        # object initialization code goes here...
    }

    package main;
    my $object = My::Moose::Class->new();

=head2 Why would you want a custom constructor?

If your object needs to verify some behaivor or internal state before it is
created, a good time to do that is when the object is being created.  Why
waste resources (CPU, memory) on objects that won't work because of missing
resources?

=head2 When would you use an Moose type constraint instead of a custom constructor?

Using type constraints via L<Moose::Util::TypeConstraints>, you can verify
simple relationships when an object is created:

    package Triangle;
    use Moose;

    has 

You would want to use the C<BUILD()> method in order to verify more complex
relationships:

    package IsoscelesTriangle;
    use Moose;

=head2 BUILD method is run only if it is defined in the object

If your object does not have a C<BUILD> method, then Moose will skip trying to
run it.

=head2 What is 'BUILDALL'?

(Taken from L<Moose::Object>)  The C<BUILDALL> method will call every BUILD
method in the inheritance hierarchy, and pass it a hash-ref of the the 
C<%params> passed to the C<new()> method.

=head1 SEE ALSO

=over 4

=item L<Moose::Object> - The base object for Moose (BUILDALL) 

=item L<Moose::Cookbook::FAQ> - Frequently asked questions about Moose 
(How do I write custom constructors with Moose?)

=item L<Moose::Cookbook::Recipe4> - Subtypes, and modeling a simple 
Company class heirarchy (Example usage of BUILD in action)

=item L<Moose::Cookbook::WTF> - For when things go wrong with Moose 
('Roles' section describes BUILD/BUILDALL)

=back

=head1 AUTHOR

Brian Manning <elspicyjack at gmail dot com>

=head1 COPYRIGHT AND LICENSE

Copyright 2006-2008 by Infinity Interactive, Inc.

L<http://www.iinteractive.com>

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

=cut