[Petal] Reviving the Petal Cookbook

[William McKee]

After Warren's excellent tips, I went digging around in the archives to
find the Cookbook.pod that I started (way back in September 2002). I've
added his two tips and reattached it to this message. I welcome your
suggestions and/or additions. It's missing more basic examples of usage
but my intention was to preserve some of the less obvious tricks as I
learn them. Perhaps over time others will contribute and make it a more
complete cookbook. Is there any interest in including this with the
Petal distribution?


William

-- 
Knowmad Services Inc.
http://www.knowmad.com
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=head1 NAME

Petal::Cookbook - Recipes for building templates with Petal

=head1 SYNOPSIS

Not Applicable

=head1 DESCRIPTION

This document contains some examples of L<Petal|Petal> template usage. 


=head1 NOTES
I tend to use Petal to generate HTML files from HTML templates. I name these templates with the .tmpl extension to help distinguish templates from static html. Nonetheless, the templates should be in a non-web accessible directory if you are using Petal for developing web applications.


=head1 BASICS

=head2 Passing a hashreference to Petal::process
An effective way to collate data to send to the Petal process command is via a hash reference. Used as follows, this technique allows you to build up your data to be passed to the template slowly:

	my $hash = { object => $some_object, number => 3 };
	$hash->{'foo'} = "bar";
	my $template = new Petal ( 'test.tmpl' );
	my $html = $template->process($hash);
	# Output the results
	print "Content-type: text/html\n\n";
	print $html;


=head1 INTERMEDIATE TIPS

=head2 Assigning attributes (submitted by Warren Smith)

Up until now, if I wanted to use petal to pre-select an item in a selectbox, I
would have to do each item twice, like so:

  <select>
    <div petal:repeat="option options">
      <option petal:condition="true: option/selected" petal:attributes="value option/value" petal:content="option/label" selected="selected">Option 1</option>
      <option petal:condition="false: option/selected" petal:attributes="value option/value" petal:content="option/label">Option 2</option>
    </div>
  </select>

  $VAR1 = [
          { value => 1, label => 'Option 1', selected => 1 },
          { value => 2, label => 'Option 2', selected => 0 },
          { value => 4, label => 'Option 3', selected => 0 },
  ];


After reading the Petal source, I found that if you use petal:attributes
to assign an attribute an undefined value, the attribute gets omitted,
thus the above code can be replaced with the simpler version below:

  <select>
    <option petal:attributes="value option/value; selected option/selected" petal:content="option/label">Option 1</option>
  </select>

  $VAR1 = [
          { value => 1, label => 'Option 1', selected => 1 },
          { value => 2, label => 'Option 2' },
          { value => 4, label => 'Option 3' },
  ];

It turns out that although not documented in Petal's documentation, this
behavior is part of the TAL specification:

         http://www.zope.org/Wikis/DevSite/Projects/ZPT/TAL

Thanks to Warren Smith for this tip and Fergal Daly for his knowledge of the TAL specification.


=head2 Generating even/odd rows (submitted by Warren Smith)
I developed a decode: modifier that works similar to Oracle's decode
statement. It should be making it into the next release of Petal::Utils. So,
making even/odd rows of a table different classes, which allows you to do
things like alter color, font-size, etc, is relatively easy.

Example:

  <table>
    <tr class="decode: repeat/even 1 'even' 'odd'">...</tr>
  </table>


Thanks to Warren Smith for this tip.


=head1 ADVANCED

=head2 Invoking methods on objects
Petal supports the ability to call an object's methods if passed in to Petal::process via the %hash. Say you wish to check whether a particular record is contained in a recordset returned from an SQL query. Using OO-Perl techniques, you could use the following technique as described by Jean-Michel:

* all your records are hashrefs which come from some database
* you have a list of them to display

Let's say that the database table looks like this:

Raters (id, first_name, last_name, relation, phone, email)

You could bless each record into a package as is:

    use MyApplication::Record::Rater;
    my @records = complicated_query_somewhere_else();
    bless $_, "MyApplication::Record::Rater" for (@records);


Your module could look like that:

    package MyApplication::Record::Rater;
    use strict;
    use warnings;
    use CGI;
    use Carp;

    sub is_current_id
    {
        my $self = shift;
        my $cgi  = CGI->new;
        my $id = $cgi->param ('rater.id');
        return unless (defined $id and $id and $id =~ /^\d+$/);
        return $id == $self->{id};
    }

    1;

Then on top of your existing data, you have a method which you can call
from Petal, i.e.

    <span petal:condition="true:record/is_current_id">
      blah blah blah...
    </span>

This trick can also be used when you have foreign keys in database
fields.

<fictious_scenario>

For example, let's imagine that you have a column called
'friend_id'. It references another 'rater' which is supposed to be a
friend of that person.

You could defined the following subroutine:

    # give me the friend record for that person
    sub friend
    {
        my $self = shift;
        my $friend_id = $self->{friend_id};
        my $sql = 'select * from rater where id = ?';
        my $sth = $::DBH_CONNECTION->prepare_cached ($sql);
        $sth->execute ($friend_id);
        my $hash = $sth->fetchrow_hashref;
        return unless (defined $hash);

        bless $hash, "MyApplication::Record::Rater";
        return $hash;
    }

Then in your template, you could do:

  <?petal:if name="true:rater/friend"?>
  Your friend is:
    $encode:rater/friend/first_name
    $encode:rater/friend/last_name
  <?petal:end?>

</fictious_scenario>

Thanks to Jean-Michel Hiver for this tip.


			NEEDS TO BE TESTED
			NEEDS TO BE TESTED
=head2 Using CGI.pm to build forms
			NEEDS TO BE TESTED
			NEEDS TO BE TESTED
Be sure to call CGI with the -compile option as follows:
	use CGI(-compile [:all]);

Calling the HTML generating methods of CGI.pm from the Petal template provides
an extremely simple means to develop forms.  For example, the ususal ratnest of
loops used to populate a checkbox group can be replaced by the simple elegant
construct below.  You can put in a dummy checkbox to give the HTML designer
something to look at.

<span petal:replace="query/checkbox_group 'Choices' choices '' 'true'">
<input name="Choices" type="checkbox" value="test">Test</input> </span>

Thanks to Kurt Stephens for this tip.



=head1 COPYRIGHT

Copyright (c) 2002-2005 Knowmad Services Inc.  All rights reserved.

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

=head1 AUTHOR

 William McKee <william at knowmad.com>.

=head1 SEE ALSO

L<Petal|Petal>, L<Petal::Utils|Petal::Utils>

=cut