Where C<classmetadata.table> will actually be the name of the current
table, and C<config.base_url> will be replaced by the appropriate URL for
-this application. But where do these values, C<config> and C<classmetadata>,
-come from, and what else can we do with them?
+this application.
-=head2 What Maypole provides to a template
-
-XXX
-
-=head2 Maypole::View::TT and other view classes
+=head2 Locating Templates
-Please note that these template variables, C<config>, C<classmetadata>,
-C<objects> and its user-friendly alias, as well as the rest of themm are
-a function of one particular view class, the default
-C<Maypole::View::TT> class. Other view classes may need to present an
-entirely different set of template variables, since the default ones
-might not make sense. The templates may look wildly different in other
-view class implementations. But that's OK, because you couldn't
-necessarily use the same templates with a different templating system
-anyway.
-
-For instance, in really dumb templating languages which can't handle
-dereferencing hashes or arrays - no wait, that's most of them - passing
-in a hash reference like C<classmetadata> won't help you since you can't
-get at any of its elements. So you'll need to take a look at the
-documentation for the appropriate view class to see what template
-variables it provides.
-
-Another feture of C<Maypole::View::TT> which may not be present in
+Another feature of C<Maypole::View::TT> which may not be present in
alternate view class implementations - although they are strongly
encouraged to provide it - is the way that templates are located.
(Remember, I B<did> say I'd tell you about that later.) Template Toolkit
of F<basket/list>, F<basket/view>, F<product/list>, F<product/view>
and any other combination of classes and actions that you can think of.
-Again, this only goes for C<Maypole::View::TT>. If, for some perverse
-reason, the Template Toolkit just isn't good enough for you, then you
-can set your own view class while configuring your application:
+=head2 What Maypole provides to a template
+
+C<Maypole::View::TT> provides quite a variety of template variables to
+the template. As these are the building blocks of your pages, it's worth
+looking at precisely what variables are available.
+
+The most important variable is called C<objects>, and is a list of all
+the objects that this page is going to deal with. For instance,
+in the template F</beer/view>, C<objects> will contain the C<BeerDB::Beer>
+object for the 23rd item in the database, while F</brewery/list> will
+fill C<objects> will all the breweries; or at least, all the breweries
+on the current page.
+
+This variable is so important that to help design templates with it,
+C<Maypole::View::TT> provides a helpful alias to it depending on
+context. For instance, if you're writing your own F</brewery/list>
+template, the data in C<objects> is also available in a template
+variable called C<breweries>. If you're working on F</brewery/view>,
+though, it's available in C<brewery>, since there's only one brewery to
+be displayed.
+
+Additionally, you can get the base URL for the application from the
+C<base> template variable; this allows you to construct links, as we
+saw earlier:
+
+ <A HREF="[% base %]/brewery/edit/[% brewery.id %]">Edit this brewery</A>
+
+You can also get at the rest of the configuration for the site with the
+C<config> variable as we saw above, and the entire request object in
+C<request>, should you really need to poke at it. (I've only found this
+useful when working with authentication modules which stash a current user
+object in C<request.user>.)
+
+To allow the construction of the "generic" templates which live in
+F<factory>, Maypole also passes in a hash called C<classmetadata>,
+which contains all sorts of useful information about the class under
+examination:
+
+=over 3
+
+=item C<table>
+
+This is the name of the table that is represented by the class.
+
+=item C<class>
+
+This is the Perl's idea of the class; you don't need this unless you're
+doing really tricky things.
+
+=item C<moniker>
+
+This is a more human-readable version of the table name, that can be
+used for display.
+
+=item C<plural>
+
+The same, but a correctly-formed plural. For instance, "breweries".
+
+=item C<columns>
+
+The list of columns for display; see the section "Customizing Generic
+CRUD Applications" in L<StandardTemplates.pod>.
+
+=item C<colnames>
+
+This is a hash mapping the database's name for a column to a more
+human-readable name. Again, see "Customizing Generic CRUD Applications>.
+
+=item C<cgi>
+
+This is a slightly trickier one. It is a hash mapping column names to
+a C<HTML::Element> suitable for entering data into a new instance of
+that class. That is, for the C<beer> table, C<classmetadata.cgi.style>
+should be a C<HTML::Element> object containing a drop-down list of
+beer styles. This is explained in L<StandardTemplates.pod>.
+
+=item C<description>
+
+This is the human-readable description provided by a class.
+
+=item C<related_accessors>
+
+This is a list of accessors which can be called on an object to get
+lists of other things that this object "has". For instance, on a
+brewery, it would return C<beers>, since calling C<brewery.beers> would
+give you a list of beers produced by the brewery. Note that this only
+caters for accessors defining one-to-many relationships, not the
+ordinary one-to-one relationships, such as C<style>.
+
+=back
+
+=head2 Other view classes
+
+Please note that these template variables, C<config>, C<classmetadata>,
+C<objects> and its user-friendly alias, as well as the rest of them are
+a function of one particular view class, the default
+C<Maypole::View::TT> class. Other view classes may need to present an
+entirely different set of template variables, since the default ones
+might not make sense. The templates may look wildly different in other
+view class implementations. But that's OK, because you couldn't
+necessarily use the same templates with a different templating system
+anyway.
+
+For instance, in really dumb templating languages which can't handle
+dereferencing hashes or arrays - no wait, that's most of them - passing
+in a hash reference like C<classmetadata> won't help you since you can't
+get at any of its elements. So you'll need to take a look at the
+documentation for the appropriate view class to see what template
+variables it provides.
+
+So if, for some perverse reason, the Template Toolkit just isn't good
+enough for you, then you can set your own view class while configuring
+your application:
package BeerDB;
use base 'Apache::MVC';
of this is that it can get into hash references and objects, and so
providing C<classmetadata>, C<config> and the Maypole request object
will work out just fine. The down side is that C<HTML::Mason> is used to
-having all the template variables it wants already at its disposal
-through CGI parameters and the like, so we have to fiddle a bit to get
-these variables into our template.
-
-XXX
+running more or less standalone, and having all the template variables
+it wants already at its disposal through CGI parameters and the like, so
+we have to fiddle a bit to get these variables into our template.
+
+The key to building view classes is C<Maypole::View::Base>. This is the
+base class that you're going to inherit from and, to be honest, it does
+pretty much everything you need. It provides a method called C<vars>
+which returns a hash of all the template variables described above, so
+it would be good to feed those into C<HTML::Mason>. It also provides a
+C<paths> method which turns returns the full filesystem path of the
+three possible template paths as shown above. Again, it would be good to
+use this as our component paths if we can. It also has some methods we
+can override if we want to, but they're not massively important, so you
+can see L<Maypole::View::Base> for more about them.
+
+The module will do the right thing for us if we agree to provide a
+method called C<template>. This is responsible for taking the Maypole
+request object (of which more later) and putting the appropriate output
+either into C<$r-E<gt>{output}> or C<$r-E<gt>{error}>, depending, of
+course, whether things are OK or whether we got an error.
+
+Thankfully, C<HTML::Mason> makes things really easy for us. We B<can>
+use multiple template roots, so we can use the C<paths> method; we
+B<can> pass in a hash full of interesting data structures, so we can use
+the C<vars> method too. In fact, we have to do very little to make
+C<Maypole::View::Mason> work. Which is somewhat annoying, because it
+makes a boring example. But it means I can leave the fun ones to you!
+
+The doing-the-templating, in Mason and in any templating system, depends on
+three things: the paths that we're going to use to find our templates, the
+template name that we've been asked to fill out, and the set of variables that
+are going to be fed to the template. We'll assemble these for reference:
+
+ sub template {
+ my ($self, $r) = @_;
+ my @paths = $self->paths($r);
+ my $template = $r->template;
+ my %vars = $self->args($r);
+
+We'll also declare somewhere to temporarily store the output:
+
+ my $output;
+
+Now comes the part where we have to actually do something
+templating-language specific, so we open up our copy of "Embedding Perl
+in HTML with Mason" and find the bit where it talks about running Mason
+standalone. We find that the first thing we need to do is create a
+C<HTML::Mason::Interp> object which knows about the component roots.
+There's a slight subtlety in that the component roots have to be
+specified as an array of arrays, with each array being a two-element
+list of label and path, like so:
+
+ comproot => [
+ [ class => "/var/www/beerdb/templates/brewery" ],
+ [ custom => "/var/www/beerdb/templates/custom" ],
+ [ factory => "/var/www/beerdb/templates/factory" ],
+ ]
+
+We also find that we can set the output method here to capture Mason's
+output into a scalar, and also that we can tell Mason to generate
+sensible error messages itself, which saves us from having to worry
+about catching errors. At the end of all this, we come up with a
+constructor for our C<HTML::Mason::Interp> object which looks like this:
+
+ my $label = "path0";
+ my $mason = HTML::Mason::Interp->new(
+ comproot => [ map { [ $label++ => $_ ] } @paths ],
+ output_method => \$output,
+ error_mode => "output"
+ );
+
+The next thing we need to do is run the template with the appropriate
+template variables. This turns out to be really easy:
+
+ $mason->exec($template, %vars);
+
+Now we've got the data in C<$output>, we can put it into the request object,
+and return a true value to indicate that we processed everything OK. (If there
+was an error, then Mason will have produced some suitable output, so we can
+pretend that everything's OK anyway.)
+
+ $r->{output} = $output;
+ return 1;
+
+And that's all we need to do. Barely twenty lines of code for the finished
+product. Wasn't that easy? Don't you feel inspired to write Maypole view
+classes for your favourite templating language? Well, don't let me stop you!
+Patches are always welcome!
projects / maypole.git / blobdiff