The Template Toolkit

We'll present a brief introduction here by deconstructing some of the templates used by Maypole applications. For more deconstruction, see Standard Templates and Actions, which is an entire chapter dealing with the factory supplied templates.

Here's a template that could be called for the front page of the example beer database application, "custom/frontpage".

    [% INCLUDE header %]

    <h2> The beer database </h2>

    <TABLE BORDER="0" ALIGN="center" WIDTH="70%">
    [% FOR table = config.display_tables %]
    <TR>
    <TD>
    <A HREF="[%table%]/list">List by [%table %]</A>
    </TD>
    </TR>
    [% END %]
    </TABLE>

The first thing to note about this is that everything outside of the Template Toolkit tags ("[%" and "%]") is output verbatim. That is, somewhere in the output you're guaranteed to see

    <h2> The beer database </h2>

    <TABLE BORDER="0" ALIGN="center" WIDTH="70%">

Inside the tags, magic happens. The first piece of magic is the "[% INCLUDE header %]" directive. This goes away and finds a file called header - don't worry about how it finds that yet, we'll come to that later on - and processes the file's contents as though they were right there in the template. Our header file happens not to contain any "[% %]" tags, but if it did, they would be processed in the same way as the ones in frontpage.

The next piece of magic is this line:

    [% FOR table = config.display_tables %]

We're seeing a lot of things here at once. "config" is where we should start looking. This is a template variable, which is what templates are all about - templating means getting data from somewhere outside and presenting it to the user in a useful way, and "config" is a prime example of data that we want to use. It's actually an object containing configuration parameters for this Maypole application, and one of the methods is "display_tables", which returns a list of the database tables that we're supposed to show. In the application, we probably said something like

    BeerDB->config->display_tables([qw[beer brewery pub style]]);

This stores the four values - "beer", "brewery", "pub" and "style" - in an array, which is placed in the config object using the accessor/mutator method "display_tables". Now we're getting them back again. Note that we're not going to show the handpump table.

The Template Toolkit's dot operator is a sort of do-the-right-thing operator; we can say "array.0" to get the first element of an array, "hash.key" to look up the "key" key in a hash, and "object.method" to call "method" on an object. So, for instance, if we said "config.display_tables.2", we'd look up the "display_tables" method in the configuration object and get our array back, then look up the 3rd element and get "pub". Thing is, you don't have to care whether "display_tables" is an object or a hash. You can pretend it's a hash if you want. The syntax is the same, and Template Toolkit knows the right thing to do.

The "FOR" loop will repeat the code four times, setting our new variable "table" to the appropriate array element. This code:

    [% FOR table = config.display_tables %]
        Hello [% table %]!
    [% END %]

will produce something like

    Hello beer!
    Hello brewery!
    Hello pub!
    Hello style!

In our case, though, we're printing out a table element linking to each database table in turn.

Here's a slightly more complicated example, adapted from factory/pager. This template is responsible for printing the little page menu at the bottom of a listing if there are more rows in the listing than we want on a single page.

    [% PROCESS macros %]
    <P ALIGN="center">Pages:
    [%
         FOREACH num = [pager.first_page .. pager.last_page];
              IF num == pager.current_page;
                "["; num; "] ";
              ELSE;
                SET args = "?page=" _ num;
                SET label = "[" _ num _ "]";
                link(classmetadata.table, "list", args, label);
              END;
         END;
    %]
    </P>

Maypole will be providing a whole bunch of variables to this template, and we'll look at them all in a moment, but the only ones we need to care about are "pager" and "classmetadata".

We start by loading in a bunch of macros. Macros are Template Toolkit's functions - you can provide them some parameters and they'll run a little sub-template based on them. The "macros" file contains some handy macros that I've found useful for constructing Maypole templates; again, these will be covered in full detail in Standard Templates and Actions.

We're going to be displaying something like this:

    Pages: [1] [2] [3] [4]

with most of those numbers being a link to the appropriate page. This mean we're going to have to have a list of numbers, and the "FOREACH" loop provides this: ("FOREACH" and "FOR" are identical, just like in Perl.)

         FOREACH num = [pager.first_page .. pager.last_page];

Here we're manually constructing an array of numbers, using the range operator ("..") to fill in all the numbers from the "first_page" (1) to the "last_page" (4). The same dot operator is used to ask the "pager" object what its "first_page" and "last_page" are.

Now we're going to be executing this loop four times, once each for "num" being set to 1, 2, 3, and 4. At some point, we'll come across the page that we're actually on right now:

      IF num == pager.current_page;

and in that case, we don't want to produce a link to it. We just want to output it as text, surrounded by square brackets:

                "["; num; "] ";

We're using string literals to output the brackets. We don't have to do that. We could say it this way:

    [% ...
      IF num == pager.current_page;
    %]
        [ [% num %] ] 
    [% ELSE %]
       ...
    [% END %]

But you know, I quite like it my way.

Now if the number we're printing isn't the number of the current page, we want to make a link. Here's how we do it:

    SET args = "?page=" _ num;
    SET label = "[" _ num _ "]";
    link(classmetadata.table, "list", args, label);

"SET" declares a new variable of our own. If there was anything called "args" before, there isn't now. It's going to be the result of our statement ""?page=" _ num". "_" is the concatenation operator, and glues "?page=" onto the front of our number. So if we want to link to page 4, then the "args" variable will contain "?page=4". Similarly, the "label" variable will be "[4]".

Now we call a macro, "link" with these two variables and the value of "classmetadata.table". This macro takes four arguments, "table", "action", "args" and "label", and constructs a link of the form

    <A HREF="[% base %]/[% table %]/[% action %][% args %]">
    [% label %]
    </A>

In our case, it'll be filled in like so:

    <A HREF="[% base %]/[% classmetadata.table %]/list?page=4">
    [ 4 ]
    </A>

Where "classmetadata.table" will actually be the name of the current table, and "base" will be replaced by the appropriate URL for this application.

Locating Templates

When you configure a Maypole application, you can tell it the base directory of your templates like so:

    BeerDB->config->template_root("/var/www/beerdb/templates");

If you don't do this, most Maypole front-ends will use the current directory, which may be what you want anyway. Off this directory, Maypole will look for a set of subdirectories.

For instance, I said we were in the middle of processing the front page and looking up a template file called header. Maypole will first look for this file in the custom subdirectory. (say, /var/www/beerdb/templates/custom) If it doesn't find one, then it looks in the factory subdirectory. If it doesn't find one there, then it gives up and dies with an error. But that's your fault, since you've called for a template which doesn't exist. Don't do that.

This behaviour means that you can provide your own site-specific templates, but if you don't do so, then you get to use a generic one provided by Maypole. Maypole's "factory setting" templates are written in such a way as to try and do the right thing no matter what your application does. They are occasionally successful at this.

Now the front page was a pretty simple example, since Maypole only looks up two directories. In most cases, it checks an additional directory, and this directory depends entirely on what Maypole is doing.

If you're writing an e-commerce application, for example, you may well have a table which represents the product catalogue and all the products you can buy. Let's call this the "product" table. You'll also have a data source which is specific to the user which contains all the products that they're buying on this particular visit to the site. In time-honoured tradition, we'll call this the "basket" table.

Now it ought to be reasonably apparent that you don't want the basket to be displayed in exactly the same way as the product catalogue. The templates for "product/list" and "basket/list" need to be different. This is where the third directory comes in. The other directory, which Maypole checks very first of all, is specific to the table that you're viewing. So if you go to "http://your.shop.com/basket/list", Maypole will look in the basket directory first for a file called list, and second in the custom directory for a site-wide list template, and then fall-back to the factory directory for a generic list template. It should be obvious that you probably want to provide all of basket/list, basket/view, product/list, product/view and any other combination of classes and actions that you can think of.

What Maypole provides to a template

objects

The most important variable is called "objects", and is a list of all the objects that this page is going to deal with. For instance, if the URL is "http://localhost/beerdb/beer/view/23", then in the template /beer/view, "objects" will contain the "BeerDB::Beer" object for the 23rd item in the database, while for the /brewery/list template, the view will fill "objects" with all the breweries; or at least, all the breweries on the current page.

breweries!

This variable is so important that to help design templates with it, "Maypole::View::TT" provides a helpful alias to it depending on context. For instance, if you're writing your own /brewery/list template, the data in "objects" is also available in a template variable called "breweries". If you're working on /brewery/view, though, it's available in "brewery", since there's only one brewery to be displayed.

base

Additionally, you can get the base URL for the application from the "base" template variable; this allows you to construct links, as we saw earlier:

    <A HREF="[% base %]/brewery/edit/[% brewery.id %]">Edit this brewery</A>

config

You can also get at the rest of the configuration for the site with the "config" variable as we saw above.

request

The entire request object is made available in "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 "request.user".)

classmetadata

To allow the construction of the "generic" templates which live in factory, Maypole also passes in a hash called "classmetadata", which contains all sorts of useful information about the class under examination:

"table"

This is the name of the table that is represented by the class.

"name"

This is the Perl's idea of the class; you don't need this unless you're doing really tricky things.

"moniker"

This is a more human-readable version of the table name, that can be used for display. "brewery" for example.

"plural"

The same, but a correctly-formed plural. For instance, "breweries".

"columns"

The list of columns for display; see the hard way section in the Beer Database chapter.

"list_columns"

As for "columns", but these are the columns to be displayed on a list page.

"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".

"cgi"

This is a slightly trickier one. It is a hash mapping column names to a "HTML::Element" suitable for entering data into a new instance of that class. That is, for the "beer" table, "classmetadata.cgi.style" should be a "HTML::Element" object containing a drop-down list of beer styles.

"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 "beers", since calling "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 "style".

Additional variables and overrides

You can pass additional data to templates by creating new variables. You'd typically do this in your view class. Just add the name of your template variable as a key to the "template_args" hash in the request object, and supply its value:

  $r->template_args->{your_variable_name} = 'some_value';

You can also override the value of any of the standard variables by giving their name as the key.

Accessing other classes

When using "Maypole::View::TT" you are reccomended to use Richard Clamp's incredibly useful Template::Plugin::Class -- see the and Template::Plugin::Class and "Maypole::View::TT" documentation for details.

Mason and MasonX views also allow you to pull in arbitary classes, see the relevent Mason and Plugin/View documentation for details.

If you are using HTML::Template you are out of luck on this front due to philosophy and architecture this templating system cannot call code, and only reads the data provided when the template is processed.

Other view classes

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 "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 Maypole::Application;
   ...
   BeerDB->setup("dbi:SQLite:t/beerdb.db");
   BeerDB->config->uri_base(http://localhost/beerdb/");
   BeerDB->config->rows_per_page(10);
   BeerDB->config->view("Maypole::View::Mason");

Where do these alternate view classes come from? Gentle reader, they come from you.

Building your own view class

Imagine you've found a brand new templating system that's much better than the Template Toolkit. I know I'm stretching your imagination a bit here, but try. You'd like to use it with Maypole, which means writing your own view class. How is it done?

We'll demonstrate by implementing a view class for HTML::Mason, although no value judgement is implied. "HTML::Mason" is a templating system which embeds pure Perl code inside its magic tags. The good side of this is that it can get into hash references and objects, and so providing "classmetadata", "config" and the Maypole request object will work out just fine. The down side is that "HTML::Mason" is used to 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 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 "vars" which returns a hash of all the template variables described above, so it would be good to feed those into "HTML::Mason". It also provides a "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 Maypole::View::Base for more about them.

The module will do the right thing for us if we agree to provide a method called "template". This is responsible for taking the Maypole request object $r (of which more later) and putting the appropriate output either into "$r->output" or "$r->error", depending, of course, whether things are OK or whether we got an error.

Thankfully, "HTML::Mason" makes things really easy for us. We can use multiple template roots, so we can use the "paths" method; we can pass in a hash full of interesting data structures, so we can use the "vars" method too. In fact, we have to do very little to make "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 "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 "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 $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!

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