| Maypole::Manual::View - Maypole View Classes |
Maypole::Manual::View - Maypole View Classes
In a large application, you will almost certainly want to customize the layout and design of the output pages. This task may even be the purview of a separate team of HTML designers rather than the programmers. Since a typical programmer will try to avoid touching HTML as much as possible and a typical designer will try to avoid touching Perl code, programmers have evolved a system of templating to separate the concerns of programming and designing.
One of the core concepts in Maypole is the view class, and this is responsible for routing the data produced in the model class into the templates produced by the designers. Of course, there are a great many possible templating systems and styles, and so there can be a great many possible Maypole view classes. Each view class will take the data from the controller, locate a template to be processed, and hand the whole lot to its preferred templating module, which will then do the hard work of filling in the template and coming up with the output.
You can choose whatever Maypole view class you want, but the default view class is the Maypole::View::TT manpage, and it feeds its data and templates to a module called the Template Toolkit.
The Template Toolkit, written by Andy Wardley, is a very powerful and
generic templating system. It provides its own little formatting language
which supports loops, conditionals, hash and array dereferences and
method calls, macro processing and a plug-in system to connect it to
external Perl modules.
Its homepage is http://www.template-toolkit.org/.
There are several good introductions to the
Template Toolkit available: you should have one installed as
the Template::Tutorial::Datafile manpage; there's one at
http://www.perl.com/pub/a/2003/07/15/nocode.html, and of course
there's the ``Badger Book'' - The Perl Template Toolkit, by Andy et al.
http://www.oreilly.com/catalog/perltt/index.html
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.
Another feature of 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 did say I'd tell you about that later.) Template Toolkit
allows whatever uses it to provide a path for template files to be
located in. Maypole::View::TT feeds it up to three possible
directories to look things up in, and it will try to find a template in
each of these in turn.
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.
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 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.
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.
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>
You can also get at the rest of the configuration for the site with the
config variable as we saw above.
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.)
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:
tablenamemonikerpluralcolumnslist_columnscolumns, but these are the columns to be displayed on a
list page.
colnamescgiHTML::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_accessorsbeers, 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.
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.
When building a frontpage, login or other template that isn't directly linked to a particular table, (and therefore it's class,) that you wish to use, you can access the classes directly.
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.
Please note that these template variables, config, classmetadata,
objects and its user-friendly alias, as well as the rest of them are
a function of one particular view class, the default
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 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.
You should probably skip this section for the first few readings of this manual. It's only intended for people extending Maypole.
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 the HTML::Mason manpage,
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 the Maypole::View::Base manpage. 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 the Maypole::View::Base manpage 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!
Contents, Next Standard Templates and Actions, Previous Maypole Model Classes,
| Maypole::Manual::View - Maypole View Classes |