From f3b1f00e41ae64fabb974d3ac073298f936457be Mon Sep 17 00:00:00 2001
From: Simon Cozens <simon@simon-cozens.org>
Date: Thu, 1 Apr 2004 16:19:26 +0000
Subject: [PATCH] Most of the view documentation.

git-svn-id: http://svn.maypole.perl.org/Maypole/trunk@103 48953598-375a-da11-a14b-00016c27c3ee
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 =head1 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 I<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 C<Maypole::View::TT>, and it feeds its data and templates
+to a module called the Template Toolkit.
+
 =head2 The Template Toolkit
 
-=head2 Maypole::View::TT
+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. There are several good introductions to the
+Template Toolkit available: you should have one installed as
+L<Template::Tutorial::Datafile>; there's one at
+L<http://www.perl.com/pub/a/2003/07/15/nocode.html>, and of course
+there's the "Badger Book" - I<The Perl Template Toolkit>, by Andy et al.
+
+We'll present a brief introduction here by deconstructing some of the 
+templates used by Maypole applications. For more deconstruction, see
+L<StandardTemplates.pod>, which is an entire chapter dealing with the
+factory supplied templates.
+
+Here's the template which is called for the front page of the standard
+beer database application, C<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 (C<[%> and C<%]>) is output verbatim. That is,
+you're guaranteed to see 
+
+    <h2> The beer database </h2>
+
+    <TABLE BORDER="0" ALIGN="center" WIDTH="70%">
+
+in the output somewhere. Inside the tags, magic happens. The first piece
+of magic is the C<[% INCLUDE header %]> directive. This goes away and
+finds a file called F<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 F<header> file happens
+not to contain any C<[% %]> tags, but if it did, they would be processed
+in the same way as the ones in F<frontpage>. 
+
+The next piece of magic is this line:
+
+    [% FOR table = config.display_tables %]
+
+We're seeing a lot of things here at once. C<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 the C<config> hash is a
+prime example of data that we want to use. It's actually the hash of
+configuration parameters for this Maypole application, and one of the
+keys in that hash is C<display_tables>, the database tables that we're
+allowed to play with. In the application, we probably said something
+like
+
+    BeerDB->config->{display_tables} = [qw[beer brewery pub style]];
+
+This stores the four values - C<beer>, C<brewery>, C<pub> and C<style> -
+in an array, which is placed in the config hash under the key
+C<display_tables>. Now we're getting them back again.
+
+The Template Toolkit's dot operator is a sort of do-the-right-thing
+operator; we can say C<array.0> to get the first element of an array,
+C<hash.key> to look up the C<key> key in a hash, and C<object.method> to
+call C<method> on an object. So, for instance, if we said
+C<config.display_tables.2>, we'd look up the C<display_tables> key in
+the configuration hash and get our array back, then look up the 2nd
+element and get C<pub>.
+
+The C<FOR> loop will repeat the code four times, setting our new
+variable C<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 F<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.moniker, "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 C<pager> and C<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 C<macros> file contains some handy macros
+that I've found useful for constructing Maypole templates; again, these
+will be covered in full detail in L<StandardTemplates.pod>.
+
+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 C<FOREACH> loop
+provides this: (C<FOREACH> and C<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 (C<..>) to fill in all the numbers from the C<first_page> (1)
+to the C<last_page> (4). The same dot operator is used to ask the C<pager>
+what its C<first_page> and C<last_page> is. Remember when we said 
+C<config.display_tables>, we were looking up the C<display_tables> key
+in the C<config> hash? Well, this time we're not looking anything up in
+a hash. C<pager> is an object, and C<first_page> is a method. Thing is,
+you don't have to care. You can pretend it's a hash if you want. The
+syntax is the same, and Template Toolkit knows the right thing to do.
+
+Now we're going to be executing this loop four times, once each for C<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;
 
-=head2 What Maypole provides to a view
+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);
+
+C<SET> declares a new variable of our own. If there was anything called
+C<args> before, there isn't now. It's going to be the result of our
+statement C<"?page=" _ num>. C<_> is the concatenation operator, and
+glues C<?page=> onto the front of our number. So if we want to link to
+page 4, then the C<args> variable will contain C<?page=4>. Similarly,
+the C<label> variable will be C<[4]>.
+
+Now we call a macro, C<link> with these two variables and the value of
+C<classmetadata.table>. This macro takes four arguments, C<table>, 
+C<action>, C<args> and C<label>, and constructs a link of the form
+
+    <A HREF="[% config.base_url %]/[% table %]/[% action %][% args %]">
+    [% label %]
+    </A>
+
+In our case, it'll be filled in like so:
+
+    <A HREF="[% config.base_url %]/[% classmetadata.table %]/list?page=4">
+    [ 4 ]
+    </A>
+
+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?
+
+=head2 What Maypole provides to a template
+
+XXX
+
+=head2 Maypole::View::TT and 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 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
+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
+allows whatever uses it to provide a path for template files to be
+located in. C<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 is generally 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 F<header>. Maypole will first look
+for this file in the F<custom> subdirectory. (say,
+F</var/www/beerdb/templates/custom>) If it doesn't find one, then it
+looks in the F<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 C<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 C<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 C<product/list> and C<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 C<http://your.shop.com/basket/list>, Maypole
+will look in the F<basket> directory first for a file called F<list>,
+and second in the F<custom> directory for a site-wide list template,
+and then fall-back to the F<factory> directory for a generic list
+template. It should be obvious that you probably want to provide all
+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:
+
+   package BeerDB;
+   use base 'Apache::MVC';
+   ...
+   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 B<you>.
 
 =head2 Building your own view class
+
+I<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 B<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 C<HTML::Mason>,
+although no value judgement is implied. C<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 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
-- 
2.39.2