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<h1 class="headline">Template::Manual::Views</h1>
<h2 class="subhead">Template Toolkit views (experimental)</h1>
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<div class="body">
<ul class="toc">
<li class=""><a href="#Overview">Overview</a></li>
<li class=""><a href="#Views_as_Template_Collectors_Providers">Views as Template Collectors/Providers</a></li>
<li class=""><a href="#Local_BLOCK_Definitions">Local BLOCK Definitions</a></li>
<li class=""><a href="#Preserving_Variable_State_within_Views">Preserving Variable State within Views</a></li>
<li class="sub"><a href="#section_Inheritance_Delegation_and_Reuse">Inheritance, Delegation and Reuse</a></li>
<li class="sub"><a href="#section_Self_Reference">Self-Reference</a></li>
<li class="sub"><a href="#section_Saving_References_to_External_Views">Saving References to External Views</a></li>
<li class="sub"><a href="#section_Views_as_Data_Presenters">Views as Data Presenters</a></li>
</ul>
</div>
</div>
<div class="pod">
<div class="section">
<div class="head">
<h1 id="Overview" onclick="switch_section(this)" title="Click title to show/hide section content.">Overview</h1>
<a href="#body" class="top" title="Back up to the top of the page" >Top</a>
</div>
<div class="body">
<p>
A view is effectively a collection of templates and/or variable
definitions which can be passed around as a self-contained unit. This
then represents a particular interface or presentation style for other
objects or items of data.
</p>
<p>
You can use views to implement custom "skins" for an application or
content set. You can use them to help simplify the presentation of common
objects or data types. You can even use then to automate the presentation
of complex data structures such as that generated in an
<code>XML::DOM</code> tree or similar. You let an iterator do the
walking, and the view does the talking (or in this case, the presenting).
Voila - you have view independant, structure shy traversal using
templates.
</p>
<p>
In general, views can be used in a number of different ways to achieve
several different things. They elegantly solve some problems which were
otherwise difficult or complicated, and make easy some things that were
previously hard.
</p>
<p>
At the moment, they're still very experimental. The directive syntax and
underlying API are likely to change quite considerably over the next
version or two. Please be very wary about building your multi-million
dollar e-commerce solutions based around this feature.
</p>
</div>
</div>
<div class="section">
<div class="head">
<h1 id="Views_as_Template_Collectors_Providers" onclick="switch_section(this)" title="Click title to show/hide section content.">Views as Template Collectors/Providers</h1>
<a href="#body" class="top" title="Back up to the top of the page" >Top</a>
</div>
<div class="body">
<p>
The <code>VIEW</code> directive starts a view definition and includes a
name by which the view can be referenced. The view definition continues
up to the matching <code>END</code> directive.
</p>
<pre><span class="tt">[% VIEW myview %]</span>
...
<span class="tt">[% END %]</span></pre>
<p>
The first role of a view is to act as a collector and provider of
templates. The <code>include()</code> method can be called on a view to
effectively do the same thing as the <code>INCLUDE</code> directive. The
template name is passed as the first argument, followed by any local
variable definitions for the template.
</p>
<pre><span class="tt">[% myview.include('header', title='The Title') %]</span>
# equivalent to
<span class="tt">[% INCLUDE header title='The Title' %]</span></pre>
<p>
Views accept a number of configuration options which can be used to
control different aspects of their behaviour. The '<code>prefix</code>'
and '<code>suffix</code>' options can be specified to add a fixed prefix
and/or suffix to the name of each template.
</p>
<pre><span class="tt">[% VIEW myview
prefix = 'my/'
suffix = '.tt2' ;
END
%]</span></pre>
<p>
Now the call
</p>
<pre><span class="tt">[% myview.include('header', title='The Title') %]</span></pre>
<p>
is equivalent to
</p>
<pre><span class="tt">[% INCLUDE my/header.tt2 title='The Title' %]</span></pre>
<p>
Views provide an <code>AUTOLOAD</code> method which maps method names to
the <code>include()</code> method. Thus, the following are all
equivalent:
</p>
<pre><span class="tt">[% myview.include('header', title='Hello World') %]</span>
<span class="tt">[% myview.include_header(title='Hello World') %]</span>
<span class="tt">[% myview.header(title='Hello World') %]</span></pre>
</div>
</div>
<div class="section">
<div class="head">
<h1 id="Local_BLOCK_Definitions" onclick="switch_section(this)" title="Click title to show/hide section content.">Local BLOCK Definitions</h1>
<a href="#body" class="top" title="Back up to the top of the page" >Top</a>
</div>
<div class="body">
<p>
A <code>VIEW</code> definition can include <code>BLOCK</code> definitions
which remain local to the view. A request for a particular template will
return a <code>BLOCK</code>, if defined, in preference to any other
template of the same name.
</p>
<pre><span class="tt">[% BLOCK foo %]</span>
public foo block
<span class="tt">[% END %]</span>
<span class="tt">[% VIEW plain %]</span>
<span class="tt">[% BLOCK foo %]</span>
plain foo block
<span class="tt">[% END %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% VIEW fancy %]</span>
<span class="tt">[% BLOCK foo %]</span>
fancy foo block
<span class="tt">[% END %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% INCLUDE foo %]</span> # public foo block
<span class="tt">[% plain.foo %]</span> # plain foo block
<span class="tt">[% fancy.foo %]</span> # fancy foo block</pre>
<p>
In addition to <code>BLOCK</code> definitions, a <code>VIEW</code> can
contain any other template directives. The entire <code>VIEW</code>
definition block is processed to initialise the view but no output is
generated (this may change RSN - and get stored as '<code>output</code>'
item, subsequently accessible as <code>[% view.output %]</code>).
However, directives that have side-effects, such as those that update a
variable, will have noticable consequences.
</p>
</div>
</div>
<div class="section">
<div class="head">
<h1 id="Preserving_Variable_State_within_Views" onclick="switch_section(this)" title="Click title to show/hide section content.">Preserving Variable State within Views</h1>
<a href="#body" class="top" title="Back up to the top of the page" >Top</a>
</div>
<div class="body">
<p>
Views can also be used to save the values of any existing variables, or
to create new ones at the point at which the view is defined. Unlike
simple template metadata (<code>META</code>) which can only contain
static string values, the view initialisation block can contain any
template directives and generate any kind of dynamic output and/or data
items.
</p>
<pre><span class="tt">[% VIEW my_web_site %]</span>
<span class="tt">[% view.title = title or 'My Cool Web Site' %]</span>
<span class="tt">[% view.author = "$abw.name, $abw.email" %]</span>
<span class="tt">[% view.sidebar = INCLUDE my/sidebar.tt2 %]</span>
<span class="tt">[% END %]</span></pre>
<p>
Note that additional data items can be specified as arguments to the
<code>VIEW</code> directive. Anything that doesn't look like a
configuration parameter is assumed to be a data item. This can be a
little hazardous, of course, because you never know when a new
configuration item might get added which interferes with your data.
</p>
<pre><span class="tt">[% VIEW my_web_site
# config options
prefix = 'my/'
# misc data
title = title or 'My Cool Web Site'
author = "$abw.name, $abw.email"
sidebar = INCLUDE my/sidebar.tt2
%]</span>
...
<span class="tt">[% END %]</span></pre>
<p>
Outside of the view definition you can access the view variables as, for
example:
</p>
<pre><span class="tt">[% my_web_site.title %]</span></pre>
<p>
One important feature is the equivalence of simple variables and
templates. You can implement the view item '<code>title</code>' as a
simple variable, a template defined in an external file, possibly with a
prefix/suffix automatically appended, or as a local <code>BLOCK</code>
definition within the <code>[% VIEW %] ... [% END %]</code> definition.
If you use the syntax above then the view will Do The Right Thing to
return the appropriate output.
</p>
<p>
At the <code>END</code> of the <code>VIEW</code> definition the view is
"sealed" to prevent you from accidentally updating any variable values.
If you attempt to change the value of a variable after the
<code>END</code> of the <code>VIEW</code> definition block then a
<code>view</code> error will be thrown.
</p>
<pre><span class="tt">[% TRY;
my_web_site.title = 'New Title';
CATCH;
error;
END
%]</span></pre>
<p>
The error above will be reported as:
</p>
<pre>view error - cannot update item in sealed view: title</pre>
<p>
The same is true if you pass a parameter to a view variable. This is
interpreted as an attempt to update the variable and will raise the same
warning.
</p>
<pre><span class="tt">[% my_web_site.title('New Title') %]</span> # view error!</pre>
<p>
You can set the <code>silent</code> parameter to have the view ignore
these parameters and simply return the variable value.
</p>
<pre><span class="tt">[% VIEW my_web_site
silent = 1
title = title or 'My Cool Web Site'
# ... ;
END
%]</span>
<span class="tt">[% my_web_site.title('Blah Blah') %]</span> # My Cool Web Site</pre>
<p>
Alternately, you can specify that a view is unsealed allowing existing
variables to be updated and new variables defined.
</p>
<pre><span class="tt">[% VIEW my_web_site
sealed = 0
title = title or 'My Cool Web Site'
# ... ;
END
%]</span>
<span class="tt">[% my_web_site.title('Blah Blah') %]</span> # Blah Blah
<span class="tt">[% my_web_site.title %]</span> # Blah Blah</pre>
<div class="subsection">
<div class="head">
<h2 id="section_Inheritance_Delegation_and_Reuse" onclick="switch_subsection(this)" title="Click title to show/hide sub-section content.">Inheritance, Delegation and Reuse</h2>
<a href="#body" class="top" title="Back up to the top of the page" >Top</a>
</div>
<div class="body">
<p>
Views can be inherited from previously defined views by use of the
<code>base</code> parameter. This example shows how a base class view is
defined which applies a <code>view/default/</code> prefix to all template
names.
</p>
<pre><span class="tt">[% VIEW my.view.default
prefix = 'view/default/';
END
%]</span></pre>
<p>
Thus the directive:
</p>
<pre><span class="tt">[% my.view.default.header(title='Hello World') %]</span></pre>
<p>
is now equivalent to:
</p>
<pre><span class="tt">[% INCLUDE view/default/header title='Hello World' %]</span></pre>
<p>
A second view can be defined which specifies the default view as a base.
</p>
<pre><span class="tt">[% VIEW my.view.fancy
base = my.view.default
prefix = 'view/fancy/';
END
%]</span></pre>
<p>
Now the directive:
</p>
<pre><span class="tt">[% my.view.fancy.header(title='Hello World') %]</span></pre>
<p>
will resolve to:
</p>
<pre><span class="tt">[% INCLUDE view/fancy/header title='Hello World' %]</span></pre>
<p>
or if that doesn't exist, it will be handled by the base view as:
</p>
<pre><span class="tt">[% INCLUDE view/default/header title='Hello World' %]</span></pre>
<p>
When a parent view is specified via the <code>base</code> parameter, the
delegation of a view to its parent for fetching templates and accessing
user defined variables is automatic. You can also implement your own
inheritance, delegation or other reuse patterns by explicitly delegating
to other views.
</p>
<pre><span class="tt">[% BLOCK foo %]</span>
public foo block
<span class="tt">[% END %]</span>
<span class="tt">[% VIEW plain %]</span>
<span class="tt">[% BLOCK foo %]</span>
<plain><span class="tt">[% PROCESS foo %]</span></plain>
<span class="tt">[% END %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% VIEW fancy %]</span>
<span class="tt">[% BLOCK foo %]</span>
<span class="tt">[% plain.foo | replace('plain', 'fancy') %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% plain.foo %]</span> # <plain>public foo block</plain>
<span class="tt">[% fancy.foo %]</span> # <fancy>public foo block</fancy></pre>
<p>
Note that the regular <code>INCLUDE/PROCESS/WRAPPER</code> directives
work entirely independantly of views and will always get the original,
unaltered template name rather than any local per-view definition.
</p>
</div>
</div> <div class="subsection">
<div class="head">
<h2 id="section_Self_Reference" onclick="switch_subsection(this)" title="Click title to show/hide sub-section content.">Self-Reference</h2>
<a href="#body" class="top" title="Back up to the top of the page" >Top</a>
</div>
<div class="body">
<p>
A reference to the view object under definition is available with the
<code>VIEW ... END</code> block by its specified name and also by the
special name '<code>view</code>' (similar to the <code>my $self =
shift;</code> in a Perl method or the '<code>this</code>' pointer in C++,
etc). The view is initially unsealed allowing any data items to be
defined and updated within the <code>VIEW ... END</code> block. The view
is automatically sealed at the end of the definition block, preventing
any view data from being subsequently changed.
</p>
<p>
(NOTE: sealing should be optional. As well as sealing a view to prevent
updates (<code>SEALED</code>), it should be possible to set an option in
the view to allow external contexts to update existing variables
(<code>UPDATE</code>) or even create totally new view variables
(<code>CREATE</code>)).
</p>
<pre><span class="tt">[% VIEW fancy %]</span>
<span class="tt">[% fancy.title = 'My Fancy Title' %]</span>
<span class="tt">[% fancy.author = 'Frank Open' %]</span>
<span class="tt">[% fancy.col = { bg => '#ffffff', bar => '#a0a0ff' } %]</span>
<span class="tt">[% END %]</span></pre>
<p>
or
</p>
<pre><span class="tt">[% VIEW fancy %]</span>
<span class="tt">[% view.title = 'My Fancy Title' %]</span>
<span class="tt">[% view.author = 'Frank Open' %]</span>
<span class="tt">[% view.col = { bg => '#ffffff', bar => '#a0a0ff' } %]</span>
<span class="tt">[% END %]</span></pre>
<p>
It makes no real difference in this case if you refer to the view by its
name, '<code>fancy</code>', or by the general name, '<code>view</code>'.
Outside of the view block, however, you should always use the given name,
'<code>fancy</code>':
</p>
<pre><span class="tt">[% fancy.title %]</span>
<span class="tt">[% fancy.author %]</span>
<span class="tt">[% fancy.col.bg %]</span></pre>
<p>
The choice of given name or '<code>view</code>' is much more important
when it comes to <code>BLOCK</code> definitions within a
<code>VIEW</code>. It is generally recommended that you use
'<code>view</code>' inside a <code>VIEW</code> definition because this is
guaranteed to be correctly defined at any point in the future when the
block gets called. The original name of the view might have long since
been changed or reused but the self-reference via '<code>view</code>'
should always be intact and valid.
</p>
<p>
Take the following VIEW as an example:
</p>
<pre><span class="tt">[% VIEW foo %]</span>
<span class="tt">[% view.title = 'Hello World' %]</span>
<span class="tt">[% BLOCK header %]</span>
Title: <span class="tt">[% view.title %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% END %]</span></pre>
<p>
Even if we rename the view, or create a new <code>foo</code> variable,
the header block still correctly accesses the <code>title</code>
attribute of the view to which it belongs. Whenever a view
<code>BLOCK</code> is processed, the <code>view</code> variable is always
updated to contain the correct reference to the view object to which it
belongs.
</p>
<pre><span class="tt">[% bar = foo %]</span>
<span class="tt">[% foo = { title => "New Foo" } %]</span> # no problem
<span class="tt">[% bar.header %]</span> # => Title: Hello World</pre>
</div>
</div> <div class="subsection">
<div class="head">
<h2 id="section_Saving_References_to_External_Views" onclick="switch_subsection(this)" title="Click title to show/hide sub-section content.">Saving References to External Views</h2>
<a href="#body" class="top" title="Back up to the top of the page" >Top</a>
</div>
<div class="body">
<p>
When it comes to view inheritance, it's always a good idea to take a
local copy of a parent or delegate view and store it as an attribute
within the view for later use. This ensures that the correct view
reference is always available, even if the external name of a view has
been changed.
</p>
<pre><span class="tt">[% VIEW plain %]</span>
...
<span class="tt">[% END %]</span>
<span class="tt">[% VIEW fancy %]</span>
<span class="tt">[% view.plain = plain %]</span>
<span class="tt">[% BLOCK foo %]</span>
<span class="tt">[% view.plain.foo | replace('plain', 'fancy') %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% plain.foo %]</span> # => <plain>public foo block</plain>
<span class="tt">[% plain = 'blah' %]</span> # no problem
<span class="tt">[% fancy.foo %]</span> # => <fancy>public foo block</fancy></pre>
</div>
</div> <div class="subsection">
<div class="head">
<h2 id="section_Views_as_Data_Presenters" onclick="switch_subsection(this)" title="Click title to show/hide sub-section content.">Views as Data Presenters</h2>
<a href="#body" class="top" title="Back up to the top of the page" >Top</a>
</div>
<div class="body">
<p>
Another key role of a view is to act as a dispatcher to automatically
apply the correct template to present a particular object or data item.
This is handled via the <code>print()</code> method.
</p>
<p>
Here's an example:
</p>
<pre><span class="tt">[% VIEW foo %]</span>
<span class="tt">[% BLOCK text %]</span>
Some text: <span class="tt">[% item %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% BLOCK hash %]</span>
a hash:
<span class="tt">[% FOREACH key = item.keys.sort -%]</span>
<span class="tt">[% key %]</span> => <span class="tt">[% item.$key %]</span>
<span class="tt">[% END -%]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% BLOCK list %]</span>
a list: <span class="tt">[% item.sort.join(', ') %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% END %]</span></pre>
<p>
We can now use the view to print text, hashes or lists. The
<code>print()</code> method includes the right template depending on the
typing of the argument (or arguments) passed.
</p>
<pre><span class="tt">[% some_text = 'I read the news today, oh boy.' %]</span>
<span class="tt">[% a_hash = { house => 'Lords', hall => 'Albert' } %]</span>
<span class="tt">[% a_list = [ 'sure', 'Nobody', 'really' ] %]</span>
<span class="tt">[% view.print(some_text) %]</span>
# Some text: I read the news today, oh boy.
<span class="tt">[% view.print(a_hash) %]</span>
# a hash:
hall => Albert
house => Lords
<span class="tt">[% view.print(a_list) %]</span>
# a list: Nobody, really, sure</pre>
<p>
You can also provide templates to print objects of any other class. The
class name is mapped to a template name with all non-word character
sequences such as '<code>::</code>' converted to a single
'<code>_</code>'.
</p>
<pre><span class="tt">[% VIEW foo %]</span>
<span class="tt">[% BLOCK Foo_Bar %]</span>
a Foo::Bar object:
thingies: <span class="tt">[% view.print(item.thingies) %]</span>
doodahs: <span class="tt">[% view.print(item.doodahs) %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% USE fubar = Foo::Bar(...) %]</span>
<span class="tt">[% foo.print(fubar) %]</span></pre>
<p>
Note how we use the view object to display various items within the
objects ('<code>thingies</code>' and '<code>doodahs</code>'). We don't
need to worry what kind of data these represent (text, list, hash, etc)
because we can let the view worry about it, automatically mapping the
data type to the correct template.
</p>
<p>
Views may define their own type => template map.
</p>
<pre><span class="tt">[% VIEW foo
map = { TEXT => 'plain_text',
ARRAY => 'show_list',
HASH => 'show_hash',
My::Module => 'template_name'
default => 'any_old_data'
}
%]</span>
<span class="tt">[% BLOCK plain_text %]</span>
...
<span class="tt">[% END %]</span>
...
<span class="tt">[% END %]</span></pre>
<p>
They can also provide a <code>default</code> map entry, specified as part
of the <code>map</code> hash or as a parameter by itself.
</p>
<pre><span class="tt">[% VIEW foo
map = { ... },
default = 'whatever'
%]</span>
...
<span class="tt">[% END %]</span></pre>
<p>
or
</p>
<pre><span class="tt">[% VIEW foo %]</span>
<span class="tt">[% view.map = { ... }
view.default = 'whatever'
%]</span>
...
<span class="tt">[% END %]</span></pre>
<p>
The <code>print()</code> method provides one more piece of magic. If you
pass it a reference to an object which provides a <code>present()</code>
method, then the method will be called passing the view as an argument.
This then gives any object a chance to determine how it should be
presented via the view.
</p>
<pre>package Foo::Bar;
...
sub present {
my ($self, $view) = @_;
return "a Foo::Bar object:\n"
. "thingies: " . $view->print($self->{ _THINGIES }) . "\n"
. "doodahs: " . $view->print($self->{ _DOODAHS }) . "\n";
}</pre>
<p>
The object is free to delve deeply into its innards and mess around with
its own private data, before presenting the relevant data via the view.
In a more complex example, a <code>present()</code> method might walk
part of a tree making calls back against the view to present different
nodes within the tree. We may not want to expose the internal structure
of the tree (because that would break encapsulation and make our
presentation code dependant on it) but we want to have some way of
walking the tree and presenting items found in a particular manner.
</p>
<p>
This is known as <i>Structure Shy Traversal</i>. Our view object doesn't
require prior knowledge about the internal structure of any data set to
be able to traverse it and present the data contained therein. The data
items themselves, via the <code>present()</code> method, can implement
the internal iterators to guide the view along the right path to
presentation happiness.
</p>
<p>
The upshot is that you can use views to greatly simplify the display of
data structures like <code>XML::DOM</code> trees. The documentation for
the <code>Template::Plugin::XML::DOM</code> module contains an example of
this. In essence, it looks something like this:
</p>
<p>
XML source:
</p>
<pre><user name="Andy Wardley">
<project id="iCan" title="iCan, but theyCan't"/>
<project id="p45" title="iDid, but theyDidn't"/>
</user></pre>
<p>
TT View:
</p>
<pre><span class="tt">[% VIEW fancy %]</span>
<span class="tt">[% BLOCK user %]</span>
User: <span class="tt">[% item.name %]</span>
<span class="tt">[% item.content(myview) %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% BLOCK project %]</span>
Project: <span class="tt">[% project.id %]</span> - <span class="tt">[% project.name %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% END %]</span></pre>
<p>
Generate view:
</p>
<pre><span class="tt">[% USE dom = XML.DOM %]</span>
<span class="tt">[% fancy.print(dom.parse(xml_source)) %]</span></pre>
<p>
Output:
</p>
<pre>User: Andy Wardley
Project: iCan - iCan, but theyCan't
Project: p45 - iDid, but theyDidn't</pre>
<p>
The same approach can be applied to many other areas. Here's an example
from the <code>File</code>/<code>Directory</code> plugins.
</p>
<pre><span class="tt">[% VIEW myview %]</span>
<span class="tt">[% BLOCK file %]</span>
- <span class="tt">[% item.name %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% BLOCK directory %]</span>
* <span class="tt">[% item.name %]</span>
<span class="tt">[% item.content(myview) FILTER indent %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% END %]</span>
<span class="tt">[% USE dir = Directory(dirpath) %]</span>
<span class="tt">[% myview.print(dir) %]</span></pre>
<p>
And here's the same approach use to convert POD documentation to any
other format via template.
</p>
<pre><span class="tt">[% # load Pod plugin and parse source file into Pod Object Model
USE Pod;
pom = Pod.parse_file(my_pod_file);
# define view to map all Pod elements to "pod/html/xxx" templates
VIEW pod2html
prefix='pod/html';
END;
# now print document via view (i.e. as HTML)
pod2html.print(pom)
%]</span></pre>
<p>
Here we simply define a template prefix for the view which causes the
view to look for <code>pod/html/head1</code>,
<code>pod/html/head2</code>, <code>pod/html/over</code> as templates to
present the different sections of the parsed Pod document.
</p>
<p>
There are some examples in the Template Toolkit test suite:
<i>t/pod.t</i> and <i>t/view.t</i> which may shed some more light on
this. See the distribution sub-directory <i>examples/pod/html</i> for
examples of Pod -> HTML templates.
</p>
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