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Template::Manual::ViewUser Contributed Perl DocumentTemplate::Manual::Views(3)

       Template::Manual::Views - Template Toolkit views	(experimental)

       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.

       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
       "XML::DOM" 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 independent, structure shy	traversal using	templates.

       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.

       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.

Views as Template Collectors/Providers
       The "VIEW" directive starts a view definition and includes a name by
       which the view can be referenced.  The view definition continues	up to
       the matching "END" directive.

	   [% VIEW myview %]
	   [% END %]

       The first role of a view	is to act as a collector and provider of
       templates.  The "include()" method can be called	on a view to
       effectively do the same thing as	the "INCLUDE" directive.  The template
       name is passed as the first argument, followed by any local variable
       definitions for the template.

	   [% myview.include('header', title='The Title') %]

	   # equivalent	to
	   [% INCLUDE header  title='The Title'	%]

       Views accept a number of	configuration options which can	be used	to
       control different aspects of their behaviour.  The '"prefix"' and
       '"suffix"' options can be specified to add a fixed prefix and/or	suffix
       to the name of each template.

	   [% VIEW myview
		prefix = 'my/'
		suffix = '.tt2'	;

       Now the call

	   [% myview.include('header', title='The Title') %]

       is equivalent to

	   [% INCLUDE my/header.tt2  title='The	Title' %]

       Views provide an	"AUTOLOAD" method which	maps method names to the
       "include()" method.  Thus, the following	are all	equivalent:

	   [% myview.include('header', title='Hello World') %]
	   [% myview.include_header(title='Hello World') %]
	   [% myview.header(title='Hello World') %]

Local BLOCK Definitions
       A "VIEW"	definition can include "BLOCK" definitions which remain	local
       to the view.   A	request	for a particular template will return a
       "BLOCK",	if defined, in preference to any other template	of the same

	   [% BLOCK foo	%]
	      public foo block
	   [% END %]

	   [% VIEW plain %]
	      [% BLOCK foo %]
	      plain foo	block
	      [% END %]
	   [% END %]

	   [% VIEW fancy %]
	      [% BLOCK foo %]
	      fancy foo	block
	      [% END %]
	   [% END %]

	   [% INCLUDE foo %]	   # public foo	block
	   [%	%]	   # plain foo block
	   [%	%]	   # fancy foo block

       In addition to "BLOCK" definitions, a "VIEW" can	contain	any other
       template	directives.  The entire	"VIEW" definition block	is processed
       to initialise the view but no output is generated (this may change RSN
       - and get stored	as '"output"' item, subsequently accessible as "[%
       view.output %]").  However, directives that have	side-effects, such as
       those that update a variable, will have noticeable consequences.

Preserving Variable State within Views
       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	("META") 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.

	   [% VIEW my_web_site %]
	      [% view.title   =	title or 'My Cool Web Site' %]
	      [%  =	"$, $"	%]
	      [% view.sidebar =	INCLUDE	my/sidebar.tt2 %]
	   [% END %]

       Note that additional data items can be specified	as arguments to	the
       "VIEW" 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.

	   [% VIEW my_web_site
		   # config options
		   prefix = 'my/'
		   # misc data
		   title   = title or 'My Cool Web Site'
		   author  = "$, $"
		   sidebar = INCLUDE my/sidebar.tt2
	   [% END %]

       Outside of the view definition you can access the view variables	as,
       for example:

	   [% my_web_site.title	%]

       One important feature is	the equivalence	of simple variables and
       templates.  You can implement the view item '"title"' as	a simple
       variable, a template defined in an external file, possibly with a
       prefix/suffix automatically appended, or	as a local "BLOCK" definition
       within the "[% VIEW %] ... [% END %]" definition.  If you use the
       syntax above then the view will Do The Right Thing to return the
       appropriate output.

       At the "END" of the "VIEW" 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	"END" of the "VIEW" definition
       block then a "view" error will be thrown.

	   [% TRY;
		my_web_site.title = 'New Title';

       The error above will be reported	as:

	   view	error -	cannot update item in sealed view: title

       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.

	   [% my_web_site.title('New Title') %]	   # view error!

       You can set the "silent"	parameter to have the view ignore these
       parameters and simply return the	variable value.

	   [% VIEW my_web_site
		   silent = 1
		   title  = title or 'My Cool Web Site'
		   # ... ;

	   [% my_web_site.title('Blah Blah') %]	  # My Cool Web	Site

       Alternately, you	can specify that a view	is unsealed allowing existing
       variables to be updated and new variables defined.

	   [% VIEW my_web_site
		   sealed = 0
		   title  = title or 'My Cool Web Site'
		   # ... ;

	   [% my_web_site.title('Blah Blah') %]	  # Blah Blah
	   [% my_web_site.title	%]		  # Blah Blah

   Inheritance,	Delegation and Reuse
       Views can be inherited from previously defined views by use of the
       "base" parameter.  This example shows how a base	class view is defined
       which applies a "view/default/" prefix to all template names.

	   [% VIEW my.view.default
		   prefix = 'view/default/';

       Thus the	directive:

	   [% my.view.default.header(title='Hello World') %]

       is now equivalent to:

	   [% INCLUDE view/default/header title='Hello World' %]

       A second	view can be defined which specifies the	default	view as	a

	   [% VIEW my.view.fancy
		   base	  = my.view.default
		   prefix = 'view/fancy/';

       Now the directive:

	   [% my.view.fancy.header(title='Hello	World')	%]

       will resolve to:

	   [% INCLUDE view/fancy/header	title='Hello World' %]

       or if that doesn't exist, it will be handled by the base	view as:

	   [% INCLUDE view/default/header title='Hello World' %]

       When a parent view is specified via the "base" 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.

	   [% BLOCK foo	%]
	      public foo block
	   [% END %]

	   [% VIEW plain %]
	      [% BLOCK foo %]
	      <plain>[%	PROCESS	foo %]</plain>
	      [% END %]
	   [% END %]

	   [% VIEW fancy %]
	      [% BLOCK foo %]
	      [% | replace('plain', 'fancy') %]
	      [% END %]
	   [% END %]

	   [%	%]     # <plain>public foo block</plain>
	   [%	%]     # <fancy>public foo block</fancy>

       Note that the regular "INCLUDE/PROCESS/WRAPPER" directives work
       entirely	independently of views and will	always get the original,
       unaltered template name rather than any local per-view definition.

       A reference to the view object under definition is available with the
       "VIEW ... END" block by its specified name and also by the special name
       '"view"'	(similar to the	"my $self = shift;" in a Perl method or	the
       '"this"'	pointer	in C++,	etc).  The view	is initially unsealed allowing
       any data	items to be defined and	updated	within the "VIEW ... END"
       block.  The view	is automatically sealed	at the end of the definition
       block, preventing any view data from being subsequently changed.

       (NOTE: sealing should be	optional.  As well as sealing a	view to
       prevent updates ("SEALED"), it should be	possible to set	an option in
       the view	to allow external contexts to update existing variables
       ("UPDATE") or even create totally new view variables ("CREATE")).

	   [% VIEW fancy %]
	      [% fancy.title  =	'My Fancy Title' %]
	      [% =	'Frank Open' %]
	      [% fancy.col    =	{ bg =>	'#ffffff', bar => '#a0a0ff' } %]
	   [% END %]


	   [% VIEW fancy %]
	      [% view.title  = 'My Fancy Title'	%]
	      [% = 'Frank Open' %]
	      [% view.col    = { bg => '#ffffff', bar => '#a0a0ff' } %]
	   [% END %]

       It makes	no real	difference in this case	if you refer to	the view by
       its name, '"fancy"', or by the general name, '"view"'.  Outside of the
       view block, however, you	should always use the given name, '"fancy"':

	   [% fancy.title  %]
	   [% %]
	   [% %]

       The choice of given name	or '"view"' is much more important when	it
       comes to	"BLOCK"	definitions within a "VIEW".  It is generally
       recommended that	you use	'"view"' inside	a "VIEW" 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 '"view"'
       should always be	intact and valid.

       Take the	following VIEW as an example:

	   [% VIEW foo %]
	      [% view.title = 'Hello World' %]
	      [% BLOCK header %]
	      Title: [%	view.title %]
	      [% END %]
	   [% END %]

       Even if we rename the view, or create a new "foo" variable, the header
       block still correctly accesses the "title" attribute of the view	to
       which it	belongs.  Whenever a view "BLOCK" is processed,	the "view"
       variable	is always updated to contain the correct reference to the view
       object to which it belongs.

	   [% bar = foo	%]
	   [% foo = { title => "New Foo" } %]  # no problem
	   [% bar.header %]		       # => Title: Hello World

   Saving References to	External Views
       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.

	   [% VIEW plain %]
	   [% END %]

	   [% VIEW fancy %]
	      [% view.plain = plain %]
	      [% BLOCK foo %]
	      [%	| replace('plain', 'fancy') %]
	      [% END %]
	   [% END %]

	   [%	%]	   # =>	<plain>public foo block</plain>
	   [% plain = 'blah' %]	   # no	problem
	   [%	%]	   # =>	<fancy>public foo block</fancy>

   Views as Data Presenters
       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 "print()" method.

       Here's an example:

	   [% VIEW foo %]

	      [% BLOCK text %]
		 Some text: [% item %]
	      [% END %]

	      [% BLOCK hash %]
		 a hash:
		 [% FOREACH key	= item.keys.sort -%]
		    [% key %] => [% item.$key %]
		 [% END	-%]
	      [% END %]

	      [% BLOCK list %]
		 a list: [% item.sort.join(', ') %]
	      [% END %]

	   [% END %]

       We can now use the view to print	text, hashes or	lists.	The "print()"
       method includes the right template depending on the typing of the
       argument	(or arguments) passed.

	   [% some_text	= 'I read the news today, oh boy.' %]
	   [% a_hash	= { house => 'Lords', hall => 'Albert' } %]
	   [% a_list	= [ 'sure', 'Nobody', 'really' ] %]

	   [% view.print(some_text) %]
			       # Some text: I read the news today, oh boy.

	   [% view.print(a_hash) %]
			       # a hash:
				    hall => Albert
				    house => Lords
	   [% view.print(a_list) %]
			       # a list: Nobody, really, sure

       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 '"::"'	converted to a single '"_"'.

	   [% VIEW foo %]
	      [% BLOCK Foo_Bar %]
		 a Foo::Bar object:
		     thingies: [% view.print(item.thingies) %]
		      doodahs: [% view.print(item.doodahs)  %]
	      [% END %]
	   [% END %]

	   [% USE fubar	= Foo::Bar(...)	%]

	   [% foo.print(fubar) %]

       Note how	we use the view	object to display various items	within the
       objects ('"thingies"' and '"doodahs"').	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.

       Views may define	their own type => template map.

	   [% VIEW foo
		map = {	TEXT  => 'plain_text',
			ARRAY => 'show_list',
			HASH  => 'show_hash',
			My::Module => 'template_name'
			default	   => 'any_old_data'
	       [% BLOCK	plain_text %]
	       [% END %]

	   [% END %]

       They can	also provide a "default" map entry, specified as part of the
       "map" hash or as	a parameter by itself.

	   [% VIEW foo
		map	= { ...	},
		default	= 'whatever'
	   [% END %]


	   [% VIEW foo %]
	      [%     =	{ ... }
		 view.default =	'whatever'
	   [% END %]

       The "print()" method provides one more piece of magic. If you pass it a
       reference to an object which provides a "present()" 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

	   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";

       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 "present()" 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.

       This is known as	Structure Shy Traversal.  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 "present()" method, can implement	the
       internal	iterators to guide the view along the right path to
       presentation happiness.

       The upshot is that you can use views to greatly simplify	the display of
       data structures like "XML::DOM" trees.  The documentation for the
       "Template::Plugin::XML::DOM" module contains an example of this.	 In
       essence,	it looks something like	this:

       XML source:

	   <user name="Andy Wardley">
	       <project	id="iCan" title="iCan, but theyCan't"/>
	       <project	id="p45"  title="iDid, but theyDidn't"/>

       TT View:

	   [% VIEW fancy %]
	      [% BLOCK user %]
		 User: [% %]
		       [% item.content(myview) %]
	      [% END %]

	      [% BLOCK project %]
		   Project: [% %] - [% %]
	      [% END %]
	   [% END %]

       Generate	view:

	   [% USE dom =	XML.DOM	%]
	   [% fancy.print(dom.parse(xml_source)) %]


		 User: Andy Wardley
		   Project: iCan - iCan, but theyCan't
		   Project: p45	- iDid,	but theyDidn't

       The same	approach can be	applied	to many	other areas.  Here's an
       example from the	"File"/"Directory" plugins.

	   [% VIEW myview %]
	      [% BLOCK file %]
		 - [%	%]
	      [% END %]

	      [% BLOCK directory %]
		 * [%	%]
		   [% item.content(myview) FILTER indent %]
	      [% END %]
	   [% END %]

	   [% USE dir =	Directory(dirpath) %]
	   [% myview.print(dir)	%]

       And here's the same approach use	to convert POD documentation to	any
       other format via	template.

	   [%  # 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

	       # now print document via	view (i.e. as HTML)

       Here we simply define a template	prefix for the view which causes the
       view to look for	"pod/html/head1", "pod/html/head2", "pod/html/over" as
       templates to present the	different sections of the parsed Pod document.

       There are some examples in the Template Toolkit test suite: t/pod.t and
       t/view.t	which may shed some more light on this.	 See the distribution
       sub-directory examples/pod/html for examples of Pod -> HTML templates.

perl v5.32.1			  2019-01-04	    Template::Manual::Views(3)

NAME | Overview | Views as Template Collectors/Providers | Local BLOCK Definitions | Preserving Variable State within Views

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