Provided by: libtemplate-perl_2.27-1_amd64 bug


       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 name.

           [% 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

           [% 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 base.

           [% 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

           [% 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

       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()"

       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 view.

           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

       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

           [%  # 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.