CHP-5>
In the previous chapters you have been introduced to the basic features of Mason, and you should have a fairly good idea by now of how you might actually go about constructing a dynamic web site from Mason components. You have seen a few of Mason's unique features, such as the autohandler mechanism, the dhandler mechanism, and the ability to pass arbitrary data between components.
In this chapter we'll go beyond the basics and learn more about advanced ways to use Mason components to design large dynamic sites. You'll learn how to define multiple components in the same text file, how to create components on the fly from Perl strings, how to manage multiple component root directories, and (finally!) how to use all of Mason's object-oriented features.
CHP-5-SECT-1>
Although we often imagine a one-to-one correspondence between text
files and Mason components, it is actually possible to define
multiple components in a single text file. This is achieved by using
a <%def></%def>
block, a special Mason
directive that defines one component from within another. The
component embedded within the
<%def>
<%def>
block:def block>
block is called a
subcomponent
subcomponents>
,
and it is visible only to the component within which it resides:
component A may not access component B's
subcomponents directly.
The subcomponent may use any of the standard Mason component
directives, such as <%args>
,
<%init>
, %
-lines, and so
on. The only exceptions are that you may not use
<%def>
or <%method>
blocks within subcomponents nor may you use
``global'' blocks like
<%once>
or
<%shared>
.
Subcomponents are most useful when you have some piece of processing to repeat several times that is used only in a certain specific situation and doesn't merit its own separate component file.
Here is an example of defining and calling a subcomponent. Note that
the component is assigned a name inside the
<%def>
tag (the name often starts with a
period, purely by convention) and that you use the regular
component-calling mechanisms ($m->comp()
or a
<& &>
tag) to invoke it.
<h2>Information about certain Minnesota cities:</h2> % my @cities = ("Young America", "Sleepy Eye", "Nisswa", "Embarrass", % "Saint Cloud", "Little Canada", "Burnsville", "Luverne"); % foreach my $name (@cities) { <hr> <& .city_info, city => $name, state => 'MN' &> % } <%def .city_info> <%args> $city $state </%args> <table border="2"> <tr> <th colspan="2"><% $city %></th> </tr> <tr> <td>Population:</td> <td><% $population %></td> </tr> <tr> <td>Coordinates:</td> <td><% "$latitude, $longitude" %></td> </tr> <tr> <td>Mayor:</td> <td><% $mayor %></td> </tr> </table> <%init> my ($population, $latitude, $longitude, $mayor) = $dbh->selectrow_array("SELECT population, latitude, longitude, mayor FROM cities WHERE city=? and state=?", undef, $city, $state); </%init> </%def>
Since a subcomponent is visible only to the component that defines, and because it has all the capabilities that regular components have, you may think of subcomponents as roughly analogous to privately scoped anonymous subroutine __FOX_NLBF__> references in Perl.
CHP-5-SECT-2>
You may encounter situations in which you want to use Mason's templating features and data management tools, but you don't want to create a full-blown component root hierarchy on disk to house your components. Perhaps you want to create a component from an isolated file or directly from a string containing the components;creating> component text.
For these situations, the Mason interpreter provides the
make_component( )
make_component( )
method>
method. It accepts a
comp_file
or comp_source
parameter (letting you create a component from a file or a string,
respectively) and returns a Component object.
# Creating a component from scratch #!/usr/bin/perl -w use strict; use HTML::Mason; my $source = <<'EOF'; <%args> $planet </%args> Hello, <% $planet %>! EOF my $interp = HTML::Mason::Interp->new( ); my $comp = $interp->make_component(comp_source => $source); $interp->exec($comp, planet => 'Neptune');
And here is a component that creates another component at runtime:
<& $comp &> <%init> my $comp = $m->interp->make_component( comp_file => '/home/slappy/my_comps/foo', ); </%init>
Of course, creating __FOX_NLBF__> components at runtime is slower than creating them ahead of time, so if you need to squeeze out all the performance you possibly can, you might need to think of a speedier method to achieve your goals. And as always, benchmark everything so you really know what the effects are.
If the compiler encounters syntax errors when attempting to compile
the __FOX_NLBF__>
component, a fatal exception will be
thrown inside the make_component( )
method. If you
want to trap these errors, you may wrap the make_component(
)
__FOX_NLBF__>
method in Perl's
eval
eval {} block
(Perl)>
{}
block, and check
$@
after the method call.
CHP-5-SECT-3>
By default, the scope of variables created within an
<%init>
<%init> blocks:init blocks>
blocks;<%init>>
block, a Perl line, or any other Mason markup sections is the entire
components;sharing
data among>
component. This is tremendously
convenient, because it lets you initialize variables in the
<%init>
block, then use their values across
the rest of the component. So most of the time, the techniques
discussed in this section won't be needed.
There is one limitation to variables created within the
<%init>
section, however: their values
won't be seen by any subcomponents you might define.
This is true for two reasons. First, the subcomponents may themselves
contain an <%init>
section, so the relevance
of the main component's
<%init>
section isn't
necessarily clear. Second, a subcomponent may actually be a
method (more on this later), in which case it is
accessible to the outside world without first calling the main
component, so the <%init>
section never has
a chance to run.
Sometimes you need to share data between a component and its
subcomponents, however, and for these situations Mason provides the
<%shared>
<%shared> block:shared block>
<%once> block:once block>
blocks;<%shared>>
blocks;<%once>>
and <%once>
blocks. A
<%shared>
block runs before the main
component or any of its methods or subcomponents and may run
initialization code. Any variables created here will be visible to
the entire main component and any of its subcomponents, including the
main component's <%init>
section, if any. The <%once>
block is
similar -- the only difference is that code in the
<%once>
block won't run
every time the component is called. It will run only when the
component itself is loaded. The initialized values will remain intact
for the lifetime of the component object, which may be until you make
changes to the component source file and Mason reloads it or until
the web server child expires and gets replaced by a new one.
A <%shared>
section is great when a
component and its subcomponents have a tight relationship and may
make complicated use of shared data. In contrast,
<%once>
sections are useful for caching
values that change infrequently but may take a long time to compute.
See A<CHP-5-EX-1>Example 5-1.