Provided by: libhtml-mason-perl_1.59-2_all 
NAME
HTML::Mason::Subclassing - Documentation on Subclassing Internal Mason classes
DESCRIPTION
This is the deep voodoo guide, for folks who want to create their own custom subclasses
for parts of Mason, such as the Request or Interp objects.
Class::Container
A number of modules in Mason are subclasses of "Class::Container". This module was
originally part of the Mason core as "HTML::Mason::Container", but Ken Williams decided to
release it separately on CPAN.
It was created to encapsulate some common behaviors for Mason objects such as parameter
validation and the creation of "contained" objects.
Basically, any Mason object which takes parameters to its constructor must inherit from
this module. Of course, since all of the classes that you might consider subclassing
already inherit from "Class::Container", you won't need to inherit from it directly.
However, you may need to use some of its methods.
So before you go further we highly recommend familiarizing yourself with
"Class::Container" and its methods. Also feel free to look at some of the Mason core
modules to see how "Class::Container" is used within Mason itself.
SUBCLASSABLE CLASSES
The following classes have been designed with subclassing in mind:
• HTML::Mason::Request
This object is your old friend $m. The request contains information about the current
request context, and provides methods for calling other components.
• HTML::Mason::Resolver
The resolver's job is to translate a component paths into an actual component. Mason
comes with a single Resolver subclass, "HTML::Mason::Resolver::File", which is used to
translate component paths into filesystem paths.
• HTML::Mason::ComponentSource
An object of this class represents a component's source. These objects are
instantiated by the resolver when it finds a component matching a given path.
• HTML::Mason::Lexer
The lexer is responsible for parsing a component. Creating a new lexer would allow
you to change Mason's component syntax.
• HTML::Mason::Compiler
The compiler takes the parsed chunks from the lexer and gives them meaning. The
default compiler, "HTML::Mason::Compiler::ToObject", turns a Mason component into a
Mason "object file", which contains actual Perl code.
• HTML::Mason::ApacheHandler
The ApacheHandler class is the bridge between the mod_perl world and Mason, primarily
Mason's Interp class.
It also provides its own "HTML::Mason::Request" and "HTML::Resolver::File" subclasses
which implement some mod_perl specific behaviors and features.
• HTML::Mason::Interp
The Interp is the core of Mason, and is primarily responsible for making all the other
objects do their jobs.
CONSTRUCTORS
If you choose to override the constructor, which is always "new" with Mason objects, that
you make sure to call the superclass's constructor and that you use the object returned by
it. A good boilerplate for an overridden constructor looks something like this:
sub new
{
my $class = shift;
my $self = $class->SUPER::new(@_);
$self->_do_some_init;
return $self;
}
Request
What to Subclass?
One important thing to know about this class is that it is actually several classes. The
first, "HTML::Mason::Request", is used when ApacheHandler is not loaded. The other,
"HTML::Mason::Request::ApacheHandler", is loaded by ApacheHandler and used to provide some
mod_perl specific features. Similar, the CGIHandler class provides its own request
subclass, "HTML::Mason::Request::CGIHandler".
It is impossible to know which one of these to subclass at compile time, since it is
possible that your subclass will be loaded before either ApacheHandler or CGIHandler.
To handle this, simply call the "alter_superclass()" method in your constructor, like
this:
sub new
{
my $class = shift;
$class->alter_superclass( $HTML::Mason::ApacheHandler::VERSION ?
'HTML::Mason::Request::ApacheHandler' :
$HTML::Mason::CGIHandler::VERSION ?
'HTML::Mason::Request::CGI' :
'HTML::Mason::Request' );
my $self = $class->SUPER::new(@_);
...
return $self;
}
It is quite important that you do this as these handler-specific subclasses provide
important functionality. The "alter_superclass()" method is implemented in the
"HTML::Mason::Request" base class, and will do the right thing even in cases of multiple
inheritance. It also cooperates with "Class::Container" to make sure that it sees changes
to the inheritance hierarchy.
The exec() method
The "exec" method is called in order to execute a request, and is the method that you are
most likely to want to override.
However, if you do override it we suggest that you make sure to call the parent class's
"exec" method to implement the actual component execution and there is no need for you to
re-implement them.
Since the "exec()" method is scalar/list context-sensitive, your "exec" method will need
to preserve that. Here is a boilerplate:
sub exec
{
my $self = shift;
... # do something cool
my @r;
if (wantarray)
{
@r = $self->SUPER::exec(@_);
}
else
{
$r[0] = $self->SUPER::exec(@_);
}
... # maybe do some cleanup
return wantarray ? @r : $r[0];
}
Subrequests
Your custom request class will also be used to implement subrequests, which are
implemented by calling "exec" just like any other method. If you only want to do certain
things in "exec" for the first request, you can simply check the value of
"$self->is_subrequest".
Examples
See the "MasonX::Request::WithApacheSession" module on CPAN.
Resolver and ComponentSource
The resolver takes a component path and figures out what component that path corresponds
to.
All resolver classes must implement two methods, "get_info" and "glob_path". The first
takes a component path and returns a new "HTML::Mason::ComponentSource" object. This
object contains information about the component, such as its last modified time and its
source. See the "HTML::Mason::ComponentSource" documentation for more details.
You may choose to provide your own ComponentSource subclass as well, if your resolver
implementation can take advantage of it.
The "glob_path" method is responsible for translating a component path like /foo/*/bar
into a list of component paths that match that glob pattern.
Lexer
The rationale for providing your own lexer would be to extend or replace Mason's syntax.
The lexer is called by the compiler via its "lex" method. The arguments it receives are
the component name, source, and the compiler object. See the Compiler class documentation
for details on what methods the lexer can call.
Compiler
See the Compiler class documentation for details on what methods a subclass of this class
needs to provide.
If you simply want to tweak Mason's existing behavior, you will probably want to subclass
"HTML::Mason::Compiler::ToObject", which is the default Compiler class. For example, if
you wanted to do something like make attributes dynamic, you could override the
"_flags_or_attr()" method in ToObject.
If you want to drastically change the behavior, you can subclass "HTML::Mason::Compiler"
instead. An example of this would be creating a compiler that generates "EmbPerl" or
"Apache::ASP" as output.
ApacheHandler
The methods that you are most likely to want to subclass are documented in the
"ApacheHandler class" documentation.
Providing an ApacheHandler subclass gives you a chance to do your own client parameter
parsing, as well as the capability of providing a different way of handling requests.
CGIHandler
Like the ApacheHandler, you could subclass this module in order to provide your own
argument processing or to step in and provide a different way to handle requests.
USING SUBCLASSES
When using your custom subclasses, we recommend that you take advantage of Mason's ability
to construct subclassed object on the fly.
For example, if you're subclassed the Interp object, you can still let the ApacheHandler
object create the Interp object for you, as long as you give it the appropriate
interp_class parameter. This is important because Mason may internally set up certain
defaults for contained objects. For example, the ApacheHandler, by default, will tell the
Interp object to use the "HTML::Mason::Request::ApacheHandler" Request subclass. If you
create an Interp object manually and you want to use that Interp object with
ApacheHandler, you'll have to specify the same Request class.
For example:
my $interp =
My::Interp->new
( request_class => 'HTML::Mason::Request::ApacheHandler',
my_new_interp_param => 42,
);
my $ah = HTML::Mason::ApacheHandler->new( interp => $interp );
It is far easier to simply do this:
my $ah =
HTML::Mason::ApacheHandler->new
( interp_class => 'My::Interp',
my_new_interp_param => 42,
);
Your new parameter, "my_new_interp_param", will still be passed to the "My::Interp"
constructor, but this also gives ApacheHandler a chance to set various parameters for the
Interp object. Of course, you can still override these defaults explicitly:
my $ah =
HTML::Mason::ApacheHandler->new
( interp_class => 'My::Interp',
resolver_class => 'My::Resolver'.
my_new_interp_param => 42,
);
If you need access to the interp object's methods directly, it will be always be available
via "$ah->interp".