HTML::Mason::Subclassing - Documentation on Subclassing Internal Mason classes
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.
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.
The following classes have been designed with subclassing in mind:
$m. The request contains
information about the current request context, and provides methods
for calling other components.
HTML::Mason::Resolver::File, which is used to translate component
paths into filesystem paths.
HTML::Mason::Compiler::ToObject,
turns a Mason component into a Mason "object file", which contains
actual Perl code.
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.
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;
}
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 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];
}
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.
See the MasonX::Request::WithApacheSession module on CPAN.
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.
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.
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.
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.
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.
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.