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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".
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