NAME
mro::EVERY - EVERY & EVERY::LAST pseudo-packages using mro.
SYNOPSIS
# EVERY & EVERY::LAST redispatch the named method into
# all classes in the object/class hierarchy which
# define the method or have a suitable can() which
# returns their AUTOLOAD to handdle the method.
# one common use: initialization in construction.
#
# construct an object then dispatch the 'initialize'
# method into any derived classes least-to-most derived
# that declare their own 'initialize' method.
#
# derived classes don't have to re-dispatch the method,
# they just handle the object and argumets on their own.
#
# Note: not finding that $object->can( $method )
# raises an exception. generic base classes should
# provide a stub if it's reasonable that none of the
# derived classes hande the method.
package MyBase;
use mro qw( c3 );
use mro::EVERY;
sub construct
{
my $proto = shift;
my $class =
bless \( my $a = '' ), blessed $proto || $proto
}
sub new
{
my $object = &construct;
$object->EVERY::LAST::initialize( @_ );
$object
}
# notice the lack of a daisy-chain call in the
# initialize. each initialize defined up the
# stack is called once.
sub initialize{};
# another common use: cleanup in destruction.
# tear down an object from the top down, calling
# 'cleanup' for most-to-least derived classes.
package Thingy;
use mro qw( dfs );
use mro::EVERY;
DESTROY
{
my $object = shift;
$object->EVERY::cleanup;
}
# again, notice the lack of a daisy-chain.
sub cleanup {}
# Dispatching to AUTOLOAD that can.
# the "autoload" switch turns on scanning for
# $proto->can( $name ) and checking for AUTOLOAD
# subs (vs. simply checking for a defined coderef
# in the package).
#
# using this approach requires properly overloading
# can() in the package.
#
# note that AUTOLOAD's can have all sorts of side
# effects, this should be used with care and where
# the handling classes really do have overloaded
# "can" methods and really do handle the named
# operation properly.
#
# lacking an overloaded can() and appropriate
# AUTOLOAD, this is a waste.
#
# nu, don't say I didn't warn you.
package RocketEngine;
use mro qw( c3 );
use parent qw( Fuel Base );
use mro::EVERY qw( autoload );
sub ignite
{
my $obj = shift;
$obj->EVERY::bottle( 'open' );
$obj->EVERY::oxydize;
}
package Base;
use mro qw( c3 );
# EVERY ends up here via parent 'Base'.
#
# can has to return something for every method
# the class can handle -- including UNIVERSAL
# and any other base classes. this is a trivial
# example that works because there are no other
# bases classes other than UNIVERSAL here.
my %can =
(
oxydize => \&AUTOLOAD
);
sub can
{
$_[0]->UNIVERSAL::can( $_[1] )
or
%can{ $_[1] }
}
sub bottle
{
# not autoloaded, found via
# UNIVERSAL::can.
...
}
our $AUTOLOAD = '';
AUTOLOD
{
# call ends up here becuase mro::EVERY can
# find that $pkg->can( 'oxydize' ) and
# also that there is an AUTOLOAD defined
# in the package (not just inherited).
my $name = ( split '::', $AUTOLOAD )[-1];
if( 'oxydize' eq $name )
{
say 'Burn, baby, burn!'
}
}
# some very, very old code may depend on using depth
# first searches. This switch turns on dfs for calls
# to mro::EVERY & mro::EVERY::LAST from the Frobnicate
# package only.
#
# Note that other packages using either pseudo-class
# will still get whatever mro they have declared and
# will search Frobnicate using whatever mro they define.
package Frobnicte;
use mro::EVERY qw( dfs );DESCRIPTION
The use of both pseudo-classes is re-dispatching an arbitrary method up or down the inheritence stack without each class in the hierarchy having to do its own re-dispatch to another. One common use of this is in initializers, which can use EVERY::LAST to walk the tree from least-to-most derived classes calling the method where it is declared in each class (vs. simply inherited). This works nicely for base class destructors also using EVERY to tear down the object from most-to-least derived layers.
An initial sanity check of '$object->can( $method )' to ensure that something in the hierarchy claims to handle the call. If not an exception is raised. Inherited methods are also skipped to avoid duplicate dispatches into the same method with the same object. For installed methods a unique check for the returned coderefs also helps avoid duplicate dispatches.
Without autoloading this is quite simple: Walk down the list from mro::get_linear_isa looking for packages the method name in their pacakge as CODE. The unique list of subrefs is dispatced in order for EVERY and in reverse for EVERY::LAST. This is pretty much the same guts as NEXT, just using mro for the package names rather than iterating on @ISA.
One final step after finding the declared (vs. inherited) methods is applying uniq() to get a distinct list. This is important in not re-dispatching the same method mulitple times up or down the stack. In the case of EVERY this finds a unique list of most- derived methods avilable; EVERY::LAST finds the least-derived going up the stack from base to derived classes.
Autoload requires a bit more work, and co-operation from the classes in overloading can() to return true for methods handled by the AUTOLOAD. In some cases it's trivial: return true for anything. If the AUTOLOAD only handles some cases then can() needs to return the correct ones. The AUTOLOAD also has to exist in the classes package space (vs. being inherited).
SEE ALSO
- mro
-
This describes the use of "dfs" & "c3" methologies for resolving class inheritence order. This module is agnostic, relying on mro::get_linea_isa which handles them properly.
- NEXT
-
Further description EVERY & EVERY::LAST.
The NEXT uses its own DFS inheritence search and is not compatible with mro. If you don't require 5.8 compatibility then this module and mro's next::method and maybe::next::method along with this one will be a reasonable substitute.
If you are dealing with existing code that uses NEXT then this may provide different result for any classes using mro( c3 ).