NAME¶
Aspect::Library::Wormhole - A wormhole between call frames
SYNOPSIS¶
package A;
sub new { bless {}, shift }
sub a { B->new->b }
package B;
sub new { bless {}, shift }
sub b { C->new->c }
package C;
sub new { bless {}, shift }
sub c { ref pop }
package main;
print ref A->new->a; # without aspect, prints C
use Aspect::Library::Wormhole;
aspect Wormhole => 'A::a', 'C::c';
print ref A->new->a; # with aspect, prints A
DESCRIPTION¶
A reusable aspect for passing objects down a call flow, without adding extra
arguments to the frames between the source and the target. It is a tool for
acquiring implicit context.
Suppose "A::a()" calls "B::b()" calls "C::c()"...
until "Z::z()".
All is well, until one day you get a requirement with a crosscutting
implication- "Z::Z()" requires one extra argument. It requires an
instance of the class "A". The very same instance on which the
method "a()" was called, high up the call chain of
"Z::z()".
Without this aspect you can either add a global $Current_A (very problematic),
or make "A::a()" send "B::b()" its $self, make
"B::b()" pass it on to "C::c()", and so on until
"Z::z()". You are forced to add many arguments to many methods.
Show me a developer who has never encountered this situation: you need to add an
argument to a long call flow, just because someone at the bottom needs it, yet
only someone on the top has it. The monkey code required
on each call
frame in the call flow,
for each argument that
each target
requires, is suffering from
EEK- Extraneous Embedded Knowledge
(<
http://citeseer.ist.psu.edu/254612.html>).
The code for the frames between the two ends of the wormhole, knows more about
the world than it should. This extraneous knowledge is embedded in each method
on the call flow, and there is no easy way to remove it.
This aspect removes the EEK by allowing you to setup a wormhole between the
source and target frames in the call flow. The only effect the wormhole has on
the call flow, is that the target gets called with one extra argument: the
calling source object. Thus the target acquires implicit context.
So this wormhole:
aspect Wormhole => 'A::a', 'Z::z';
Means: before the method "Z::z()" is called,
if
"A::a()" exists in the call flow,
then append one argument to
the argument list of "Z::z()". The argument appended is the calling
"A" object.
No method in the call flow is required to pass the source object, but
"Z::z()" will still receive it.
+--------+ +--------+
| source | +--------+ +--------+ | target |
+--------+--> | B::b() |--> | C::c() |--> ...--> +--------+
| A::a() | +--------+ +--------+ | Z::z() |
+--------+ +--------+
. ,
| /|\
| / | \
| |
+------------- The Bajoran Wormhole -------------+
USING¶
The aspect constructor takes two pointcut specs, a source and a target. The spec
can be a string (full sub name), a regex (sub will match if rexep matches), or
a coderef (called with sub name, will match if returns true).
For example, this will append a calling "Printer" to any call to a sub
defined on "Page", if it is in the call flow of
"Printer::print":
aspect Wormhole => 'Printer::Print', qr/^Page::/;
AUTHORS¶
Adam Kennedy <adamk@cpan.org>
Marcel Gruenauer <marcel@cpan.org>
Ran Eilam <eilara@cpan.org>
COPYRIGHT¶
Copyright 2001 by Marcel Gruenauer
Some parts copyright 2009 - 2013 Adam Kennedy.
This library is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.