Tie::Watch
Watch(3) User Contributed Perl Documentation Watch(3)
NAME
Tie::Watch - place watchpoints on Perl variables.
SYNOPSIS
use Tie::Watch;
$watch = Tie::Watch->new(
-variable => \$frog,
-debug => 1,
-shadow => 0,
-fetch => [\&fetch, ’arg1’, ’arg2’, ..., ’argn’],
-store => \&store,
-destroy => sub {print "Final value=$frog.\n"},
}
%vinfo = $watch->Info;
$args = $watch->Args(-fetch);
$val = $watch->Fetch;
print "val=", $watch->Say($val), ".\n";
$watch->Store(’Hello’);
$watch->Unwatch;
DESCRIPTION
This class module binds one or more subroutines of your devising to a
Perl variable. All variables can have FETCH, STORE and DESTROY call-
backs. Additionally, arrays can define CLEAR, DELETE, EXISTS, EXTEND,
FETCHSIZE, POP, PUSH, SHIFT, SPLICE, STORESIZE and UNSHIFT callbacks,
and hashes can define CLEAR, DELETE, EXISTS, FIRSTKEY and NEXTKEY
callbacks. If these term are unfamiliar to you, I really suggest you
read perltie.
With Tie::Watch you can:
. alter a variable’s value
. prevent a variable’s value from being changed
. invoke a Perl/Tk callback when a variable changes
. trace references to a variable
Callback format is patterned after the Perl/Tk scheme: supply either a
code reference, or, supply an array reference and pass the callback
code reference in the first element of the array, followed by callback
arguments. (See examples in the Synopsis, above.)
Tie::Watch provides default callbacks for any that you fail to spec-
ify. Other than negatively impacting performance, they perform the
standard action that you’d expect, so the variable behaves "normally".
Once you override a default callback, perhaps to insert debug code
like print statements, your callback normally finishes by calling the
underlying (overridden) method. But you don’t have to!
To map a tied method name to a default callback name simply lowercase
the tied method name and uppercase its first character. So FETCH
becomes Fetch, NEXTKEY becomes Nextkey, etcetera.
Here are two callbacks for a scalar. The FETCH (read) callback does
nothing other than illustrate the fact that it returns the value to
assign the variable. The STORE (write) callback uppercases the vari-
able and returns it. In all cases the callback must return the cor-
rect read or write value - typically, it does this by invoking the
underlying method.
my $fetch_scalar = sub {
my($self) = @_;
$self->Fetch;
};
my $store_scalar = sub {
my($self, $new_val) = @_;
$self->Store(uc $new_val);
};
Here are FETCH and STORE callbacks for either an array or hash. They
do essentially the same thing as the scalar callbacks, but provide a
little more information.
my $fetch = sub {
my($self, $key) = @_;
my $val = $self->Fetch($key);
print "In fetch callback, key=$key, val=", $self->Say($val);
my $args = $self->Args(-fetch);
print ", args=(’", join("’, ’", @$args), "’)" if $args;
print ".\n";
$val;
};
my $store = sub {
my($self, $key, $new_val) = @_;
my $val = $self->Fetch($key);
$new_val = uc $new_val;
$self->Store($key, $new_val);
print "In store callback, key=$key, val=", $self->Say($val),
", new_val=", $self->Say($new_val);
my $args = $self->Args(-store);
print ", args=(’", join("’, ’", @$args), "’)" if $args;
print ".\n";
$new_val;
};
In all cases, the first parameter is a reference to the Watch object,
used to invoke the following class methods.
METHODS
$watch = Tie::Watch->new(-options => values);
The watchpoint constructor method that accepts option/value pairs
to create and configure the Watch object. The only required
option is -variable.
-variable is a reference to a scalar, array or hash variable.
-debug (default 0) is 1 to activate debug print statements inter-
nal to Tie::Watch.
-shadow (default 1) is 0 to disable array and hash shadowing. To
prevent infinite recursion Tie::Watch maintains parallel variables
for arrays and hashes. When the watchpoint is created the paral-
lel shadow variable is initialized with the watched variable’s
contents, and when the watchpoint is deleted the shadow variable
is copied to the original variable. Thus, changes made during the
watch process are not lost. Shadowing is on my default. If you
disable shadowing any changes made to an array or hash are lost
when the watchpoint is deleted.
Specify any of the following relevant callback parameters, in the
format described above: -fetch, -store, -destroy. Additionally
for arrays: -clear, -extend, -fetchsize, -pop, -push, -shift,
-splice, -storesize and -unshift. Additionally for hashes:
-clear, -delete, -exists, -firstkey and -nextkey.
$args = $watch->Args(-fetch);
Returns a reference to a list of arguments for the specified call-
back, or undefined if none.
$watch->Fetch(); $watch->Fetch($key);
Returns a variable’s current value. $key is required for an array
or hash.
%vinfo = $watch->Info();
Returns a hash detailing the internals of the Watch object, with
these keys:
%vinfo = {
-variable => SCALAR(0x200737f8)
-debug => ’0’
-shadow => ’1’
-value => ’HELLO SCALAR’
-destroy => ARRAY(0x200f86cc)
-fetch => ARRAY(0x200f8558)
-store => ARRAY(0x200f85a0)
-legible => above data formatted as a list of string, for printing
}
For array and hash Watch objects, the -value key is replaced with
a -ptr key which is a reference to the parallel array or hash.
Additionally, for an array or hash, there are key/value pairs for
all the variable specific callbacks.
$watch->Say($val);
Used mainly for debugging, it returns $val in quotes if required,
or the string "undefined" for undefined values.
$watch->Store($new_val); $watch->Store($key, $new_val);
Store a variable’s new value. $key is required for an array or
hash.
$watch->Unwatch();
Stop watching the variable.
EFFICIENCY CONSIDERATIONS
If you can live using the class methods provided, please do so. You
can meddle with the object hash directly and improved watch perfor-
mance, at the risk of your code breaking in the future.
AUTHOR
Stephen O. Lidie
HISTORY
lusol@Lehigh.EDU, LUCC, 96/05/30
. Original version 0.92 release, based on the Trace module from Hans Mulder,
and ideas from Tim Bunce.
lusol@Lehigh.EDU, LUCC, 96/12/25
. Version 0.96, release two inner references detected by Perl 5.004.
lusol@Lehigh.EDU, LUCC, 97/01/11
. Version 0.97, fix Makefile.PL and MANIFEST (thanks Andreas Koenig).
Make sure test.pl doesn’t fail if Tk isn’t installed.
Stephen.O.Lidie@Lehigh.EDU, Lehigh University Computing Center, 97/10/03
. Version 0.98, implement -shadow option for arrays and hashes.
Stephen.O.Lidie@Lehigh.EDU, Lehigh University Computing Center, 98/02/11
. Version 0.99, finally, with Perl 5.004_57, we can completely watch arrays.
With tied array support this module is essentially complete, so its been
optimized for speed at the expense of clarity - sorry about that. The
Delete() method has been renamed Unwatch() because it conflicts with the
builtin delete().
Stephen.O.Lidie@Lehigh.EDU, Lehigh University Computing Center, 99/04/04
. Version 1.0, for Perl 5.005_03, update Makefile.PL for ActiveState, and
add two examples (one for Perl/Tk).
sol0@lehigh.edu, Lehigh University Computing Center, 2003/06/07
. Version 1.1, for Perl 5.8, can trace a reference now, patch from Slaven
Rezic.
sol0@lehigh.edu, Lehigh University Computing Center, 2005/05/17
. Version 1.2, for Perl 5.8, per Rob Seegel’s suggestion, support array
DELETE and EXISTS.
COPYRIGHT
Copyright (C) 1996 - 2005 Stephen O. Lidie. All rights reserved.
This program is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.
perl v5.8.8 2005-05-18 Watch(3)
