Tie::Watch(3pm) perl/Tk Documentation Tie::Watch(3pm)
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
callbacks. 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 specify.
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 variable
and returns it. In all cases the callback must return the correct 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 internal
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 parallel
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
callback, 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 performance, 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.
Tk804.036 2025-04-13 Tie::Watch(3pm)
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