tie::watch(3)

NAME

Tie::Watch - place watchpoints on Perl variables.

SYNOPSIS

use Tie::Watch;
$watch = Tie::Watch->new(
    -variable => rog,
    -debug    => 1,
    -shadow   => 0,
    -fetch    => [fetch, 'arg1', 'arg2', ..., 'argn'],
    -store    => store,
    -destroy  => sub {print "Final value=$frog.0},
}
%vinfo = $watch->Info;
$args  = $watch->Args(-fetch);
$val   = $watch->Fetch;
print "val=", $watch->Say($val), ".0;
$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, EXTEND, FETCHSIZE, POP, PUSH, SHIFT, SPLICE, STORESIZE and UNSHIFT callbacks, and hashes can define CLEAR, DELETE, EXISTS, FIRSTKEY and NEXTKEY call backs. If these term are unfamiliar to you, I really sug
gest 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 refer ence 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 perfor mance, 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 sim ply lowercase the tied method name and uppercase its first character. So FETCH becomes Fetch, NEXTKEY becomes Nex tkey, 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 ".0;
$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 ".0;
$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 state
ments 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 parame
ters, 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@Lehigh.EDU

HISTORY

lusol@Lehigh.EDU, LUCC, 96/05/30
. Original version 0.92 release, based on the Trace mod
ule 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 An
dreas 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 com
pletely watch arrays.: With tied array support this module is essentially com; plete, 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).

COPYRIGHT

This program is free software; you can redistribute it
and/or modify it under the same terms as Perl itself.

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