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Tie::Watch(3)			      perl/Tk Documentation			    Tie::Watch(3)

	Tie::Watch - place watchpoints on Perl variables.

	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";

       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
       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,

       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

	my $fetch_scalar = sub {
	    my($self) = @_;

	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";

	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";

       In all cases, the first parameter is a reference to the Watch object, used to invoke the
       following class 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

       $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.

	   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.

	   Stop watching the variable.

       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.

       Stephen O. Lidie

	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

	sol0@lehigh.edu, Lehigh University Computing Center, 2005/05/17
	. Version 1.2, for Perl 5.8, per Rob Seegel's suggestion, support array

       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.

Tk804.03				    2014-06-10				    Tie::Watch(3)
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