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From: [email protected] (Stephen O. Lidie)
Newsgroups: comp.lang.perl.announce,comp.lang.perl.misc
Subject: ANNOUNCE:  Tie-Watch-0.95
Followup-To: comp.lang.perl.misc
Date: 27 Jul 1996 23:34:45 GMT
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I've uploaded Tie-Watch-0.95.tar.gz to CPAN, under authors/id/LUSOL.  I'd like
to move from beta to a 1.0 "production" version, so comments are welcomed.

Here's the pod:

package Tie::Watch;

=head1 NAME

Tie::Watch() - place watchpoints on Perl variables.

=head1 SYNOPSIS

use Tie::Watch;

$watch = Tie::Watch->new(
    -variable => \$frog,
    -debug    => 1,
    -fetch    => [\&fetch, 'arg1', 'arg2', ..., 'argn'],
    -store    => \&store,
    -destroy  => sub {print "Final value=$frog.\n"}
);
%vinfo = $watch->Info;
$args  = $watch->Args(-fetch);
$val   = $watch->Fetch;
print "val=", $watch->Say($val), ".\n";
$watch->Store('Hello');
$watch->Delete;

=head1 DESCRIPTION

This class module binds subroutine(s) of your devising to a Perl variable.
All variables can have FETCH, STORE and DESTROY callbacks.  Additionally,
hashes can define CLEAR, DELETE, EXISTS, FIRSTKEY and NEXTKEY callbacks.
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".

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.

my $fetch_scalar = sub {
    my($self) = @ARG;
    $self->Fetch;
};

my $store_scalar = sub {
    my($self, $new_val) = @ARG;
    $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) = @ARG;
    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) = @ARG;
    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.  You
can use this to invoke useful class methods.

=head1 METHODS

=head2 $watch = Tie::Watch->new(-options => values);

-variable  = a *reference* to a scalar, array or hash variable.

-debug     = 1 to activate debug print statements internal to Tie::Watch.

Specify any of the following relevant callback parameters, in the format
described above: -fetch -store -destroy -clear -delete -exists -firstkey
and/or -nextkey.

=head2 $args = $watch->Args(-fetch);

Returns a reference to a list of arguments for the specified callback, or
undef() if none.

=head2 $watch->Delete;

Stop watching the variable.

=head2 $watch->Fetch;  $watch->Fetch($key);

Return a variable's current value.  $key is required for an array or hash.

=head2 %vinfo = $watch->Info;

Returns a hash detailing the internals of the Watch object, with these keys:

%vinfo = {
    -variable =>  SCALAR(0x200737f8)
    -fetch    =>  ARRAY(0x200f8558)
    -store    =>  ARRAY(0x200f85a0)
    -destroy  =>  ARRAY(0x200f86cc)
    -debug    =>  '1'
    -value    =>  'HELLO SCALAR'
    -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
hashes, there are key/value pairs to the hash-specific callback options.

=head2 $watch->Say($val);

Used mainly for debugging, it returns $val in quotes if required, or
returns the string "undefined" for undefined values.

=head2 $watch->Store($new_val);  $watch->Store($key, $new_val);

Store a variable's new value.  $key is required for an array or hash.

=head1 EFFICIENCY CONSIDERATIONS

If you can live using the class methods provided, please do so.  You can
meddle with the object hash directly and improve watch performance, at
the risk of your code breaking in the future.

=cut

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