1Tie::Watch(3)                perl/Tk Documentation               Tie::Watch(3)
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NAME

6        Tie::Watch - place watchpoints on Perl variables.
7

SYNOPSIS

9        use Tie::Watch;
10
11        $watch = Tie::Watch->new(
12            -variable => \$frog,
13            -debug    => 1,
14            -shadow   => 0,
15            -fetch    => [\&fetch, 'arg1', 'arg2', ..., 'argn'],
16            -store    => \&store,
17            -destroy  => sub {print "Final value=$frog.\n"},
18        }
19        %vinfo = $watch->Info;
20        $args  = $watch->Args(-fetch);
21        $val   = $watch->Fetch;
22        print "val=", $watch->Say($val), ".\n";
23        $watch->Store('Hello');
24        $watch->Unwatch;
25

DESCRIPTION

27       This class module binds one or more subroutines of your devising to a
28       Perl variable.  All variables can have FETCH, STORE and DESTROY
29       callbacks.  Additionally, arrays can define CLEAR, DELETE, EXISTS,
30       EXTEND, FETCHSIZE, POP, PUSH, SHIFT, SPLICE, STORESIZE and UNSHIFT
31       callbacks, and hashes can define CLEAR, DELETE, EXISTS, FIRSTKEY and
32       NEXTKEY callbacks.  If these term are unfamiliar to you, I really
33       suggest you read perltie.
34
35       With Tie::Watch you can:
36
37        . alter a variable's value
38        . prevent a variable's value from being changed
39        . invoke a Perl/Tk callback when a variable changes
40        . trace references to a variable
41
42       Callback format is patterned after the Perl/Tk scheme: supply either a
43       code reference, or, supply an array reference and pass the callback
44       code reference in the first element of the array, followed by callback
45       arguments.  (See examples in the Synopsis, above.)
46
47       Tie::Watch provides default callbacks for any that you fail to specify.
48       Other than negatively impacting performance, they perform the standard
49       action that you'd expect, so the variable behaves "normally".  Once you
50       override a default callback, perhaps to insert debug code like print
51       statements, your callback normally finishes by calling the underlying
52       (overridden) method.  But you don't have to!
53
54       To map a tied method name to a default callback name simply lowercase
55       the tied method name and uppercase its first character.  So FETCH
56       becomes Fetch, NEXTKEY becomes Nextkey, etcetera.
57
58       Here are two callbacks for a scalar. The FETCH (read) callback does
59       nothing other than illustrate the fact that it returns the value to
60       assign the variable.  The STORE (write) callback uppercases the
61       variable and returns it.  In all cases the callback must return the
62       correct read or write value - typically, it does this by invoking the
63       underlying method.
64
65        my $fetch_scalar = sub {
66            my($self) = @_;
67            $self->Fetch;
68        };
69
70        my $store_scalar = sub {
71            my($self, $new_val) = @_;
72            $self->Store(uc $new_val);
73        };
74
75       Here are FETCH and STORE callbacks for either an array or hash.  They
76       do essentially the same thing as the scalar callbacks, but provide a
77       little more information.
78
79        my $fetch = sub {
80            my($self, $key) = @_;
81            my $val = $self->Fetch($key);
82            print "In fetch callback, key=$key, val=", $self->Say($val);
83            my $args = $self->Args(-fetch);
84            print ", args=('", join("', '",  @$args), "')" if $args;
85            print ".\n";
86            $val;
87        };
88
89        my $store = sub {
90            my($self, $key, $new_val) = @_;
91            my $val = $self->Fetch($key);
92            $new_val = uc $new_val;
93            $self->Store($key, $new_val);
94            print "In store callback, key=$key, val=", $self->Say($val),
95              ", new_val=", $self->Say($new_val);
96            my $args = $self->Args(-store);
97            print ", args=('", join("', '",  @$args), "')" if $args;
98            print ".\n";
99            $new_val;
100        };
101
102       In all cases, the first parameter is a reference to the Watch object,
103       used to invoke the following class methods.
104

METHODS

106       $watch = Tie::Watch->new(-options => values);
107           The watchpoint constructor method that accepts option/value pairs
108           to create and configure the Watch object.  The only required option
109           is -variable.
110
111           -variable is a reference to a scalar, array or hash variable.
112
113           -debug (default 0) is 1 to activate debug print statements internal
114           to Tie::Watch.
115
116           -shadow (default 1) is 0 to disable array and hash shadowing.  To
117           prevent infinite recursion Tie::Watch maintains parallel variables
118           for arrays and hashes.  When the watchpoint is created the parallel
119           shadow variable is initialized with the watched variable's
120           contents, and when the watchpoint is deleted the shadow variable is
121           copied to the original variable.  Thus, changes made during the
122           watch process are not lost.  Shadowing is on my default.  If you
123           disable shadowing any changes made to an array or hash are lost
124           when the watchpoint is deleted.
125
126           Specify any of the following relevant callback parameters, in the
127           format described above: -fetch, -store, -destroy.  Additionally for
128           arrays: -clear, -extend, -fetchsize, -pop, -push, -shift, -splice,
129           -storesize and -unshift.  Additionally for hashes: -clear, -delete,
130           -exists, -firstkey and -nextkey.
131
132       $args = $watch->Args(-fetch);
133           Returns a reference to a list of arguments for the specified
134           callback, or undefined if none.
135
136       $watch->Fetch();  $watch->Fetch($key);
137           Returns a variable's current value.  $key is required for an array
138           or hash.
139
140       %vinfo = $watch->Info();
141           Returns a hash detailing the internals of the Watch object, with
142           these keys:
143
144            %vinfo = {
145                -variable =>  SCALAR(0x200737f8)
146                -debug    =>  '0'
147                -shadow   =>  '1'
148                -value    =>  'HELLO SCALAR'
149                -destroy  =>  ARRAY(0x200f86cc)
150                -fetch    =>  ARRAY(0x200f8558)
151                -store    =>  ARRAY(0x200f85a0)
152                -legible  =>  above data formatted as a list of string, for printing
153            }
154
155           For array and hash Watch objects, the -value key is replaced with a
156           -ptr key which is a reference to the parallel array or hash.
157           Additionally, for an array or hash, there are key/value pairs for
158           all the variable specific callbacks.
159
160       $watch->Say($val);
161           Used mainly for debugging, it returns $val in quotes if required,
162           or the string "undefined" for undefined values.
163
164       $watch->Store($new_val);  $watch->Store($key, $new_val);
165           Store a variable's new value.  $key is required for an array or
166           hash.
167
168       $watch->Unwatch();
169           Stop watching the variable.
170

EFFICIENCY CONSIDERATIONS

172       If you can live using the class methods provided, please do so.  You
173       can meddle with the object hash directly and improved watch
174       performance, at the risk of your code breaking in the future.
175

AUTHOR

177       Stephen O. Lidie
178

HISTORY

180        lusol@Lehigh.EDU, LUCC, 96/05/30
181        . Original version 0.92 release, based on the Trace module from Hans Mulder,
182          and ideas from Tim Bunce.
183
184        lusol@Lehigh.EDU, LUCC, 96/12/25
185        . Version 0.96, release two inner references detected by Perl 5.004.
186
187        lusol@Lehigh.EDU, LUCC, 97/01/11
188        . Version 0.97, fix Makefile.PL and MANIFEST (thanks Andreas Koenig).
189          Make sure test.pl doesn't fail if Tk isn't installed.
190
191        Stephen.O.Lidie@Lehigh.EDU, Lehigh University Computing Center, 97/10/03
192        . Version 0.98, implement -shadow option for arrays and hashes.
193
194        Stephen.O.Lidie@Lehigh.EDU, Lehigh University Computing Center, 98/02/11
195        . Version 0.99, finally, with Perl 5.004_57, we can completely watch arrays.
196          With tied array support this module is essentially complete, so its been
197          optimized for speed at the expense of clarity - sorry about that. The
198          Delete() method has been renamed Unwatch() because it conflicts with the
199          builtin delete().
200
201        Stephen.O.Lidie@Lehigh.EDU, Lehigh University Computing Center, 99/04/04
202        . Version 1.0, for Perl 5.005_03, update Makefile.PL for ActiveState, and
203          add two examples (one for Perl/Tk).
204
205        sol0@lehigh.edu, Lehigh University Computing Center, 2003/06/07
206        . Version 1.1, for Perl 5.8, can trace a reference now, patch from Slaven
207          Rezic.
208
209        sol0@lehigh.edu, Lehigh University Computing Center, 2005/05/17
210        . Version 1.2, for Perl 5.8, per Rob Seegel's suggestion, support array
211          DELETE and EXISTS.
212
214       Copyright (C) 1996 - 2005 Stephen O. Lidie. All rights reserved.
215
216       This program is free software; you can redistribute it and/or modify it
217       under the same terms as Perl itself.
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221Tk804.036                         2022-01-21                     Tie::Watch(3)
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