1callbacks(3)          User Contributed Perl Documentation         callbacks(3)
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NAME

6       Tk::callbacks - Specifying code for Tk to call.
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SYNOPSIS

9       One can specify a callback in one of the following ways:
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11       Without arguments:
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13           ... => \&subname, ...
14           ... => sub { ... }, ...
15           ... => 'methodname', ...
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17       or with arguments:
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19           ... => [ \&subname, args ... ], ...
20           ... => [ sub { ... }, args... ], ...
21           ... => [ 'methodname', args... ], ...
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DESCRIPTION

24       Perl/Tk has a callback, where Tcl/Tk has a command string (i.e. a
25       fragment of Tcl to be executed).  A perl/Tk callback can take one of
26       the following basic forms:
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28       ·   Reference to a subroutine "\&subname"
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30       ·   Anonymous subroutine (closure) "sub { ... }"
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32       ·   A method name 'methodname'
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34       Any of these can be provided with arguments by enclosing them and the
35       arguments in []. Here are some examples:
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37       $mw->bind($class, "<Delete>" => 'Delete');
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39       This will call $widget->Delete, the $widget being provided (by bind) as
40       the one where the Delete key was pressed.
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42       While having bind provide a widget object for you is ideal in many
43       cases it can be irritating in others. Using the list form this
44       behaviour can be modified:
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46       $a->bind("<Delete>",[$b => 'Delete']);
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48       because the first element $b is an object bind will call $b->Delete.
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50       Note that method/object ordering only matters for "bind" callbacks, the
51       auto-quoting in perl5.001 makes the first of these a little more
52       readable:
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54           $w->configure(-yscrollcommand => [ set => $ysb]);
55           $w->configure(-yscrollcommand => [ $ysb => 'set' ]);
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57       but both will call $ysb->set(args provided by Tk)
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59       Another use of arguments allows you to write generalized methods which
60       are easier to re-use:
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62           $a->bind("<Next>",['Next','Page']);
63           $a->bind("<Down>",['Next','Line']);
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65       This will call $a->Next('Page') or $a->Next('Line') respectively.
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67       Note that the contents of the "[]" are evaluated by perl when the
68       callback is created. It is often desirable for the arguments provided
69       to the callback to depend on the details of the event which caused it
70       to be executed. To allow for this callbacks can be nested using the
71       "Ev(...)" "constructor".  "Ev(...)" inserts callback objects into the
72       argument list. When perl/Tk glue code is preparing the argument list
73       for the callback it is about to call it spots these special objects and
74       recursively applies the callback process to them.
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EXAMPLES

77           $entry->bind('<Return>' => [$w , 'validate', Ev(['get'])]);
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79           $toplevel->bind('all', '<Visibility>', [\&unobscure, Ev('s')]);
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81           $mw->bind($class, '<Down>', ['SetCursor', Ev('UpDownLine',1)]);
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SEE ALSO

84       Tk::bind Tk::after Tk::options Tk::fileevent
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KEYWORDS

87       callback, closure, anonymous subroutine, bind
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91perl v5.30.1                      2020-01-30                      callbacks(3)