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Using advanced widgets in Perl/Tk
By Philipp K. Janert - 2004-10-14 Page:  1 2 3

Simplified data entry: Tk::DateEntry and Tk::PathEntry

The Tk::DateEntry and the Tk::PathEntry widgets simplify the input of structured data (namely dates and file paths, respectively), by providing a display of valid input and allowing the user to select from them.

The DateEntry widget displays a text input field with an adjacent button. Clicking the button displays a calendar in a drop down menu, and selecting a date using the mouse enters the corresponding string into the text input field.

Figure 3. The DateEntry widget
Figure 3. The DateEntry widget

Listing 3 shows the code associated with this example. Selecting Convert calculates the number of seconds since the beginning of the Unix epoch and displays them in the Label widget atop the text input field.

Listing 3: Using the DateEntry widget


use Tk;
use Tk::DateEntry;

use Time::Local;

%idx_for_mon = ( JAN=>1, FEB=>2, MAR=>3, APR=> 4, MAY=> 5, JUN=> 6,
		 JUL=>7, AUG=>8, SEP=>9, OCT=>10, NOV=>11, DEC=>12 );

$input = '01-APR-2004'; # Initial value for display

$mw = MainWindow->new();
$mw->geometry( '200x80' );
$mw->resizable( 0, 0 );

$label = $mw->Label( -text=>'' )->pack;
$entry = $mw->DateEntry( -textvariable=>\$input, -width=>11,
			 -parsecmd=>\&parse, -formatcmd=>\&format )->pack;

$mw->Button( -text=>'Quit', -command=>sub{ exit } )->pack( -side=>'right' );
$mw->Button( -text=>'Convert',
	     -command=>sub{ convert( $input, $label ) } )->pack( -side=>'left' );

MainLoop;

# called on dropdown with content of \$textvariable, must return ( $yr, $mon, $day )
sub parse {
  my ( $day, $mon, $yr ) = split '-', $_[0];
  return ( $yr, $idx_for_mon{$mon}, $day );
}

# called on user selection with ($yr, $mon, $day), must return formatted string
sub format {
  my ( $yr, $mon, $day ) = @_;
  my %mon_for_idx = reverse %idx_for_mon;
  return sprintf( "%02d-%s-%2d", $day, $mon_for_idx{ $mon }, $yr );
}

# perform the conversion to epoch seconds when the corresponding button is pressed
sub convert {
  my ( $input, $label ) = @_;
  my ( $yr, $mon, $day ) = parse( $input );
  my $output = "Epoch seconds: " . timelocal( 0, 0, 0, $day, $mon-1, $yr-1900 );
  $label->configure( -text => $output );
}
    

Date input is hard because there are so many ways to represent the same date in string form. The DateEntry provides three standard date formats (MM/DD/YYYY, YYYY/MM/DD, and DD/MM/YYYY), which can be selected using the -dateformat option. If a different date format is desired, the programmer has to provide the conversion routines explicitly, using the callbacks -parsecmd and -formatcmd. In the example above, we use a custom date format, displaying months using 3-letter acronyms. When parsing the input string into its numerical constituents, we use the hash %idx_for_mon, which holds the numerical index of each month (1..12) given its acronym. When a user selects a date from the drop down menu, it has to be formatted into the corresponding string, requiring the opposite lookup, namely the acronym given the index. We build up such a data structure on the fly in the format routine, using the reverse command. Since this command expects an array, the original hash is unwound into an array in such a way that the values follow their respective keys in the array. This array is then reversed (so that now the former keys follow their values) and transformed back into a hash. This trick works here, because both keys and values are unique (again, for more information on this, see Learning Perl, which is listed in Resources).

The convert function takes the variable containing the input string, as well as a reference to the Label widget as parameters, so that it can change the value shown by the Label. Here, we do not pass a reference to the callback and the values of the parameters in an anonymous array to the -command attribute; instead, we invoke the callback function directly, from within an anonymous subroutine (a closure). The reason has to do with variable scoping; see the Sidebar for a full explanation.

Finally, the Tk::PathEntry widget is very simple: it provides a text input field for path names -- but with a twist! Similar to the behavior of tcsh or the Emacs mini-buffer, pressing the Tab key will complete the contents of the input field as far as possible, and will pop up a list box of possible choices if the current contents cannot be completed unambiguously. Bizarrely, the color of the list box cannot be changed -- unless one wants to edit the code of the underlying Perl module.

Figure 4. The PathEntry widget
Figure 4. The PathEntry widget

Listing 4: Using the PathEntry widget

use Tk;
use Tk::PathEntry;

use Cwd;

$path = cwd();

$mw = MainWindow->new();
$mw->geometry( '300x80' );
$mw->resizable( 0, 0 );

$mw->PathEntry( -textvariable=>\$path )->pack;
$mw->Label( -textvariable=>\$path, -foreground=>'blue' )->pack;
$mw->Button( -text=>'Quit', -command=>sub{ exit } )->pack;

MainLoop;
    

What makes PathEntry so interesting is that it is merely a widget, and not a dialog box. It can (in fact, it has to) be combined with other widgets in a program. Therefore, it provides a very lightweight way to add file selection capabilities to an application.

Conclusion

These are just a few of the more "advanced" widgets for the Perl/Tk GUI toolkit that allow developers to create richer, more powerful user interfaces using Perl. All the widgets discussed here are freely available as user contributions from CPAN.



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First published by IBM developerWorks


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