Provided by: libcurses-widgets-perl_1.997-7_all 
NAME
Curses::Widget::Tutorial::Creation -- Widget Creation Tutorial
POD VERSION
$Id: Creation.pod,v 0.3 2002/11/04 00:45:06 corliss Exp corliss $
DESCRIPTION
Creating a custom widget is as easy as creating a descendant class of Curses::Widget and
defining as few as four methods:
Method Purpose
====================================================
_conf Validates configurations options and
initialises the internal state/data
_content Renders the widget according to the
current state
_cursor Renders the widget cursor according to the
current state
input_key Updates the state information according
to the passed character input
BASIC MODULE STRUCTURE
A decent code template for custom widgets would start with the following (we'll call our
new widget MyWidget):
package MyWidget;
use strict;
use vars qw($VERSION @ISA);
use Curses;
use Curses::Widget;
($VERSION) = (q$Revision: 0.3 $ =~ /(\d+(?:\.(\d+))+)/);
@ISA = qw(Curses::Widget);
Please note that the use Curses::Widget; statment provides more than just a base class to
inherit methods from, it also imports standard functions for use in the module:
Function Purpose
===========================================================
select_colour Initialises new colour pairs, and returns
the appropriate colour pair number, for use
with $wh->attrset(COLOR_PAIR($n)) calls.
select_color, the American English spelling,
also works.
scankey This blocks until a key is pressed, and that
key returned.
textwrap Splits the text given into lines no longer
than the column limit specified.
See the Curses::Widget pod for the specific syntax.
Descendent classes will automatically know the following fields (as used by the new or
get/setField methods):
Field Type Description
===========================================================
Y int Y coordinate of upper left corner of widget
X int X coordinate of upper left corner of widget
LINES int Number of lines in the content area of the widget
COLUMNS int Number of columsn inthe content area
BORDER boolean Whether to surround the widget with a box
CAPTION string Caption to display on top of border
FOREGROUND string Default foreground colour
BACKGROUND string Default background colour
BORDERCOL string Default border foreground colour
CAPTIONCOL string Default caption foreground colour
The colours, if not specified during widget instantiation, will default to the colours in
colour pair 0 (the terminal default). Borders will only be drawn if BORDER is true, and
that decision is made in the default _border method, not in the draw method. The _caption
method also decides internally whether or not to draw itself according to the value of
BORDER.
METHOD SEMANTICS
The _conf method is called by the class constructor (provided by Curses::Widget, unless
you override it here as well). Widget objects should be created with all configuration
options passed in a hash ref:
$widget = Curses::Widget::MyWidget->new({
OPTION1 => $value1,
OPTION2 => $value2,
[. . .]
});
The configuration hash is dereferenced and passed as arguments to the _conf method inside
of the new constructor:
$rv = $self->_conf(%$conf);
Because of this, the _conf method should probably begin along these lines:
sub _conf {
my $self = shift;
my %conf = (
OPTION1 => default1,
OPTION2 => default2,
[. . .],
@_
);
my $err = 0;
# Validate and initialise the widget's state
# and store in the %conf hash
# Always include the following
$err = 1 unless $self->SUPER::_conf(%conf);
return ($err == 0) ? 1 : 0;
}
You should perform any initialisation and validation of the configuration options here.
This routine is expected to return a true or false value, depending on whether or not any
critical errors were found. A false value will prevent the new constructor from returning
an object reference, causing the instantiation request to fail.
The last two lines of code should always be included in this subroutine. The call to the
parent class' _conf method stores the final initialised state information in %conf in the
object field CONF, after initialising many of the standard colour fields, should they have
been left undefined. You can retrieve and update the state information via $self->{CONF}.
A copy of that state information will be stored in $self->{OCONF}, and can be restored
with a call to reset, a method provided by Curses::Widgets.
The second method you should override is the _content method. This method, as mentioned
above, is responsible for rendering the widget according to its state information. This
method should handle one arguments:
$widget->_content($cwh);
The argument will be a window handle to the content area of the widget. You should always
layout your widget with the upper left corner as (0, 0), since the draw method is
responsible for allocating any extra space needed for borders and captions.
If your widget doesn't support borders and/or captions you can do one of two things:
override those methods (_border and _caption) to immediately return without doing
anything, or override the draw method to exclude those calls. Typically, the former
method of handling this would be preferred.
The third method you need to override is the _cursor method. This accepts the same window
handle as the _content method. The default draw method will only call this method if it
was called with a true active argument.
Neither of these two methods will need to allocate, refresh, or destroy window handles,
just print the content. The windows will already be erased and initialised to specified
foreground/background pairs, and those settings saved via the _save method. If at any
time you need to reset the window handle's current cursor back to those settings you can
call _restore:
$self->_restore($dwh);
In fact, in order to make the state of the window handle more predictable for descendent
classes you should probably call _restore at the end of each of these methods.
The final method that should be overridden is the input_key method. This expects a single
argument, that being the keystroke captured by the keyboard scanning function. It uses
that value to update (if it's not rejected) the widget's state information. A rough
skeleton for this function would be as follows:
sub input_key {
my $self = shift;
my $key = shift;
my $conf = $self->{CONF};
# validate/update state information
}
CONCLUSION
That, in a nutshell, is all there is to creating a custom widget. For a working example
which uses the structure noted above, look at the TextField or ButtonSet widgets. Both
consist of nothing more than the routines listed above.
HISTORY
2001/07/07 -- First draft. 2002/11/01 -- Updated for reworked internals.
AUTHOR/COPYRIGHT
(c) 2001 Arthur Corliss (corliss@digitalmages.com),