oracular (3) Curses::Widgets::Tutorial::Creation.3pm.gz
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),