
=head1 NAME

Tk::Menu::Item - Base class for Menu items

=for pm Tk/Menu/Item.pm

=for category Implementation

=head1 SYNOPSIS

   require Tk::Menu::Item;

   my $but = $menu->Button(...);
   $but->configure(...);
   my $what = $but->cget();

   package Whatever;
   require Tk::Menu::Item;
   @ISA = qw(Tk::Menu::Item);

   sub PreInit
   {
    my ($class,$menu,$info) = @_;
    $info->{'-xxxxx'} = ...
    my $y = delete $info->{'-yyyy'};
   }

=head1 DESCRIPTION

Tk::Menu::Item is the base class from which Tk::Menu::Button,
Tk::Menu::Cascade, Tk::Menu::Radiobutton and Tk::Menu::Checkbutton are derived.
There is also a Tk::Menu::Separator.

Constructors are declared so that $menu-E<gt>Button(...) etc. do what you would
expect.

The C<-label> option is pre-processed allowing ~ to be prefixed to the character
to derive a C<-underline> value. Thus

    $menu->Button(-label => 'Goto ~Home',...)

    is equivalent to

    $menu->Button(-label => 'Goto Home', -underline => 6, ...)

The C<Cascade> menu item creates a sub-menu and accepts
these options:

=over 4

=item B<-menuitems>

A list of items for the sub-menu.
Within this list (which is also accepted by Menu and Menubutton) the first
two elements of each item should be the "constructor" name and the label:

    -menuitems => [
                   [Button      => '~Quit', -command => [destroy => $mw]],
                   [Checkbutton => '~Oil',  -variable => \$oil],
                  ]

=item B<-postcommand>

A callback to be invoked before posting the menu.

=item B<-tearoff>

Specifies whether sub-menu can be torn-off or not.

=item B<-menuvar>

Scalar reference that will be set to the newly-created sub-menu.

=back

The returned object is currently a blessed reference to an array of two items:
the containing Menu and the 'label'.
Methods C<configure> and C<cget> are mapped onto underlying C<entryconfigure>
and C<entrycget>.

The main purpose of the OO interface is to allow derived item classes to
be defined which pre-set the options used to create a more basic item.

=head1 BUGS

This OO interface is very new. Using the label as the "key" is a problem
for separaror items which don't have one. The alternative would be to
use an index into the menu but that is a problem if items are deleted
(or inserted other than at the end).

There should probably be a PostInit entry point too, or a more widget like
defered 'configure'.

=cut

