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  • hi james!

    some time ago i posted the following to the perl-aspects list (which is so quiet as to say it's defunct by now... not much interest in aspects, it seems). i just remembered it as you talked about aspects & gui.



    Here's an idea. Aspects can be used to explicitly, in a declarative
    way, specify relationships between objects. In a way, that's what
    design patterns do.

    So we're looking at a fictitious set of user interface widgets;
    there's a Widget class from which all widgets derive. We have a
    Listbox class that can hold a list of options, one of which is
    selected. We also have a Textbox class. And there's a Dialog class
    that holds both a textbox and a listbox.

    So far so good. If you look at the implementation of all those
    classes you'll notice that they don't know anything about each

    How these classes interact with each other is specified explicitly
    using an aspect. This particular aspect consists of three pieces
    of advice:

    (a1) When a listbox or textbox is assigned to a dialog, said box
    is told who its parent is.

    (a2) When an option on the listbox is selected, its dialog's
    associated textbox is told about that text as well.

    (a3) When a textbox's text is set, it is told to redraw or update
    itself, which in this simple example just prints a line to STDOUT.

    So the basic ideas here is that aspects can specify interrelationships
    between objects in a declarative way.

    Ideas? Comments?


    use warnings;
    use strict;
    use Aspect::Attribute;

    package Widget;
    use Class::MethodMaker
            new_with_init => 'new',
            get_set => [ -eiffel => 'parent' ];

    sub init {
                    my $self = shift;
                    while (my ($prop, $value) = splice(@_, 0, 2)) {
                                    $prop = "set_$prop" if $self->can("set_$prop");
                                    $self->$prop(ref $value eq 'ARRAY' ? @$value : $value);

    sub update {}

    package Dialog;
    use base 'Widget';
    use Class::MethodMaker
            get_set => [ -eiffel => qw/listbox textbox/ ];

    sub update {
                    my $self = shift;

    package Listbox;
    use base 'Widget';
    use Class::MethodMaker
            list => 'options',
            get_set => [ -eiffel => 'select' ];

    package Textbox;
    use base 'Widget';
    use Class::MethodMaker
            get_set => [ -eiffel => 'text' ];

    sub update { printf "Text [ %20s ]\n", $_[0]->text }

    package main;

    sub a1 : Before(qr/^Dialog::set_(list|box)box$/) { $_[1]->set_parent($_[0]) }
    sub a2 : Before('Listbox::set_select') {
                    $_[0]->parent->textbox->set_text($_[0]->options_index($_[1])) }
    sub a3 : After('Textbox::set_text') { $_[0]->update }

    # Now the test...

    my $listbox = Listbox->new(options => [ qw/Courier Helvetica Monaco/ ]);

    my $font_dialog = Dialog->new(
            listbox => $listbox,
            textbox => Textbox->new,

    for (0..2) {
                    print "selecting option $_\n";



    selecting option 0
    Text [ Courier ]
    selecting option 1
    Text [ Helvetica ]
    selecting option 2
    Text [ Monaco ]