package Model;   # mix in to model classes that are
                 # implemented as blessed hashes

# This is called by a View
sub addDependent
{
    my $self = shift;
    my $dependent = shift;
    my $dependents = (self->{dependents} ||= []);
    push @$dependents, $dependent;
}

# This is called by a View
sub removeDependent
{
    my $self = shift;
    my $dependent = shift;
    my $dependents = $self->{dependents} || return;
    @$dependents = grep { $_ != $dependent } @$dependents;
}

# This is called by the Model
# you can pass additional data as needed
sub _changed
{
    my $self = shift;
    my $dependents = $self->{dependents} || return;
    foreach my $dependent ( @$dependents )
    {
        $dependent->update($self, @_);
    }
}

package View;   # mix in to observer classes of any kind

sub update  # stub: do nothing
{
    my ($self, $model, @aspectData) = @_;
}


##</code><code>##

package MyModel;
@MyModel::ISA = qw(SomethingElse Model);

sub changeSomething
{
    my $self = shift;
    my $value = shift;
    $self->{something} = $value;
    $self->_changed('something', $value);
}
package MyView;

sub update
{
    my ($self, $model, $aspect, $otherStuff) = @_;
    # hey! $model has changed its $aspect!
}

package main;

my $view = bless {}, 'MyView';
my $model = bless {}, 'MyModel';

$model->addDependent($view);

# now the following
# calls $view->update($model, 'something', 123);
$model->changeSomething(123);