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) = @_;
}