You can do this using required methods in the role and treating them as delegated "events". This is actually a pretty common technique in the Cocoa framework used by Mac OSX and the iPhone OS. Here is how that might look. 

package RolePage;
use Moose::Role;

has textFont => (
  is      => 'rw',
  isa     => 'Str',
  default => 'Courier',
  trigger => sub {
    my $self = shift;
    $self->text_attribute_changed
  }
);

has textSize => (
  is      => 'rw',
  isa     => 'Int',
  default => 12,
  trigger => sub {
    my $self = shift;
    $self->text_attribute_changed
  }
);

requires 'text_attribute_changed';

[download]
Then in your class you can do: 
package CairoPage;

use Moose;
use Cairo;
with 'RolePage';

has cairoFont => (
  is  => 'bare',
  isa => 'Cairo::ScaledFont',
);

sub text_attribute_changed {
    # ... do whatever you need to here ...
}

[download]

-stvn

In reply to Re: Moose: possible way to implement trigger in other packages? by stvn
in thread Moose: possible way to implement trigger in other packages? by llancet

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