thanks to http://stackoverflow.com/users/1010632/david-barker now I got the answer : this is my observer :

class NewsObserver {

protected $events;

public function __construct(Dispatcher $dispatcher)
{
    $this->events = $dispatcher;
}

public function saving($model)
{
    if ($model->isDirty(['publish']))
    {
        $this->changeState($model->publish);
    }
}	

public function changeState($value)
{
    echo 'event fired and I got it';
}

}

and this is my model

use Illuminate\Events\Dispatcher; class News extends Magniloquent {

protected $fillable = [];
public $table = 'news';


public static function boot()
{
    parent::boot();

    News::observe(new NewsObserver(new Dispatcher));
}	

}