怎么独立改变php设计模式 Mediator (中介者模式)的交互

/**
* 中介者模式
*
* 用一个中介对象来封装一系列的对象交互,使各对象不需要显式地相互引用从而使其耦合松散,而且可以独立地改变它们之间的交互
*/
abstract class Mediator
{
abstract public function send($message,$colleague);
}
abstract class Colleague
{
private $_mediator = null;
public function Colleague($mediator)
{
$this->_mediator = $mediator;
}
public function send($message)
{
$this->_mediator->send($message,$this);
}
abstract public function notify($message);
}
class ConcreteMediator extends Mediator
{
private $_colleague1 = null;
private $_colleague2 = null;
public function send($message,$colleague)
{
if($colleague == $this->_colleague1)
{
$this->_colleague1->notify($message);
} else {
$this->_colleague2->notify($message);
}
}
public function set($colleague1,$colleague2)
{
$this->_colleague1 = $colleague1;
$this->_colleague2 = $colleague2;
}
}
class Colleague1 extends Colleague
{
public function notify($message)
{
echo "Colleague1 Message is :".$message."
";
}
}
class Colleague2 extends Colleague
{
public function notify($message)
{
echo "Colleague2 Message is :".$message."
";
}
}
//
$objMediator = new ConcreteMediator();
$objC1 = new Colleague1($objMediator);
$objC2 = new Colleague2($objMediator);
$objMediator->set($objC1,$objC2);
$objC1->send("to c2 from c1");
$objC2->send("to c1 from c2");