行为型设计模式-中介者 Mediator

简介

通过引入一个中介者，将对象之间两两交互的情景，改为每个对象都是与中介者交互，即将网状关系改成了星状关系。缺点是中介者逻辑会很臃肿。

当系统中出现多对多的情况时，应该先考虑设计是否合理，而尽量不要直接使用中介者。

角色

• 抽象Mediator中介者
• 具体Mediator中介者
• 抽象Component
• 具体Component

类图

中介者需要了解所有Component，而且notify中要编写处理所有可能发生的情况，Mediator是一个非常臃肿的类。

代码

interface Mediator
{
    public function notify(object $sender, string $event): void;
}

class ConcreteMediator implements Mediator
{
    private $component1;

    private $component2;

    public function __construct(Component1 $c1, Component2 $c2)
    {
        $this->component1 = $c1;
        $this->component1->setMediator($this);
        $this->component2 = $c2;
        $this->component2->setMediator($this);
    }

    public function notify(object $sender, string $event): void
    {
        if ($event == "A") {
            echo "Mediator reacts on A and triggers following operations:\n";
            $this->component2->doC();
        }

        if ($event == "D") {
            echo "Mediator reacts on D and triggers following operations:\n";
            $this->component1->doB();
            $this->component2->doC();
        }
    }
}

class BaseComponent
{
    protected $mediator;

    public function __construct(Mediator $mediator = null)
    {
        $this->mediator = $mediator;
    }

    public function setMediator(Mediator $mediator): void
    {
        $this->mediator = $mediator;
    }
}

class Component1 extends BaseComponent
{
    public function doA(): void
    {
        echo "Component 1 does A.\n";
        $this->mediator->notify($this, "A");
    }

    public function doB(): void
    {
        echo "Component 1 does B.\n";
        $this->mediator->notify($this, "B");
    }
}

class Component2 extends BaseComponent
{
    public function doC(): void
    {
        echo "Component 2 does C.\n";
        $this->mediator->notify($this, "C");
    }

    public function doD(): void
    {
        echo "Component 2 does D.\n";
        $this->mediator->notify($this, "D");
    }
}

$c1 = new Component1();
$c2 = new Component2();
$mediator = new ConcreteMediator($c1, $c2);

echo "Client triggers operation A.\n";
$c1->doA();

echo "\n";
echo "Client triggers operation D.\n";
$c2->doD();

output:

Client triggers operation A.
Component 1 does A.
Mediator reacts on A and triggers following operations:
Component 2 does C.

Client triggers operation D.
Component 2 does D.
Mediator reacts on D and triggers following operations:
Component 1 does B.
Component 2 does C.