【PHP 設計模式】觀察者模式 Observer Pattern

/**
 * Class Island.
 */
class Island implements SplSubject
{
    /**
     * 用來存放觀察者名單。
     * 
     * @var SplObjectStorage
     */
    protected SplObjectStorage $observers;

    /**
     * Island constructor.
     */
    public function __construct()
    {
        $this->observers = new SplObjectStorage();
    }

    /**
     * 賦予觀察者物件。
     * 
     * @param SplObserver $observer
     */
    public function attach(SplObserver $observer)
    {
        $this->sendMessages("有玩家加入了！");
        $this->observers->attach($observer);
    }

    /**
     * 抽離觀察者物件。
     * 
     * @param SplObserver $observer
     */
    public function detach(SplObserver $observer)
    {
        $this->observers->detach($observer);
        $this->sendMessages("有玩家離開了！");
    }

    /**
     * 通知觀察者。
     */
    public function notify()
    {
        foreach ($this->observers as $observer) {
            $observer->update();
        }
    }

    /**
     * 發佈訊息給所有的觀察者。
     * 
     * @param string $message
     */
    public function sendMessages(string $message)
    {
        foreach ($this->observers as $observer) {
            $observer->sendMessage($message);
        }
    }
}

/**
 * Class PlayerObserver.
 */
class PlayerObserver implements SplObserver
{
    /**
     * @var string
     */
    protected string $user;

    /**
     * @var SplSubject[]
     */
    protected array $observers = [];
    
    /**
     * PlayerObserver constructor.
     * 
     * @param string $user
     */
    public function __construct(string $user)
    {
        $this->user = $user;
    }

    /**
     * @param SplSubject $subject
     */
    public function update(SplSubject $subject)
    {
        // TODO: Implement update() method.
    }

    /**
     * @param string $message
     */
    public function sendMessage(string $message)
    {
        echo "[$this->user 收到通知] $message";
    }
}

SplSubject {
    /* Methods */
    abstract public attach ( SplObserver $observer ) : void
    abstract public detach ( SplObserver $observer ) : void
    abstract public notify ( void ) : void
}

SplObserver {
    /* Methods */
    abstract public update ( SplSubject $subject ) : void
}

SplObjectStorage implements Countable , Iterator , Serializable , ArrayAccess {
    /* Methods */
    public addAll ( SplObjectStorage $storage ) : void
    public attach ( object $object [, mixed $data = NULL ] ) : void
    public contains ( object $object ) : bool
    public count ( void ) : int
    public current ( void ) : object
    public detach ( object $object ) : void
    public getHash ( object $object ) : string
    public getInfo ( void ) : mixed
    public key ( void ) : int
    public next ( void ) : void
    public offsetExists ( object $object ) : bool
    public offsetGet ( object $object ) : mixed
    public offsetSet ( object $object [, mixed $data = NULL ] ) : void
    public offsetUnset ( object $object ) : void
    public removeAll ( SplObjectStorage $storage ) : void
    public removeAllExcept ( SplObjectStorage $storage ) : void
    public rewind ( void ) : void
    public serialize ( void ) : string
    public setInfo ( mixed $data ) : void
    public unserialize ( string $serialized ) : void
    public valid ( void ) : bool
}

/**
 * Class ObserverPatternTest.
 */
class ObserverPatternTest extends TestCase
{
    /**
     * @test
     */
    public function test_observer()
    {
        $this->expectOutputString(implode(array(
            "[Player A 收到通知] 有玩家加入了！",
            "[Player A 收到通知] 有玩家加入了！",
            "[Player B 收到通知] 有玩家加入了！",
            "[Player A 收到通知] 有玩家離開了！",
            "[Player C 收到通知] 有玩家離開了！",
        )));
        
        /**
         * 建立一個島嶼
         */
        $island = new Island();

        /**
         * Player A 加入了這座島嶼
         * 加入前島上沒有玩家
         * 所以沒有叮咚通知
         */
        $playerA = new PlayerObserver('Player A');
        $island->attach($playerA);

        /**
         * Player B 加入了這座島嶼
         * 加入前島上有 1 位玩家
         * 扣除自己後，會有 A 收到叮咚通知
         */
        $playerB = new PlayerObserver('Player B');
        $island->attach($playerB);

        /**
         * Player C 加入了這座島嶼
         * 加入前島上有 2 位玩家
         * 扣除自己後，會有 A、B 收到叮咚通知
         */
        $playerC = new PlayerObserver('Player C');
        $island->attach($playerC);

        /**
         * Island_B 離開了這座島嶼
         * 離開前島上有 3 位玩家
         * 扣除自己後，會有 A、C 收到叮咚通知
         */
        $island->detach($playerB);
    }
}