使用閉包來實現觀察者模式

通常，觀察者是在對被觀察物件發生動作時呼叫特定方法的類。在某些情況下，閉包足以實現觀察者設計模式。

以下是此類實現的詳細示例。讓我們首先宣告一個類，其目的是在其屬性發生更改時通知觀察者。

<?php

class ObservedStuff implements SplSubject
{
    protected $property;
    protected $observers = [];

    public function attach(SplObserver $observer)
    {
        $this->observers[] = $observer;
        return $this;
    }

    public function detach(SplObserver $observer)
    {
        if (false !== $key = array_search($observer, $this->observers, true)) {
            unset($this->observers[$key]);
        }
    }

    public function notify()
    {
        foreach ($this->observers as $observer) {
            $observer->update($this);
        }
    }

    public function getProperty()
    {
        return $this->property;
    }

    public function setProperty($property)
    {
        $this->property = $property;
        $this->notify();
    }
}

然後，讓我們宣告代表不同觀察者的類。

<?php

class NamedObserver implements SplObserver
{
    protected $name;
    protected $closure;

    public function __construct(Closure $closure, $name)
    {
        $this->closure = $closure->bindTo($this, $this);
        $this->name = $name;
    }

    public function update(SplSubject $subject)
    {
        $closure = $this->closure;
        $closure($subject);
    }
}

讓我們最終測試一下：

<?php

$o = new ObservedStuff;

$observer1 = function(SplSubject $subject) {
    echo $this->name, ' has been notified! New property value: ', $subject->getProperty(), "\n";
};

$observer2 = function(SplSubject $subject) {
    echo $this->name, ' has been notified! New property value: ', $subject->getProperty(), "\n";
};

$o->attach(new NamedObserver($observer1, 'Observer1'))
  ->attach(new NamedObserver($observer2, 'Observer2'));

$o->setProperty('Hello world!');
// Shows:
// Observer1 has been notified! New property value: Hello world!
// Observer2 has been notified! New property value: Hello world!

請注意，此示例有效，因為觀察者具有相同的性質（它們都是命名觀察者。）