Example
In general, an observer is a class with a specific method being called when an action on the observed object occurs. In certain situations, closures can be enough to implement the observer design pattern.
Here is a detailed example of such an implementation. Let's first declare a class whose purpose is to notify observers when its property is changed.
<?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();
}
}
Then, let's declare the class that will represent the different observers.
<?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);
}
}
Let's finally test this:
<?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!
Note that this example works because the observers share the same nature (they are both "named observers.")
