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Observer

A PHP library that implements the observer pattern.

Summary

This library provides an implementation of the observer pattern. You can use it to create other libraries such as event managers, state machines, MVC frameworks, and even provide a plugin system for application.

Requirement

• PHP >= 5.3.9
• Phine Exception >= 1.0.0

Installation

Via Composer:

$ composer require "phine/observer=~2.0"

Usage

To create a subject, you will need to either create your own implementation of SubjectInterface, or use the bundled Subject class.

use Phine\Observer\Subject;

$subject = new Subject();

Observing

You will then need to create your own implementation of ObserverInterface to observe changes made to the subject. You may use multiple instances of the observer implementation, or even the same instance multiple times.

use Phine\Observer\ObserverInterface;
use Phine\Observer\SubjectInterface;

// register a few instances
$subject->registerObserver(new Message('First'));
$subject->registerObserver(new Message('Second'));

// register the same one twice
$reuse = new Message('Third');

$subject->registerObserver($reuse);
$subject->registerObserver($reuse);

// notify all observers of an update
$subject->notifyObservers();

/**
 * Simply echos a message when updated.
 */
class Message implements ObserverInterface
{
    /**
     * The message to echo.
     *
     * @var string
     */
    private $message;

    /**
     * Sets the message to echo on update.
     *
     * @param string $message The message.
     */
    public function __construct($message)
    {
        $this->message = $message;
    }

    /**
     * {@inheritDoc}
     */
    public function receiveUpdate(SubjectInterface $subject)
    {
        echo $this->message, "\n";
    }
}

With the example above, you can expect the following output:

First
Second
Third
Third

Prioritizing Observers

Implementations of SubjectInterface support prioritizing observers during registration (registerObserver()). By default, shown in all the examples provided, the priority is SubjectInterface::FIRST_PRIORITY (which is 0, zero). You may, however, specify your own priority:

$subject->registerObserver(new Message('A'), SubjectInterface::LAST_PRIORITY);
$subject->registerObserver(new Message('B'), 789);
$subject->registerObserver(new Message('C'), 123);
$subject->registerObserver(new Message('D'), 456);
$subject->registerObserver(new Message('E'), SubjectInterface::FIRST_PRIORITY);

With the above example, you can expect the following output:

E
C
D
B
A

When a subject updates its observers it begins at priority 0 (zero), and works its way to PHP_INT_MAX (the lowest possible priority). If multiple observers are registered using the same provider, they will be updated in the order that they were registered.

Interrupting an Update

When a subject is in the process of updating its registered observers, an observer may interrupt the subject. An interrupt is performed by an observer when it calls the SubjectInterface::interruptUpdate() method.

use Phine\Observer\Exception\ReasonException;

/**
 * Simply interrupts the subject in the middle of an update.
 */
class InterruptingCow implements ObserverInterface
{
    /**
     * Interrupts the update.
     */
    public function receiveUpdate(SubjectInterface $subject)
    {
        // do some work

        $subject->interruptUpdate(
            new ReasonException('MOOOOO')
        );

        // do some final work
    }
}

Using the following example:

// create a new subject
$subject = new Subject();

// register some observers
$subject->registerObserver(new Message('So what did the interrupt cow say?'));
$subject->registerObserver(new InterruptingCow());
$subject->registerObserver(new Message('We never get this far.'));

// notify the observers
$subject->notifyObservers();

You can expect the following output:

So what did the interrupting cow say?
PHP Fatal error:  Uncaught exception '[...]' with message 'MOOOOO' [...]
[...]

Observers are not required to provide a reason (instance of ReasonException), but it will definitely help during the debugging process if one is given.

Collections of Subjects

There may be occasions where you will need to manage a collection of subjects. The library provides two ways of doing so: Collection and ArrayCollection. The Collection will associate an individual subject with a specific unique identifier.

use Phine\Observer\Collection;

// create a new collection
$collection = new Collection();

// register a few subjects
$collection->registerSubject('one', new Subject());
$collection->registerSubject('two', new Subject());
$collection->registerSubject('three', new Subject());

You can then retrieve the subjects or replace them as needed.

// replace one
$collection->registerSubject('two', new Subject());

// update observers of another
$collection->getSubject('three')->notifyObservers();

The ArrayCollection class provides a leaner way of managing subject registrations. It is an extension of the Collection class that supports array access through ArrayCollectionInterface.

use Phine\Observer\ArrayCollection;

// create a new collection
$collection = new ArrayCollection();

// register a few subjects
$collection['one'] = new Subject();
$collection['two'] = new Subject();
$collection['three'] = new Subject();

Like the regular Collection class, you can also replace and retrieve individual subjects.

// replace one
$collection['two'] = new Subject();

// update observers of another
$collection['three']->notifyObservers();

Documentation

You can find the API documentation here.

License

This library is available under the MIT license.