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What is KVO   Installation   Usage   API

What is KVO ?

KVO (Key Value Observing) is a design pattern which allows an object to get notified about changes.
It allow you keep your object synchronized each other without creating a hard link thanks to Subject/Observer design pattern.
KVC (Key Value Coding) and KVO (Key Value Observing) are heavily used in Cocoa Framework (Objective-C)

Installation

The simplest way to install the library is to use Composer:

composer require macfja/php-kvo

Usage

class Downloader extends AbstractObservable
{
  protected $progress;
  public function getProgress()
  {
    return $this->progress;
  }
  protected function receiveCallback($newProgress)
  {
    $this->setValueForKey('progress', $newProgress);
    // ... do something with the data
  }
  public function download()
  {
    // ... start the download
  }
}

class ProgressDisplay implements Listener
{
  public function observeValueForKeyPath($keyPath, $object, $change, &$context)
  {
    if ($keyPath == 'progress') {
      echo sprintf('Download in progress (%d%%)%s', $change[Observer::CHANGE_NEW], PHP_EOL);
    }
  }
}

$downloader = new Downloader();
$progress = new ProgressDisplay();
$downloader->addObserverForKey($progress, 'progress', Observer::OPTION_NEW|Observer::OPTION_INITIAL);
$downloader->download()

A complete examples can be found in the directory examples.

API

API of Observable interface

Implemented in AbstractObservable, Proxy.
A trait for quick implementation is available: ObservableTrait

API of Observable::addObserverForKey method

This method allow you to subscribe to key value changes notification.

API, The Observable::addObserverForKey options list.

• Observer::OPTION_NEW, Indicates that the change array should provide the new attribute value, if applicable.

• Observer::OPTION_OLD, Indicates that the change array should contain the old attribute value, if applicable.

• Observer::OPTION_INITIAL, If specified, a notification should be sent to the observer immediately, before the observer registration method even returns.

  The change array in the notification will always contain an Observer::CHANGE_NEW entry if Observer::OPTION_NEW is also specified but will never contain an Observer::CHANGE_OLD or Observer::CHANGE_REQUESTED entry.
  (In an initial notification the current value of the observed property may be old, but it's new to the observer.)

• Observer::OPTION_PRIOR, Whether separate notifications should be sent to the observer before and after each change, instead of a single notification after the change.

  The change array in a notification sent before a change always contains an Observer::CHANGE_PRIOR entry whose value is true, but never contains an Observer::CHANGE_NEW entry. When this option is specified the change array in a notification sent after a change contains the same entries that it would contain if this option were not specified. You can use this option when the observer's own key-value observing-compliance requires it to invoke the willChangeValueForKey method for one of its own properties, and the value of that property depends on the value of the observed object's property.

API of Observable::willChangeValueForKey method

This method trigger a notification for all Listener that registered for a key with the option Observer::OPTION_PRIOR.
This method should be call before the key value change.

API of Observable::didChangeValueForKey method

This method trigger a notification for all Listener that registered for a key without the option Observer::OPTION_PRIOR.
This method should be call after the key value change.

API of Observer::setValueForKey method

This method change the value of a key and handle the call of methods Observable::willChangeValueForKey and Observable::didChangeValueForKey.

API, the Observable::willChangeValueForKey and Observable::didChangeValueForKey source value

• Observer::SOURCE_SETTER, If the value of the key was changed from a setter method. (Used by Proxy class)
• Observer::SOURCE_PROPERTY, If the value of the key was changed from a direct property change (public class property). (Used by Proxy class)
• Observer::SOURCE_CUSTOM, If the value was change without a setter, or from a direct property access.

Note: You can use your own source type, it's just a string.

Note: Observer::SOURCE_INITIAL, If the Listener has the option Observer::OPTION_INITIAL, then when registered, this source is used

API of Listener interface

API of Listener::observeValueForKeyPath method

This method is call every time an observed key is modified.

API, The Listener::observeValueForKeyPath change array

• Observer::CHANGE_NEW, If the Observer::OPTION_NEW was specified when the observer was registered, the value of this key is the new value for the attribute.

• Observer::CHANGE_OLD, If the Observer::OPTION_OLD was specified when the observer was registered, the value of this key is the value before the attribute was changed.

• Observer::CHANGE_PRIOR, If the option Observer::OPTION_PRIOR was specified when the observer was registered this notification is sent prior to a change.

  The change array contains an Observer::CHANGE_PRIOR entry whose value is true if the nofication is before the change, or false if the notification is after.