Observer

Intent

When an object changes, notify all interested parties

Motivation

events, synchronisation, ...

Implementation

Use hooks pattern at and after to implement the observer

No implementation needed in the Subject class

Example

//initialisation
$hs = new hooks();
$observer = new interested_party();
$change_method = array($object, 'setter');

//setup change point(can be done externally from $change method)
$hs->at('change', $change_method);

//express interest
$hs->after('change', array($obesrver, 'callback'));

$hs->run($arg);

Note

The precise implementation of the hooks may differ, depending on functionality, abstraction, sugar, ... Have a look at the drupal hooks as well.