I’ve been reading Head First Design Patterns recently and have decided to write some of the patterns as PHP examples for my own benefit. The first one that I’ve decided to code up is the Observer Pattern. The formal definition of the Observer Pattern is:
The observer pattern (a subset of the asynchronous publish/subscribe pattern) is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems.
As systems become more loosely coupled making sure that when an event happens all systems that require knowledge of these updates are informed. For example, a blog post, after saving a post we may need to update a search engine (e.g. Lucene), update our sitemap, tags, email subscribed users, etc. The observer pattern allows developers to add additional listeners without editing their observable object. By injecting observers (i.e. a search engine update observer, a sitemap generator, etc) into a subject (i.e. blog post editing system) we can allow the it to perform all the necessary updates without any changes.