Overview #

In this tutorial we work in the modelling of a basic observer pattern - one of the Gang of Four behavioral patterns.

Video #

Scenario #

Lets think of a real world scenario.

Use Case #

We assume we have several news channels, such as orf.at, nzz.ch or nytimes.com that publish news from all over the world. The news channels get their messages from news agencies, such as Reuters or APA. However, these news agencies neither now upfront about the news channels nor how many of them want to be notified. Rather, the news channels subscribe to news from news agencies and expect to be notified by them as soon as important news come up. The NewsChannels unsubscribe from the NewsAgency when they do not want to be notified about news anymore.

Such a scenario is a nice example for illustrating the observer pattern.

Modelling #

Lets start with modelling the observer pattern in a class diagram.

Classes #

Initially, we have the NewsAgency class and the NewsChannel class.

Identity #

NewsAgencies and NewsChannels have a name that gives the instances of these classes a human-readable identity.

Constructors #

The name is initialized by the constructor of each class.

Notify #

NewsAgencies broadcast news and NewsChannels are notified about news.

Subscribe #

To allow NewsChannels to subscribe to news from NewsAgencies, NewsAgencies have a subscribe method.

Unsubscribe #

In addition, news agencies have an unsubscribe method, so that NewsChannels can decide not get notified by the NewsAgency anymore.

Association #

For a NewsAgency to be able to notify all the registered NewsChannels, it is associated with the respective NewsChannels.

Observer Pattern #

In terms of the Gang of Four (GoF) patterns, the NewsAgency is the Observable and the NewsChannel is the Observer.

Publisher / Subscriber #

Other terms related to this concept are publisher for the NewsAgency and subscriber for the NewsChannel.

Summary #

With this nice class diagram we have modeled a basic observer.