Use cases

We generally use the Observer pattern when we want to inform/update one or more objects (observers/subscribers) about a change that happened to another object (subject/publisher/observable). The number of observers as well as who the observers are may vary and can be changed dynamically (at runtime).

We can think of many cases where Observer can be useful. One such case was already mentioned at the start of this chapter: news feeds. Whether it is RSS, Atom, or another format, the idea is the same; you follow a feed, and every time it is updated, you receive a notification about the update [Zlobin13, page 60].

The same concept exists in social networking. If you are connected to another person using a social networking service, and your connection updates something, you are notified about it. It doesn't matter if the connection is a Twitter user that you follow, a real friend on Facebook, or a business colleague on LinkedIn.

Event-driven systems are another example where Observer can...