The Observer pattern

The Observer pattern describes a publish-subscribe relationship between a single object, the publisher, which is also known as the subject or observable, and one or more objects, the subscribers, also known as observers. So, the subject notifies the subscribers of any state changes, typically by calling one of their methods.

The ideas behind the Observer pattern are the same as those behind the separation of concerns principle, that is, to increase decoupling between the publisher and subscribers, and to make it easy to add/remove subscribers at runtime.

Real-world examples

Dynamics in an auction are similar to the behavior of the Observer pattern. Every auction bidder has a number paddle that is raised whenever they want to place a bid. Whenever the paddle is raised by a bidder, the auctioneer acts as the subject by updating the price of the bid and broadcasting the new price to all bidders (subscribers).

In software, we can cite at least two examples...