Event listeners

The observer 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, in "event driven" software. Most modern languages such as C# have built in "event" constructs which implement the observer pattern components, for easy programming and short code.

The observer pattern is also a key part in the familiar model–view–controller (MVC) architectural pattern. The observer pattern is implemented in numerous programming libraries and systems, including almost all GUI toolkits.

The Observer design pattern is one of the twenty-three well-known "Gang of Four" design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

The Observer pattern addresses the following problems:

Defining a one-to-many dependency between objects by defining one object (subject) that updates the state of dependent objects directly is inflexible because it commits (tightly couples) the subject to particular dependent objects. Tightly coupled objects are hard to implement, change, test, and reuse because they refer to and know about (how to update) many different objects with different interfaces.

The sole responsibility of a subject is to maintain a list of observers and to notify them of state changes by calling their update() operation.
The responsibility of observers is to register (and unregister) themselves on a subject (to get notified of state changes) and to update their state (synchronize their state with subject's state) when they are notified.
This makes subject and observers loosely coupled. Subject and observers have no explicit knowledge of each other. Observers can be added and removed independently at run-time.
This notification-registration interaction is also known as publish-subscribe.

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