Observer pattern

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.

Description

The Observer pattern is mainly used to implement distributed event handling systems.

It is also a key part in the model–view–controller (MVC) architectural pattern.

The observer pattern is implemented in numerous programming libraries and systems, including almost all GUI toolkits.

Memory leaks

The observer pattern can cause memory leaks, known as the lapsed listener problem.

The observer pattern requires both explicit registration and explicit deregistration (as in the dispose pattern), because the subject holds strong references to the observers, keeping them alive.

Leakage can be prevented by the subject holding weak references to the observers.

Related patterns

Related patterns include:

• Dispose pattern
• Publish–subscribe pattern
• Mediator pattern
• Singleton pattern

See also

• Lapsed listener problem
• Model–view–controller
• Observable versus promise
• ReactiveX
• Software design pattern
• Zen Cart observer

External links

• Observer pattern @ Wikipedia