Editing Observer pattern (section)

{{Short description|Software design pattern based on an event-updated object with a list of dependents}}

In [[software design]] and [[Software engineering|engineering]], the '''observer pattern''' is a [[software design pattern]] in which an [[Object (computer science)#Objects in object-oriented programming|object]], named the '''''subject''''', maintains a list of its dependents, called '''''observers''''', and notifies them automatically of any [[Event (computing)|state changes]], usually by calling one of their [[Method (computer science)|methods]].

It is often used for implementing distributed [[event handling|event-handling]] systems in [[Event-driven programming|event-driven software]]. In such systems, the subject is usually named a "stream of events" or "stream source of events" while the observers are called "sinks of events." The stream nomenclature alludes to a physical setup in which the observers are physically separated and have no control over the emitted events from the subject/stream source. This pattern thus suits any process by which data arrives from some input that is not available to the [[CPU]] at [[Booting|startup]], but instead may arrive at arbitrary or indeterminate times ([[HTTP request]]s, [[GPIO]] data, [[user input]] from [[peripheral]]s and [[distributed database]]s, etc.).