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Concept
Observer pattern
Summary
In software design and engineering, the observer pattern is a software design pattern in which an object, named 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 often used for implementing distributed event-handling systems in 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 startup, but instead arrives seemingly at random (HTTP requests, GPIO data, user input from peripherals, distributed databases and blockchains, etc.).
Most modern programming languages comprise built-in event constructs implementing the observer-pattern components. While not mandatory, most observer implementations use background threads listening for subject events and other support mechanisms provided by the kernel.
The observer design pattern is a behavioural pattern listed among the 23 well-known "Gang of Four" design patterns that address recurring design challenges in order to design flexible and reusable object-oriented software, yielding objects that are easier to implement, change, test and reuse.
The observer pattern addresses the following problems:
A one-to-many dependency between objects should be defined without making the objects tightly coupled.
When one object changes state, an open-ended number of dependent objects should be updated automatically.
An object can notify multiple other objects.
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 couples the subject to particular dependent objects.
Official source
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