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Observer pattern

Software design pattern based on an event-updated object with a list of dependents

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This pattern creates a one-to-many dependency where multiple observers can listen to a single subject, but the coupling is typically synchronous and direct—the subject calls observer methods when changes occur, though asynchronous implementations using event queues are possible. Unlike the publish-subscribe pattern, there is no intermediary broker; the subject and observers have direct references to each other.

It is commonly used to implement event handling systems in event-driven programming, particularly in-process systems like GUI toolkits or MVC frameworks. This makes the pattern well-suited to processing data that arrives unpredictably—such as user input, HTTP requests, GPIO signals, updates from distributed databases, or changes in a GUI model.

Overview

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 requirements:

  • 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.

The naive approach would be for one object (subject) to directly call specific methods on each dependent object. This creates tight coupling because the subject must know the concrete types and specific interfaces of all dependent objects, making the code inflexible and hard to extend. However, this direct approach may be preferable in performance-critical scenarios (such as low-level kernel structures or real-time systems) where the overhead of abstraction is unacceptable and compile-time optimization is crucial.

The observer pattern provides a more flexible alternative by establishing a standard notification protocol:

  1. Define Subject and Observer objects with standardized interfaces.
  2. When a subject changes state, all registered observers are notified and updated automatically.
  3. The subject manages its own state while also maintaining a list of observers and notifying them of state changes by calling their update() operation.
  4. The responsibility of observers is to register and unregister themselves with a subject (in order to be notified of state changes) and to update their state (to synchronize it with the subject's state) when they are notified.

This approach makes subject and observers loosely coupled through interface standardization. The subject only needs to know that observers implement the update() method—it has no knowledge of observers' concrete types or internal implementation details. Observers can be added and removed independently at run time.

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