Observer Design Pattern

The observer pattern (sometimes known as publish/subscribe) is a design pattern used in computer programming to observe the state of an object in a program. It is related to the principle of Implicit invocation.

The essence of this pattern is that one or more objects (called observers or listeners) are registered (or register themselves) to observe an event which may be raised by the observed object (the subject). (The object which may raise an event generally maintains a collection of the observers.)

When the event is raised each observer receives a callback (which may be either a virtual function of the observer class or a function pointer (more generally a function object or "functor") passed as an argument to the listener registration method). The notify function may also be passed some parameters (generally information about the event that is occurring) which can be used by the observer.

Each concrete observer implements the notify function and as a consequence defines its own behavior when the notification occurs.

The subject normally has a register method for adding a new listener and an unregister method for removing an observer from the list of objects to be notified when the event is raised. It may also have methods for temporarily disabling and then reenabling calls to prevent inefficient cascading of a number of related updates. This is sometimes referred to as event compression.

In systems where the observer pattern is used extensively, a mechanism to prevent looping is usually implemented. Suppose that A updates B when event X is raised, and B subsequently updates A inside of the handler for event X in B in such a way that it causes A to raise event X again. It is desirable in such situations that A not raise event X again while event X is already in the process of being raised.

Typical usages

The typical usages of the observer pattern:

• Listening for an external event (such as a user action).
• Listening for changes of the value of a property of an object.

Note that often callbacks called in response of a property value change also change values of some properties, so sometimes causing an event cascade. See this article for a discussion about using observer pattern for watching over changes of properties and updating other properties accordingly.

The observer pattern is also very often associated with the Model-view-controller (MVC) paradigm. In MVC, the observer pattern is used to create a loose coupling between the model and the view. Typically, a modification in the model triggers the notification of model observers which are actually the views.