Observer 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.

This pattern is mainly used as a poor man’s event handling system. In some programming languages, the issues addressed by this pattern are handled in the native event handling syntax.

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.)

The UML diagram below illustrates this structure:

When the event is raised each observer receives a callback (which may be either a virtual function of the observer class (called notify on the diagram) 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). See Event-driven programming.
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.

Pseudocode

This pseudocode, written in a Python-esque syntax, demonstrates the observer pattern.

class Listener:
    def initialise(self):
        subject.register(self)

    def notify(self, event):
        event.process()

class Subject:
    def initialise(self):
        self.listeners = []

    def register(self, listener):
        self.listeners.append(listener)

    def unregister(self, listener):
        self.listeners.remove(listener)

    def notify_listeners(self, event):
        for listener in self.listeners:
            listener.notify(event)

subject = Subject()
listenerA = Listener()
listenerB = Listener()
#the subject now has two listeners registered to it.

Implementations

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

Some of the most notable implementations of this pattern:

The Java Swing library makes extensive use of the observer pattern for event management
Boost.Signals, an extension of the C++ STL providing a signal/slot model
The Qt C++ framework's signal/slot model
libsigc++ - the C++ signalling template library.
sigslot - C++ Signal/Slot Library
XLObject - Template-based C++ signal/slot model patterned after Qt.
GLib - an implementation of objects and signals/callbacks in C. (This library has many bindings to other programming languages.)
Exploring the Observer Design Pattern - the C Sharp (C#) and Visual Basic .NET implementation, using delegates and the Event pattern
Using the Observer Pattern, a discussion and implementation in REALbasic
flash.events, a package in ActionScript 3.0, (following from the mx.events package in ActionScript 2.0).

External links