Observer pattern

The observer pattern is a software design pattern in which an object, called 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 mainly used to implement distributed event handling systems, in "event driven" software. Most modern languages such as Java and C# have built in "event" constructs which implement the observer pattern components, for easy programming and short code.

The observer pattern is also a key part in the familiar model–view–controller (MVC) architectural pattern. The observer pattern is implemented in numerous programming libraries and systems, including almost all GUI toolkits.

Overview

The Observer design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

What problems can the Observer design pattern solve?

• A one-to-many dependency between objects should be defined without making the objects tightly coupled.
• It should be ensured that when one object changes state an open-ended number of dependent objects are updated automatically.
• It should be possible that one object can notify an open-ended number of 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 commits (tightly couples) the subject to particular dependent objects. Tightly coupled objects are hard to implement, change, test, and reuse because they refer to and know about (how to update) many different objects with different interfaces.

What solution does the Observer design pattern describe?

• Define Subject and Observer objects
• so that when a subject changes state, all registered observers are notified and updated automatically.

The sole responsibility of a subject is to maintain a list of observers and to notify them of state changes by calling their update() operation.
The responsibility of observers is to register (and unregister) themselves on a subject (to get notified of state changes) and to update their state (synchronize their state with subject's state) when they are notified.
This makes subject and observers loosely coupled. Subject and observers have no explicit knowledge of each other. Observers can be added and removed independently at run-time.
This notification-registration interaction is also known as publish-subscribe.

See also the UML class and sequence diagram below.

Strong vs. Weak reference

The observer pattern can cause memory leaks, known as the lapsed listener problem, because in basic implementation it requires both explicit registration and explicit deregistration, as in the dispose pattern, because the subject holds strong references to the observers, keeping them alive. This can be prevented by the subject holding weak references to the observers.

Coupling and typical pub-sub implementations

Typically, the observer pattern is implemented with the "subject" (which is being "observed") being part of the object whose state change is being observed, to be communicated to the observers upon occurrence. This type of implementation is considered "tightly coupled", forcing both the observers and the subject to be aware of each other and have access to their internal parts, creating possible issues of scalability, speed, message recovery and maintenance (also called event or notification loss), the lack of flexibility in conditional dispersion and possible hindrance to desired security measures. In some (non-polling) implementations of the publish-subscribe pattern (also called the pub-sub pattern), this is solved by creating a dedicated "message queue" server and at times an extra "message handler" object, as added stages between the observer and the observed object whose state is being checked, thus "decoupling" the software components. In these cases, the message queue server is accessed by the observers with the observer pattern, "subscribing to certain messages" knowing only about the expected message (or not, in some cases), but knowing nothing about the message sender itself, and the sender may know nothing about the receivers. Other implementations of the publish-subscribe pattern, which achieve a similar effect of notification and communication to interested parties, do not use the observer pattern altogether.

Still, in early implementations of multi-window operating systems like OS2 and Windows, the terms "publish-subscribe pattern" and "event driven software development" were used as a synonym for the observer pattern.

The observer pattern, as described in the GOF book, is a very basic concept and does not deal with observance removal or with any conditional or complex logic handling to be done by the observed "subject" before or after notifying the observers. The pattern also does not deal with recording the "events", the asynchronous passing of the notifications or guaranteeing their being received. These concerns are typically dealt with in message queueing systems of which the observer pattern is only a small part.

Related patterns: Publish–subscribe pattern, mediator, singleton.

Structure

UML class and sequence diagram

A sample UML class and sequence diagram for the Observer design pattern.

In the above UML class diagram, the Subject class doesn't update the state of dependent objects directly. Instead, Subject refers to the Observer interface (update()) for updating state, which makes the Subject independent of how the state of dependent objects is updated. The Observer1 and Observer2 classes implement the Observer interface by synchronizing their state with subject's state.

The UML sequence diagram shows the run-time interactions: The Observer1 and Observer2 objects call attach(this) on Subject1 to register themselves. Assuming that the state of Subject1 changes, Subject1 calls notify() on itself.
notify() calls update() on the registered Observer1 and Observer2 objects, which request the changed data (getState()) from Subject1 to update (synchronize) their state.

UML class diagram

UML class diagram of Observer pattern

Example

Below is an example written in Java that takes keyboard input and treats each input line as an event. The example is built upon the library classes java.util.Observer and java.util.Observable. When a string is supplied from System.in, the method notifyObservers is then called, in order to notify all observers of the event's occurrence, in the form of an invocation of their 'update' methods.

import java.util.Observable;
import java.util.Scanner;

class EventSource extends Observable implements Runnable {
    public void run() {
        while (true) {
            String response = new Scanner(System.in).next();
            setChanged();
            notifyObservers(response);
        }
    }
}

import java.util.Observable;
import java.util.Observer;

public class MyApp {
    public static void main(String[] args) {
        System.out.println("Enter Text: ");
        EventSource eventSource = new EventSource();

        eventSource.addObserver(new Observer() {
            public void update(Observable obj, Object arg) {
                System.out.println("Received response: " + arg);
            }
        });

        new Thread(eventSource).start();
    }
}
/*
interface Observer{
    public void update(Observable obj, Object arg);
}*/

A similar example in Python:

class Observable:
    def __init__(self):
        self.__observers = []
    
    def register_observer(self, observer):
        self.__observers.append(observer)
    
    def notify_observers(self, *args, **kwargs):
        for observer in self.__observers:
            observer.notify(self, *args, **kwargs)

class Observer:
    def __init__(self, observable):
        observable.register_observer(self)
    
    def notify(self, observable, *args, **kwargs):
        print('Got', args, kwargs, 'From', observable)


subject = Observable()
observer = Observer(subject)
subject.notify_observers('test')

See also

• Implicit invocation
• Client–server model
• The observer pattern is often used in the entity–component–system pattern

External links

• Observer implementations in various languages at Wikibooks