Observer
Observer Pattern
The Observer is a design pattern used in computer programming. The Observer pattern has two parts, the subject and the observer. The pattern has a one-to-many dependency between a subject object and the observer object(s). All the observer objects are updated when a subject changes state. It is used extensively in the AWT/Swing Listeners in Java.
UML Example
![[1]](http://www.dofactory.com/Patterns/Diagrams/observer.gif){kind=link}
Structure
Subject: Knows its observers and provides an interface for attaching and detaching observers
Observer: Defines an updating interface for objects that should be notified
ConcreteSubject: Stores states of ConcreteObservers objects. Notifies other objects when a change has been made.
ConcreteObserver: Holds a reference to the ConcreteSubjects object.
Code Examples
C# .NET
// Observer pattern -- Structural example
using System;
using System.Collections;
namespace DoFactory.GangOfFour.Observer.Structural
{
// MainApp test application
class MainApp
{
static void Main()
{
// Configure Observer pattern
ConcreteSubject s = new ConcreteSubject();
s.Attach(new ConcreteObserver(s,"X"));
s.Attach(new ConcreteObserver(s,"Y"));
s.Attach(new ConcreteObserver(s,"Z"));
// Change subject and notify observers
s.SubjectState = "ABC";
s.Notify();
// Wait for user
Console.Read();
}
}
// "Subject"
abstract class Subject
{
private ArrayList observers = new ArrayList();
public void Attach(Observer observer)
{
observers.Add(observer);
}
public void Detach(Observer observer)
{
observers.Remove(observer);
}
public void Notify()
{
foreach (Observer o in observers)
{
o.Update();
}
}
}
// "ConcreteSubject"
class ConcreteSubject : Subject
{
private string subjectState;
// Property
public string SubjectState
{
get{ return subjectState; }
set{ subjectState = value; }
}
}
// "Observer"
abstract class Observer
{
public abstract void Update();
}
// "ConcreteObserver"
class ConcreteObserver : Observer
{
private string name;
private string observerState;
private ConcreteSubject subject;
// Constructor
public ConcreteObserver(
ConcreteSubject subject, string name)
{
this.subject = subject;
this.name = name;
}
public override void Update()
{
observerState = subject.SubjectState;
Console.WriteLine("Observer {0}'s new state is {1}",
name, observerState);
}
// Property
public ConcreteSubject Subject
{
get { return subject; }
set { subject = value; }
}
}
}
Java
import java.util.Observable;
import java.util.Observer;
public class MessageBoard extends Observable {
private String message;
public String getMessage() {
return message;
}
public void changeMessage(String message) {
this.message = message;
setChanged();
notifyObservers(message);
}
public static void main(String[] args) {
MessageBoard board = new MessageBoard();
Student bob = new Student();
Student joe = new Student();
board.addObserver(bob);
board.addObserver(joe);
board.changeMessage("More Homework!");
}
}
class Student implements Observer {
public void update(Observable o, Object arg) {
System.out.println("Message board changed: " + arg);
}
}
Slightly more in depth Observer Pattern in Java
//[C] 2002 Sun Microsystems, Inc.---
import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.util.ArrayList;
import java.util.Iterator;
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;
public class RunObserverPattern {
public static void main(String[] arguments) {
System.out.println("Example for the Observer pattern");
System.out.println("This demonstration uses a central observable");
System.out.println(" object to send change notifications to several");
System.out.println(" JPanels in a GUI. Each JPanel is an Observer,");
System.out.println(" receiving notifcations when there has been some");
System.out.println(" change in the shared Task that is being edited.");
System.out.println();
System.out.println("Creating the ObserverGui");
ObserverGui application = new ObserverGui();
application.createGui();
}
}
class Task {
private String name = "";
private String notes = "";
private double timeRequired;
public Task() {
}
public Task(String newName, String newNotes, double newTimeRequired) {
name = newName;
notes = newNotes;
timeRequired = newTimeRequired;
}
public String getName() {
return name;
}
public String getNotes() {
return notes;
}
public double getTimeRequired() {
return timeRequired;
}
public void setName(String newName) {
name = newName;
}
public void setTimeRequired(double newTimeRequired) {
timeRequired = newTimeRequired;
}
public void setNotes(String newNotes) {
notes = newNotes;
}
public String toString() {
return name + " " + notes;
}
}
class TaskChangeObservable {
private ArrayList observers = new ArrayList();
public void addTaskChangeObserver(TaskChangeObserver observer) {
if (!observers.contains(observer)) {
observers.add(observer);
}
}
public void removeTaskChangeObserver(TaskChangeObserver observer) {
observers.remove(observer);
}
public void selectTask(Task task) {
Iterator elements = observers.iterator();
while (elements.hasNext()) {
((TaskChangeObserver) elements.next()).taskSelected(task);
}
}
public void addTask(Task task) {
Iterator elements = observers.iterator();
while (elements.hasNext()) {
((TaskChangeObserver) elements.next()).taskAdded(task);
}
}
public void updateTask(Task task) {
Iterator elements = observers.iterator();
while (elements.hasNext()) {
((TaskChangeObserver) elements.next()).taskChanged(task);
}
}
}
interface TaskChangeObserver {
public void taskAdded(Task task);
public void taskChanged(Task task);
public void taskSelected(Task task);
}
class TaskEditorPanel extends JPanel implements ActionListener,
TaskChangeObserver {
private JPanel controlPanel, editPanel;
private JButton add, update, exit;
private JTextField taskName, taskNotes, taskTime;
private TaskChangeObservable notifier;
private Task editTask;
public TaskEditorPanel(TaskChangeObservable newNotifier) {
notifier = newNotifier;
createGui();
}
public void createGui() {
setLayout(new BorderLayout());
editPanel = new JPanel();
editPanel.setLayout(new GridLayout(3, 2));
taskName = new JTextField(20);
taskNotes = new JTextField(20);
taskTime = new JTextField(20);
editPanel.add(new JLabel("Task Name"));
editPanel.add(taskName);
editPanel.add(new JLabel("Task Notes"));
editPanel.add(taskNotes);
editPanel.add(new JLabel("Time Required"));
editPanel.add(taskTime);
controlPanel = new JPanel();
add = new JButton("Add Task");
update = new JButton("Update Task");
exit = new JButton("Exit");
controlPanel.add(add);
controlPanel.add(update);
controlPanel.add(exit);
add.addActionListener(this);
update.addActionListener(this);
exit.addActionListener(this);
add(controlPanel, BorderLayout.SOUTH);
add(editPanel, BorderLayout.CENTER);
}
public void setTaskChangeObservable(TaskChangeObservable newNotifier) {
notifier = newNotifier;
}
public void actionPerformed(ActionEvent event) {
Object source = event.getSource();
if (source == add) {
double timeRequired = 0.0;
try {
timeRequired = Double.parseDouble(taskTime.getText());
} catch (NumberFormatException exc) {
}
notifier.addTask(new Task(taskName.getText(), taskNotes.getText(),
timeRequired));
} else if (source == update) {
editTask.setName(taskName.getText());
editTask.setNotes(taskNotes.getText());
try {
editTask
.setTimeRequired(Double.parseDouble(taskTime.getText()));
} catch (NumberFormatException exc) {
}
notifier.updateTask(editTask);
} else if (source == exit) {
System.exit(0);
}
}
public void taskAdded(Task task) {
}
public void taskChanged(Task task) {
}
public void taskSelected(Task task) {
editTask = task;
taskName.setText(task.getName());
taskNotes.setText(task.getNotes());
taskTime.setText("" + task.getTimeRequired());
}
}
class TaskHistoryPanel extends JPanel implements TaskChangeObserver {
private JTextArea displayRegion;
public TaskHistoryPanel() {
createGui();
}
public void createGui() {
setLayout(new BorderLayout());
displayRegion = new JTextArea(10, 40);
displayRegion.setEditable(false);
add(new JScrollPane(displayRegion));
}
public void taskAdded(Task task) {
displayRegion.append("Created task " + task + "\n");
}
public void taskChanged(Task task) {
displayRegion.append("Updated task " + task + "\n");
}
public void taskSelected(Task task) {
displayRegion.append("Selected task " + task + "\n");
}
}
class TaskSelectorPanel extends JPanel implements ActionListener,
TaskChangeObserver {
private JComboBox selector = new JComboBox();
private TaskChangeObservable notifier;
public TaskSelectorPanel(TaskChangeObservable newNotifier) {
notifier = newNotifier;
createGui();
}
public void createGui() {
selector = new JComboBox();
selector.addActionListener(this);
add(selector);
}
public void actionPerformed(ActionEvent evt) {
notifier.selectTask((Task) selector.getSelectedItem());
}
public void setTaskChangeObservable(TaskChangeObservable newNotifier) {
notifier = newNotifier;
}
public void taskAdded(Task task) {
selector.addItem(task);
}
public void taskChanged(Task task) {
}
public void taskSelected(Task task) {
}
}
class ObserverGui {
public void createGui() {
JFrame mainFrame = new JFrame("Observer Pattern Example");
Container content = mainFrame.getContentPane();
content.setLayout(new BoxLayout(content, BoxLayout.Y_AXIS));
TaskChangeObservable observable = new TaskChangeObservable();
TaskSelectorPanel select = new TaskSelectorPanel(observable);
TaskHistoryPanel history = new TaskHistoryPanel();
TaskEditorPanel edit = new TaskEditorPanel(observable);
observable.addTaskChangeObserver(select);
observable.addTaskChangeObserver(history);
observable.addTaskChangeObserver(edit);
observable.addTask(new Task());
content.add(select);
content.add(history);
content.add(edit);
mainFrame.addWindowListener(new WindowCloseManager());
mainFrame.pack();
mainFrame.setVisible(true);
}
private class WindowCloseManager extends WindowAdapter {
public void windowClosing(WindowEvent evt) {
System.exit(0);
}
}
}
Real World Example
Here is some code from the Mozilla Internet Browser V 1.7a
/* -*- Mode: C; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* ***** BEGIN LICENSE BLOCK *****
* Version: NPL 1.1/GPL 2.0/LGPL 2.1
*
* The contents of this file are subject to the Netscape Public License
* Version 1.1 (the "License"); you may not use this file except in
* compliance with the License. You may obtain a copy of the License at
* http://www.mozilla.org/NPL/
*
* Software distributed under the License is distributed on an "AS IS" basis,
* WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
* for the specific language governing rights and limitations under the
* License.
*
* The Original Code is mozilla.org code.
*
* The Initial Developer of the Original Code is
* Netscape Communications Corporation.
* Portions created by the Initial Developer are Copyright (C) 1998
* the Initial Developer. All Rights Reserved.
*
* Contributor(s):
*
* Alternatively, the contents of this file may be used under the terms of
* either the GNU General Public License Version 2 or later (the "GPL"), or
* the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
* in which case the provisions of the GPL or the LGPL are applicable instead
* of those above. If you wish to allow use of your version of this file only
* under the terms of either the GPL or the LGPL, and not to allow others to
* use your version of this file under the terms of the NPL, indicate your
* decision by deleting the provisions above and replace them with the notice
* and other provisions required by the GPL or the LGPL. If you do not delete
* the provisions above, a recipient may use your version of this file under
* the terms of any one of the NPL, the GPL or the LGPL.
*
* ***** END LICENSE BLOCK ***** */
#include "prmem.h"
#include "xp_obs.h"
#include "prclist.h"
#include "prtypes.h"
#define MK_OUT_OF_MEMORY -1
typedef struct Observer
{
struct PRCListStr mLink;
XP_ObserverProc mCallback;
void* mClosure;
} Observer;
#define NextObserver(_obsptr_) ((Observer*)(_obsptr_)->mLink.next)
#define ObserverLinks(_obsptr_) (&((_obsptr_)->mLink))
struct OpaqueObserverList
{
Observer* mObserverList;
XP_Observable mObservable;
PRBool mNotificationEnabled;
};
/*
Creates a new XP_Observable, to which you can add observers,
who are notified when XP_NotifyObservers is called.
Observerer notification is enabled by default.
*/
NS_Error
XP_NewObserverList(
XP_Observable inObservable,
XP_ObserverList* outObserverList )
{
NS_Error result = 0;
PR_ASSERT(outObserverList != NULL);
*outObserverList = PR_MALLOC(sizeof(struct OpaqueObserverList));
if (*outObserverList != NULL)
{
(*outObserverList)->mObserverList = NULL;
(*outObserverList)->mObservable = inObservable;
(*outObserverList)->mNotificationEnabled = PR_TRUE;
}
else {
result = MK_OUT_OF_MEMORY;
}
return result;
}
/*
Disposes of an XP_Observable. Does nothing with
its observers.
*/
void
XP_DisposeObserverList(
XP_ObserverList inObserverList )
{
Observer *obs, *next = NULL;
PR_ASSERT(inObserverList != NULL);
for (obs = inObserverList->mObserverList; obs != NULL; )
{
next = NextObserver(obs);
if (next == obs) {
break;
}
PR_REMOVE_LINK(ObserverLinks(obs));
inObserverList->mObserverList = next;
PR_FREEIF(obs);
obs = inObserverList->mObserverList;
}
PR_FREEIF(inObserverList);
}
/*
XP_GetObserverListObservable
*/
XP_Observable
XP_GetObserverListObservable(
XP_ObserverList inObserverList)
{
PR_ASSERT(inObserverList != NULL);
return inObserverList->mObservable;
}
void
XP_SetObserverListObservable(
XP_ObserverList inObserverList,
XP_Observable inObservable )
{
PR_ASSERT(inObserverList != NULL);
inObserverList->mObservable = inObservable;
}
/*
Registers a function pointer and void* closure
as an "observer", which will be called whenever
XP_NotifyObservers is called an observer notification
is enabled.
*/
NS_Error
XP_AddObserver(
XP_ObserverList inObserverList,
XP_ObserverProc inObserver,
void* inClosure )
{
Observer* obs;
NS_Error result = 0;
PR_ASSERT(inObserverList != NULL);
if (inObserverList == NULL) {
return -1;
}
obs = PR_MALLOC(sizeof (Observer));
if (obs != NULL)
{
obs->mCallback = inObserver;
obs->mClosure = inClosure;
if (inObserverList->mObserverList == NULL)
{
PR_INIT_CLIST(ObserverLinks(obs));
inObserverList->mObserverList = obs;
}
else {
PR_INSERT_AFTER(ObserverLinks(obs), ObserverLinks(inObserverList->mObserverList));
}
} else {
result = MK_OUT_OF_MEMORY;
}
return result;
}
/*
Removes a registered observer. If there are duplicate
(XP_ObserverProc/void* closure) pairs registered,
it is undefined which one will be removed.
Returns false if the observer is not registered.
*/
PRBool
XP_RemoveObserver(
XP_ObserverList inObserverList,
XP_ObserverProc inObserver,
void* inClosure )
{
PRBool result = PR_FALSE;
if ( inObserverList->mObserverList != NULL )
{
Observer *tail = (Observer*) PR_LIST_TAIL(ObserverLinks(inObserverList->mObserverList));
Observer *obs = inObserverList->mObserverList;
do
{
if (obs->mCallback == inObserver && obs->mClosure == inClosure)
{
PR_REMOVE_LINK(ObserverLinks(obs));
if (obs == inObserverList->mObserverList)
{
Observer* next = NextObserver(obs);
inObserverList->mObserverList = (next != obs) ? next : NULL;
}
PR_FREEIF(obs);
result = PR_TRUE;
break;
}
obs = NextObserver(obs);
} while (obs != tail);
}
return result;
}
/*
If observer notification is enabled for this XP_Observable,
this will call each registered observer proc, passing it
the given message and void* ioData, in addition to the
observer's closure void*.
There is no defined order in which observers are called.
*/
void
XP_NotifyObservers(
XP_ObserverList inObserverList,
XP_ObservableMsg inMessage,
void* ioData )
{
Observer* obs;
Observer *tail;
Observer *temp;
PRBool done = PR_FALSE;
if ( ! inObserverList->mNotificationEnabled ||
inObserverList->mObserverList == NULL)
{
return;
}
obs = inObserverList->mObserverList;
tail = (Observer *) PR_LIST_TAIL(ObserverLinks(obs));
do
{
if (obs == tail) done = PR_TRUE;
(obs->mCallback)(inObserverList->mObservable, inMessage, ioData, obs->mClosure);
if (done != PR_TRUE) {
temp = inObserverList->mObserverList; /* just in case this callback free this observable
entry. */
if (temp != obs) {
obs = temp;
tail = (Observer *) PR_LIST_TAIL(ObserverLinks(obs));
}
else
obs = NextObserver(obs);
}
} while (done != PR_TRUE);
}
/*
When called, subsequent calls to XP_NotifyObservers
will do nothing until XP_EnableObserverNotification
is called.
*/
void
XP_DisableObserverNotification(
XP_ObserverList inObserverList )
{
inObserverList->mNotificationEnabled = PR_FALSE;
}
/*
Enables calling observers when XP_NotifyObservers
is invoked.
*/
void
XP_EnableObserverNotification(
XP_ObserverList inObserverList )
{
inObserverList->mNotificationEnabled = PR_TRUE;
}
/*
Returns true if observer notification is enabled.
*/
PRBool
XP_IsObserverNotificationEnabled(
XP_ObserverList inObserverList )
{
return inObserverList->mNotificationEnabled;
}
Drawbacks
Abstract coupling between subject and observer: All a subject knows is that there are observers. Since the subject and observer aren't tightly coupled, the two objects can be on different layers in a system.
Support for broadcast communication: When a subject sends out a notification it does not specify a receiver. The subject just broadcasts its current change of state.
Unexpected updates: Since observers do not know about the other observers which may or may not be watching the subject, there may be a lot of updates to the observes and the objects which could result in an endless loop.