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 notified and updated when a subject changes state.

UML Example

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. Sends a notification when a change has occurred ConcreteObserver: Maintains a reference to the ConcreteSubjects object. Synchronizes itself with the ConcreteSubject's state.

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);
    }
  }
}

Drawbacks

  1.  Abstract coupling between subject and observer: All a subject knows is that it has a list of its observers. Because subject and observer aren't tightly coupled, they can belong to different layers of abstraction in a system.
  2. Support for broadcast communication: The notification that a subject sends doesn't have to specify its receiver. The subject doesn't care about how many observers are observing it, all it does is a broadcast of its current change of its state.
  3. Unexpected updates: A cascade of of updates to observers and their dependent objects may occur when an observer does a change to a subject. This occurs because the observers do not know of other observers.