Computer Science Design Patterns/Observer

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[edit] Example in Python

The observer pattern in Python:

class AbstractSubject:
    def register(self, listener):
        raise NotImplementedError("Must subclass me")
 
    def unregister(self, listener):
        raise NotImplementedError("Must subclass me")
 
    def notify_listeners(self, event):
        raise NotImplementedError("Must subclass me")
 
class Listener:
    def __init__(self, name, subject):
        self.name = name
        subject.register(self)
 
    def notify(self, event):
        print self.name, "received event", event
 
class Subject(AbstractSubject):
    def __init__(self):
        self.listeners = []
        self.data = None
 
    def getUserAction(self):
        self.data = raw_input('Enter something to do:')
        return self.data
 
    # Implement abstract Class AbstractSubject
 
    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)
 
 
if __name__=="__main__":
    # make a subject object to spy on
    subject = Subject()
 
    # register two listeners to monitor it.
    listenerA = Listener("<listener A>", subject)
    listenerB = Listener("<listener B>", subject)
 
    # simulated event
    subject.notify_listeners ("<event 1>")
    # outputs:
    #     <listener A> received event <event 1>
    #     <listener B> received event <event 1>
 
    action = subject.getUserAction()
    subject.notify_listeners(action)
    #Enter something to do:hello
    # outputs:
    #     <listener A> received event hello
    #     <listener B> received event hello

The observer pattern can be implemented more succinctly in Python using function decorators.

[edit] Java

Below is an example 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 notifyObserver is then called, in order to notify all observers of the event's occurrence, in the form of an invocation of their 'update' methods - in our example, ResponseHandler.update(...).

The file myapp.java contains a main() method that might be used in order to run the code.

/* File Name : EventSource.java */
 
package obs;
import java.util.Observable;          //Observable is here
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
 
public class EventSource extends Observable implements Runnable 
{
    public void run()
    {
        try
        {   
            final InputStreamReader isr = new InputStreamReader( System.in );
            final BufferedReader br = new BufferedReader( isr );
            while( true )
            {
                final String response = br.readLine();
                setChanged();
                notifyObservers( response );
            }
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
    }
}

/* File Name: ResponseHandler.java */
 
package obs;
 
import java.util.Observable;
import java.util.Observer;  /* this is Event Handler */
 
public class ResponseHandler implements Observer
{
    private String resp;
    public void update (Observable obj, Object arg)
    {
        if (arg instanceof String)
        {
            resp = (String) arg;
            System.out.println("\nReceived Response: "+ resp );
        }
    }
}

/* Filename : myapp.java */
/* This is main program */
 
package obs;
 
public class MyApp
{
    public static void main(String args[])
    {            
        System.out.println("Enter Text >");
 
        // create an event source - reads from stdin
        final EventSource evSrc = new EventSource();
 
        // create an observer
        final ResponseHandler respHandler = new ResponseHandler();
 
        // subscribe the observer to the event source
        evSrc.addObserver( respHandler );
 
        // starts the event thread
        Thread thread = new Thread(evSrc);
        thread.start();
    }
}

[edit] C# - Traditional Method

C# and the other .NET Framework languages do not typically require a full implementation of the Observer pattern using interfaces and concrete objects. Here is an example of using them, however.

 
 
using System;
using System.Collections;
 
namespace Wikipedia.Patterns.Observer
{
        // IObserver --> interface for the observer
        public interface IObserver
        {
                // called by the subject to update the observer of any change
                // The method parameters can be modified to fit certain criteria
                void Update(string message);
        }
 
        public class Subject
        {
                // use array list implementation for collection of observers
                private ArrayList observers;
 
                // constructor
                public Subject()
                {
                        observers = new ArrayList();
                }
 
                public void Register(IObserver observer)
                {
                        // if list does not contain observer, add
                        if (!observers.Contains(observer))
                        {
                                observers.Add(observer);
                        }
                }
 
                public void Unregister(IObserver observer)
                {
                        // if observer is in the list, remove
                        observers.Remove(observer);
                }
 
                public void Notify(string message)
                {
                        // call update method for every observer
                        foreach (IObserver observer in observers)
                        {
                                observer.Update(message);
                        }
                }
        }
 
        // Observer1 --> Implements the IObserver
        public class Observer1 : IObserver
        {
                public void Update(string message)
                {
                        Console.WriteLine("Observer1:" + message);
                }
        }
 
        // Observer2 --> Implements the IObserver
        public class Observer2 : IObserver
        {
                public void Update(string message)
                {
                        Console.WriteLine("Observer2:" + message);
                }
        }
 
        // Test class
        public class ObserverTester
        {
                [STAThread]
                public static void Main()
                {
                        Subject mySubject = new Subject();
                        IObserver myObserver1 = new Observer1();
                        IObserver myObserver2 = new Observer2();
 
                        // register observers
                        mySubject.Register(myObserver1);
                        mySubject.Register(myObserver2);
 
                        mySubject.Notify("message 1");
                        mySubject.Notify("message 2");
                }
        }
}

[edit] C# - Using Events

The alternative to using concrete and abstract observers and publishers in C# and other .NET Framework languages, such as Visual Basic, is to use events. The event model is supported via delegates that define the method signature that should be used to capture events. Consequently, delegates provide the mediation otherwise provided by the abstract observer, the methods themselves provide the concrete observer, the concrete subject is the class defining the event, and the subject is the event system built into the base class library. It is the preferred method of accomplishing the Observer pattern in .NET applications.

using System;
 
// First, declare a delegate type that will be used to fire events.
//  This is the same delegate as System.EventHandler.
//  This delegate serves as the abstract observer.
//  It does not provide the implementation, but merely the contract.
public delegate void EventHandler(object sender, EventArgs e);
 
// Next, declare a published event.  This serves as the concrete subject.
//  Note that the abstract subject is handled implicitly by the runtime.
public class Button
{
    // The EventHandler contract is part of the event declaration.
    public event EventHandler Clicked;
 
    // By convention, .NET events are fired from descendant classes by a virtual method,
    //  allowing descendant classes to handle the event invocation without subscribing
    //  to the event itself.
    protected virtual void OnClicked(EventArgs e)
    {
        if (Clicked != null) 
            Clicked(this, e); // implicitly calls all observers/subscribers
    }
}
 
// Then in an observing class, you are able to attach and detach from the events:
public class Window
{
    private Button okButton;
 
    public Window()
    {
        okButton = new Button();
        // This is an attach function.  Detaching is accomplished with -=.
        // Note that it is invalid to use the assignment operator - events are multicast
        //  and can have multiple observers.
        okButton.Clicked += new EventHandler(okButton_Clicked);
    }
 
    private void okButton_Clicked(object sender, EventArgs e)
    {
        // This method is called when Clicked(this, e) is called within the Button class
        //  unless it has been detached.
    }
}

[edit] AS3

// Main Class
package {
    import flash.display.MovieClip;
 
    public class Main extends MovieClip {
        private var _cs:ConcreteSubject = new ConcreteSubject();
        private var _co1:ConcreteObserver1 = new ConcreteObserver1();
        private var _co2:ConcreteObserver2 = new ConcreteObserver2();
 
        public function Main() {
            _cs.registerObserver(_co1);
            _cs.registerObserver(_co2);
 
            _cs.changeState(10);
            _cs.changeState(99);
 
            _cs.unRegisterObserver(_co1);
 
            _cs.changeState(17);
 
            _co1 = null;
        }
    }
}
 
// Interface Subject
package {
    public interface ISubject {
        function registerObserver(o:IObserver):void;
 
        function unRegisterObserver(o:IObserver):void;
 
        function updateObservers():void;
 
        function changeState(newState:uint):void;
    }
}
 
// Interface Observer
package {
    public interface IObserver {
        function update(newState:uint):void;
    }
}
 
// Concrete Subject
package {
    public class ConcreteSubject implements ISubject {
        private var _observersList:Array = new Array();
        private var _currentState:uint;
 
        public function ConcreteSubject() {
        }
 
        public function registerObserver(o:IObserver):void {
            _observersList.push( o );
            _observersList[_observersList.length-1].update(_currentState); // update newly registered
        }
 
        public function unRegisterObserver(o:IObserver):void {
            _observersList.splice( _observersList.indexOf( o ), 1 );
        }
 
        public function updateObservers():void {
            for( var i:uint = 0; i<_observersList.length; i++) {
                _observersList[i].update(_currentState);
            }
        }
 
        public function changeState(newState:uint):void {
            _currentState = newState;
            updateObservers();
        }
    }
}
 
// Concrete Observer 1
package {
    public class ConcreteObserver1 implements IObserver {
        public function ConcreteObserver1() {
        }
 
        public function update(newState:uint):void {
            trace( "co1: "+newState );
        }
 
        // other Observer specific methods
    }
}
 
// Concrete Observer 2
package {
    public class ConcreteObserver2 implements IObserver {
        public function ConcreteObserver2() {
        }
 
        public function update(newState:uint):void {
            trace( "co2: "+newState );
        }
 
        // other Observer specific methods
    }
}

[edit] PHP

class STUDENT

<?php
class Student implements SplObserver {
 
        protected $type = "Student";
        private   $name;
        private   $address;
        private   $telephone;
        private   $email;
        private   $_classes = array();
 
        public function __construct($name)
        {
                $this->name = $name;
        }
 
        public function GET_type()
        {
                return $this->type;
        }
 
        public function GET_name()
        {
                return $this->name;
        }
 
        public function GET_email()
        {
                return $this->email;
        }
 
        public function GET_telephone()
        {
                return $this->telephone;
        }
 
        public function update(SplSubject $object)
        {
                $object->SET_log("Comes from ".$this->name.": I'm a student of ".$object->GET_materia());
        }
 
}
 
?>

class TEACHER

<?php
class Teacher implements SplObserver {
 
        protected $type = "Teacher";
        private   $name;
        private   $address;
        private   $telephone;
        private   $email;
        private   $_classes = array();
 
        public function __construct($name)
        {
                $this->name = $name;
        }
 
        public function GET_type()
        {
                return $this->type;
        }
 
        public function GET_name()
        {
                return $this->name;
        }
 
        public function GET_email()
        {
                return $this->email;
        }
 
        public function GET_telephone()
        {
                return $this->name;
        }
 
        public function update(SplSubject $object)
        {
                $object->SET_log("Comes from ".$this->name.": I teach in ".$object->GET_materia());
        }
 
}
 
?>

Class SUBJECT

<?php
 
class Subject implements SplSubject {
 
        private $name_materia;
        private $_observers = array();
        private $_log = array();
 
        function __construct($name)
        {
                $this->name_materia = $name;
                $this->_log[]       = "Subject $name was included";
        }
 
        /* Add an observer */
        public function attach(SplObserver $classes) {
                $this->_classes[] = $classes;
                $this->_log[]     = " The ".$classes->GET_type()." ".$classes->GET_name()." was included";
        }
 
        /* Remove an observer */
        public function detach(SplObserver $classes) {
                foreach ($this->_classes as $key => $obj) {
                        if ($obj == $classes) {
                                unset($this->_classes[$key]);
                                $this->_log[] = " The ".$classes->GET_type()." ".$classes->GET_name()." was removed";
                        }
                }
        }
 
        /* Notificate an observer */
        public function notify(){
                foreach ($this->_classes as $classes){
                        $classes->update($this);
                }
        }
 
        public function GET_materia()
        {
                return $this->name_materia;
        }
 
        function SET_log($valor)
        {
                $this->_log[] = $valor ;
        }
 
        function GET_log()
        {
                return $this->_log;
        }
 
 
}
?>

Application

<?php
require_once("teacher.class.php");
require_once("student.class.php");
require_once("subject.class.php");
 
$subject  = new Subject("Math");
$marcus   = new Teacher("Marcus Brasizza");
$rafael   = new Student("Rafael");
$vinicius = new Student("Vinicius");
 
// Include observers in the math Subject
$subject->attach($rafael);
$subject->attach($vinicius);
$subject->attach($marcus);
 
$subject2 = new Subject("English");
$renato   = new Teacher("Renato");
$fabio    = new Student("Fabio");
$tiago    = new Student("Tiago");
 
// Include observers in the english Subject
$subject2->attach($renato);
$subject2->attach($vinicius);
$subject2->attach($fabio);
$subject2->attach($tiago);
 
// Remove the instance "Rafael from subject"
$subject->detach($rafael);
 
// Notify both subjects
$subject->notify();
$subject2->notify();
 
echo "First Subject <br>";
echo "<pre>";
print_r($subject->GET_log());
echo "</pre>";
echo "<hr>";
echo "Second Subject <br>";
echo "<pre>";
print_r($subject2->GET_log());
echo "</pre>";
?>

OUTPUT

First Subject

Array
(
    [0] =>  Subject Math was included
    [1] =>  The Student Rafael was included
    [2] =>  The Student Vinicius was included
    [3] =>  The Teacher Marcus Brasizza was included
    [4] =>  The Student Rafael was removed
    [5] => Comes from Vinicius: I'm a student of Math
    [6] => Comes from Marcus Brasizza: I teach in Math
)

Second Subject

Array
(
    [0] =>  Subject English was included
    [1] =>  The Teacher Renato was included
    [2] =>  The Student Vinicius was included
    [3] =>  The Student Fabio was included
    [4] =>  The Student Tiago was included
    [5] => Comes from Renato: I teach in English
    [6] => Comes from Vinicius: I'm a student of English
    [7] => Comes from Fabio: I'm a student of English
    [8] => Comes from Tiago: I'm a student of English
)

[edit] Ruby

In Ruby, use the standard Observable mixin. For documentation and an example, see http://www.ruby-doc.org/stdlib/libdoc/observer/rdoc/index.html

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