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I have been reading about data binding in windows phone 7 development and got curious about how .net implements the observer pattern. I spent some time understanding the flow and decided to write a blog post about it.

.NET platform offers IObserver and IObservable interfaces to quickly get up and running with the observer pattern. We know the theory – there is an independent subject, a observable aimed towards monitoring that subject and some observers which are very keen to receive updates whenever something interesting happens to the subject. The observers know that the observable monitors the subject, so they subscribe to the observable for notifications. The observable accepts all these subscriptions, keeps them in a safe place. Then he keeps watching and watching. When something interesting happens to the subject, he quickly gets to the list of subscribers and then notifies them one by one.

Compare this scenario to the day before Eid Ul Fitr. Tomorrow is Eid if we see the moon. Now, you have decided to go to the roof and keep waiting there until the moon pops up. Knowing this, some of your friends (who are so computer freak that they don’t even want to leave their PCs to watch the moon of Eid Ul Fitr) called you and asked you to notify them if you see the moon.

Here:
The Moon == Subject
You == Observable
Your nerd friends == Observers

Here the observable keeps watching for the subject, when the subject pops up in the sky, the observable calls the observers – that’s it, simply that’s the observer pattern.

To implement this pattern, we define a Subject object. It has a counter. Every time you view the counter, it gets increased by one. We are doing this little magic to track any changes in the subject.

Then we define the SubjectObservable object which implements the IObservable. It accepts a subject in it’s constructor. This is not mandatory but we did this to track the subject in a better fashion. It provides a Subscribe() method which is called by the observers to subscribe to the observers. The method must return an IDisposable object. We do a trick and create a custom object implementing the IDisposable interface. In the constructor we accept the list of observers recorded by the observable and the instance of the observer passed for subscription. In the Dispose() method we remove the observer from the observer list. So when the Dispose() method of the disposer is called, the observer is removed from the observer list. The observable will no longer send any notifications to the removed observer.

The SubjectObserver is pretty simple. It has OnNext(), OnCompleted() and OnError() methods. The OnNext() is called by the observable to send the current state of the subject. Besides the abstract methods, we also define a constructor to accept a unique name instance and an instance of an observable to follow. When the OnNext() is called, we print out the current counter value of the subject. We also define a Dispose() method which shall invoke the Dispose() method of the disposer, removing itself from the observer list of the observable.

Well, if all these theories seem too complex to you. Do read the codes below. I have tried to add relevant comments. If you have any feedback, please do make a comment.

using System;
using System.Collections.Generic;
using System.Linq;


namespace Test
{
    class Program
    {
        public static void Main(string[] args)
        {
            // Create a subject and the observable, inject the subject into the observable
            Subject subject = new Subject();
            SubjectObservable observable = new SubjectObservable(subject);

            //Create two observers
            SubjectObserver zeus = new SubjectObserver(observable, "Zeus");
            SubjectObserver ares = new SubjectObserver(observable, "Ares");
            
            // Run a loop and trigger the observer
            for (int i = 0; i < 5; i++)
            {
                Console.WriteLine("Counter: " + subject.Counter);
                observable.inspectSubject();

            }

            Console.ReadLine();
        }
    }

    //
    // Class Subject - It has a counter object that increases everytime you inspect it
    //

    class Subject
    {
        private int _counter;
        public int Counter
        {
            get
            {
                return ++_counter;
            }
        }

    }

    //
    // Class Subject Observable - The observable object that observes the subject and notifies the observers
    //

    class SubjectObservable : IObservable<Subject>
    {
        // List of observers and the instance of subject
        private List<IObserver<Subject>> _subjectObservers;
        private Subject _subject;

        // Accept a subject instance which we shall observe
        public SubjectObservable(Subject subject)
        {
            _subjectObservers = new List<IObserver<Subject>>();
            _subject = subject;

        }

        // This method allows the observers to attach themselves. It returns a disposer object to the observer
        // which the observer can utilize to unsubscribe
        public IDisposable Subscribe(IObserver<Subject> observer)
        {
            if (!_subjectObservers.Contains(observer))
            {
                _subjectObservers.Add(observer);
            }

            return new Disposer(_subjectObservers, observer);
        }

        // This method is used to inspect the subject over time. Usually used with a timer or an event
        public void inspectSubject()
        {
            foreach (IObserver<Subject> observer in _subjectObservers)
            {
                observer.OnNext(_subject);
            }
        }

        //
        // Private class Disposer: Implements the IDisposable. Observable returns an instance to the observer for easy unsubscription
        //
        private class Disposer : IDisposable
        {
            // The observers list recieved from the observable
            private List<IObserver<Subject>> _subjectObservers;
            // The observer instance to unsubscribe
            private IObserver<Subject> _observer;

            public Disposer(List<IObserver<Subject>> _subObservers, IObserver<Subject> observer)
            {
                _subjectObservers = _subObservers;
                _observer = observer;
            }

            public void Dispose()
            {
                if (_subjectObservers.Contains(_observer))
                {
                    _subjectObservers.Remove(_observer);
                }
            }
        }
    }

    //
    // Class Subject Observer - The object that attaches itself to a observable and recieves notifications
    //

    class SubjectObserver : IObserver<Subject>
    {
        // The disposer returned by the observable when subscribing
        private IDisposable _disposer;

        // Unique instance name to track the observer
        private string _instaceName;

        public SubjectObserver(IObservable<Subject> provider, string instanceName)
        {
            if (provider != null)
            {
                _disposer = provider.Subscribe(this);
                _instaceName = instanceName;
            }
        }

        // The observable invokes this method to pass the Subject object to the observer
        public void OnNext(Subject value)
        {
            Console.WriteLine("[" + _instaceName + "] " + value.Counter);
        }

        // Usually called when a transmission is complete. Not implemented.
        public void OnCompleted()
        {
            throw new NotImplementedException();
        }

        // Usually called when there was an error. Didn't implement.
        public void OnError(Exception error)
        {
            throw new NotImplementedException();
        }

        // Invoke the disposer recieved from the observable. This will unsubscribe
        public void Dispose()
        {
            _disposer.Dispose();
        }
    }

}

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using System;

using System.Collections.Generic;

using System.Linq;

namespace Test

{

class Program

{

public static void Main(string[] args)

{

// Create a subject and the observable, inject the subject into the observable

Subject subject = new Subject();

SubjectObservable observable = new SubjectObservable(subject);

//Create two observers

SubjectObserver zeus = new SubjectObserver(observable, "Zeus");

SubjectObserver ares = new SubjectObserver(observable, "Ares");

// Run a loop and trigger the observer

for (int i = 0; i < 5; i++)

{

Console.WriteLine("Counter: " + subject.Counter);

observable.inspectSubject();

}

Console.ReadLine();

}

// Class Subject - It has a counter object that increases everytime you inspect it

class Subject

{

private int _counter;

public int Counter

{

get

{

return ++_counter;

}

// Class Subject Observable - The observable object that observes the subject and notifies the observers

class SubjectObservable : IObservable<Subject>

{

// List of observers and the instance of subject

private List<IObserver<Subject>> _subjectObservers;

private Subject _subject;

// Accept a subject instance which we shall observe

public SubjectObservable(Subject subject)

{

_subjectObservers = new List<IObserver<Subject>>();

_subject = subject;

}

// This method allows the observers to attach themselves. It returns a disposer object to the observer

// which the observer can utilize to unsubscribe

public IDisposable Subscribe(IObserver<Subject> observer)

{

if (!_subjectObservers.Contains(observer))

{

_subjectObservers.Add(observer);

}

return new Disposer(_subjectObservers, observer);

}

// This method is used to inspect the subject over time. Usually used with a timer or an event

public void inspectSubject()

{

foreach (IObserver<Subject> observer in _subjectObservers)

{

observer.OnNext(_subject);

}

// Private class Disposer: Implements the IDisposable. Observable returns an instance to the observer for easy unsubscription

private class Disposer : IDisposable

{

// The observers list recieved from the observable

private List<IObserver<Subject>> _subjectObservers;

// The observer instance to unsubscribe

private IObserver<Subject> _observer;

public Disposer(List<IObserver<Subject>> _subObservers, IObserver<Subject> observer)

{

_subjectObservers = _subObservers;

_observer = observer;

}

public void Dispose()

{

if (_subjectObservers.Contains(_observer))

{

_subjectObservers.Remove(_observer);

}

// Class Subject Observer - The object that attaches itself to a observable and recieves notifications

class SubjectObserver : IObserver<Subject>

{

// The disposer returned by the observable when subscribing

private IDisposable _disposer;

// Unique instance name to track the observer

private string _instaceName;

public SubjectObserver(IObservable<Subject> provider, string instanceName)

{

if (provider != null)

{

_disposer = provider.Subscribe(this);

_instaceName = instanceName;

}

// The observable invokes this method to pass the Subject object to the observer

public void OnNext(Subject value)

{

Console.WriteLine("[" + _instaceName + "] " + value.Counter);

}

// Usually called when a transmission is complete. Not implemented.

public void OnCompleted()

{

throw new NotImplementedException();

}

// Usually called when there was an error. Didn't implement.

public void OnError(Exception error)

{

throw new NotImplementedException();

}

// Invoke the disposer recieved from the observable. This will unsubscribe

public void Dispose()

{

_disposer.Dispose();

}

Okay, so every time we read the value of the counter data, the value increases. The subject is inspected by the observable and the observable passes the subject to the observers. Each observer again prints the counter and increases the value by one. This continues. 🙂

When I started into the world of .NET based web development, I often encountered the term “Entity Framework Code First” or “EF Code First”. Where as I could no where find the option to create “Code First” entities in the visual studio. 🙁 How disappointing! 😛 I could create ADO.NET Entity Data Model but that would create .edmx files which looked all cryptic to me! I was just beginning .NET and my background is heavily based on PHP and Python where nothing is “Visual”, we do it all in codes.

And then after going through some of the resources online, I finally understood what’s going on. The ADO.NET Entity framework has two popular approaches – “Database first” and “Code first”. As the names suggest, in the first approach you write your models, generate databases, map the database with your models etc. Here database comes first. On the other hand, in the “code first” approach, you write plain classes which are identical to your database objects. Then following specific convention, you magically map the defined classes with the database. No need to write (I mean generate) complex xml maps or all those mess.

While I agree that both the approach have their own benefits, strengths and all those positive things, I found the code first approach easier to get started as a novice. So how does it work? Let’s see!

Say, we already have a table defined somewhere. For convenience, let’s use the following SQL script to generate a table:

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE TABLE [dbo].[Students](
	[ID] [int] IDENTITY(1,1) NOT NULL,
	[Name] [text] NULL
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

GO

SET ANSI_NULLS ON

SET QUOTED_IDENTIFIER ON

CREATE TABLE [dbo].[Students](

[ID] [int] IDENTITY(1,1) NOT NULL,

[Name] [text] NULL

) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

I am not going to elaborate the process but the direction is to first create a database and then use the above sql codes to generate a table. It has one ID field that is auto incremented and the Name field. The simplest table structure I could think of! 😀

Now, in code first approach, convention is prioritized over configuration. Hence, we must define an identical class. Here it goes:

//Students.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace LearnMVC3.Models
{
    public class Student
    {
        public virtual int ID { get; set; }
        public virtual string Name { get; set; }

    }
}

//Students.cs

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

namespace LearnMVC3.Models

{

public class Student

{

public virtual int ID { get; set; }

public virtual string Name { get; set; }

}

Remember those “virtual” identifiers are crucial. They have a lot to do with the magic that keeps things going. Another interesting thing to notice is that the Student object doesn’t derive from any other object.

Now, we need a database context. We shall define a class that will extend the “DbContext” class from the Entity Framework. The code should be like this:

//StudentsDb.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;


namespace LearnMVC3.Models
{
    public class StudentsDb : DbContext
    {
        public StudentsDb() : base("name=DatbaseConnection") { }
        public DbSet<Student> Students { get; set; }
        
    }
}

//StudentsDb.cs

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Data.Entity;

using System.Data.Entity.Infrastructure;

namespace LearnMVC3.Models

{

public class StudentsDb : DbContext

{

public StudentsDb() : base("name=DatbaseConnection") { }

public DbSet<Student> Students { get; set; }

}

One thing to notice is that we must pass a database connection string to the DbContext class. In this case, I have a connection string defined in my configuration file and I am passing the name of the connection to the DbContext class.

With this, we’re ready to use the Entities we have generated. So, how do we use them? Let’s see some codes:

StudentsDb db = new StudentsDb();
Student st = new Student();
st.Name = "Masnun";

db.Students.Add(st);
db.SaveChanges();

StudentsDb db = new StudentsDb();

Student st = new Student();

st.Name = "Masnun";

db.Students.Add(st);

db.SaveChanges();

The above codes will insert a entry in the database when run. Give it a try! 🙂

PS: I am just beginning .NET and C#. If you have any comments, suggestions or plain feedback, please do feel free to leave in the comments. I would appreciate your contribution to my personal development 🙂

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