Implementing the Observer Pattern in C#: Code Samples

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.

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. 🙂


4 Comments Implementing the Observer Pattern in C#: Code Samples

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