In my application, I was trying to fetch tweets by sending an Ajax request to Twitter server. Once the tweets are available, I wanted all my observers to be notified with the recent tweets. Here, my Subject is 'Twitter' which is responsible for fetching the tweets and notifying all its observers whenever the data is available. Also, it is responsible for maintaining a list of observers who are interested in receiving notifications. The way I modeled this application is to define a generic class 'Subject' and a class 'Twitter' which would extend the Subject class.
Here the Subject class defines an array 'observerList' and two methods 'addObserver' and 'notifyObserver'. The addObserver method would add the observer object to the array and notifyObserver would iterate through the list of observers and send notifications whenever there is a new message. Now the Twitter class can extend the Subject class:
The constructor of Twitter class would call the parent constructor and then add the observer object (by invoking addObserver) to the list. Now it's ready to fetch the tweets for the provided handlerName. The init function then sends an Ajax request to fetch the tweets and once the data is received it would call the notifyObserver function. Note that the notifyObserver function is defined in the Subject class. The implementation of the Observer is pretty simple:
The Observer needs to provide the function (or the handler) that should be called when the Subject has a new message. In this case the updateFn would log the messages that it receives from the Subject.