Model real world observer in the CF?

I'll put together some course material for my companions to desktop on design patterns in ColdFusion. I'm having a hard time coming up with an real realistic example of what situations would be a good candidate to be resolved by a model of an observer in a CF web app.  There are certainly many examples of how to implement the model in the CF, which is great, except that I don't really find a lot of scenarios that the examples are based off of meaning in a web application environment - many of them are based on Java examples where references to the observer/observable objects are maintained for life (session) of the application.

In a CF web app, this seems as it would only make sense if you were dealing with Singletons for objects of the observer and the observed so that their references to exist outside the scope of a single page from loading.  However, I'm having a conceptual block to come with a pressing problem on why you need an observer as part of a single page model CF.

I'd like to hear other devs CF come into contact with any real world examples.  I'm sure that I could probably something like LogBox refer to as an example of the behavior, but I was hoping for a very simple example that I could use, put a code in front of my developers.

Thanks in advance!

-Michael

A few years ago, when I, too, was grappling with design patterns, I found a few useful items from Microsoft MSDN. One of them, the paper by Doug Purdy and Jeffrey Richter on the model of the observer, is very good and contains an example of the stock market.

I don't see how you arrive at singletons for observers. More on that in a moment. The essence of the observer model exceeds the programming language, syntax or implementation details. All what you need to know is in the scheme of traditional patterns (which I reproduce here of Wikipedia on the model of the observer).

Indeed, the diagram itself is very suggestive and can help you find examples of your own. There are 2 main roles, the observer and the object. There is a dependence as little as possible (i.e. loose coupling) between them. This allows either to change their internal mechanisms without affecting the other.

At its most simple, with only 2 roles, the observer observes the subject. The subject has to turn standby on a dynamic resource which the observer wants to know.

In most practical applications, there is more than one observer interested in the dynamics of resources. The subject has the responsibility to inform each of the resource, or any change observer. To be able to do so, it maintains a list of observers. Each observer has a responsibility to be notified by the subject. That's all. It is said that any system that works in this way, apply the model of the observer.

The example of most common practice is a newspaper publisher and the subscribers. The Publisher made the object, the observer subscriber, and news from the dynamics of resources. In fact, much like Hoover became synonymous for vacuum cleaners, so too has Publisher-Subscriber become a synonym for the model of the observer.

Let go of the need for a coupling between the observer and the object means that none of them should use shared or global variables. These variables create tighter form of coupling possible. So back to the issue of giving observer using a singleton or not. I wouldn't do it. Absolutely not. A singleton is an application-scope variable, so shared and would be pairing.

To keep the loose coupling, it is usually recommended to implement the observer and the object as an interface. Classes representing the actual topic and concrete observer would then be the attention of implementers of the respective interfaces. Each of these concrete classes could radically change its internal structure, and the outside world would not notice. This is because the respective interface for the observer and the subject remains intact.

Now to the Enigma of a practical example of the observer model in the context of a future ColdFusion page. In the model of the observer, the subject is of necessity an earpiece, listening on a dynamic resource object. Therefore, the ColdFusion page that represents the object must be a listener CFC. There is already an example built into ColdFusion. It's the DirectoryWatcher event gateway. Use inheritance to adapt to the CFC to include features in order to maintain a list of observers in the database. Include, for example, requests to inform observers by email whenever a change occurs in a directory.

Tags: ColdFusion

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