The Gang of Four divide up the Proxy design pattern into three types: remote, virtual, and protection. In our book, Chandima and I used a remote proxy in the Symmetric Proxy example(Chapter 13). Each player in a game over the Internet was able to make simultaneous moves using the remote proxy representing an opponent on their own system. That leaves the virtual and protection proxies to cover, and we’ll start with the virtual proxy in this article.
Avoiding Repetition
Our university uses a software package called Blackboard for course administration. It’s quite handy, and I use it a good deal. As a Web-based application requiring a login, the initial login is time-consuming as the application gathers up all of the information it has stored for all of my classes. However, if I quit the program and return within a certain time window, I don’t have to go through login again. More importantly, the application does not have to reload all of the materials for me again. When you consider that hundreds of faculty and thousands of students are all using the same system on a daily basis, the ability to re-use previously loaded materials is a huge savings.
For the most part, design patterns, do not improve the performance of your programs with a few exceptions, such as the Flyweight pattern previously discussed on this blog. I believe that the virtual proxy design pattern is another one of those patterns that improves the performance of your program because your application is not constantly re-doing something expensive (time-consuming) that’s already been done. Most of the examples, including the one provided by GoF, are of loading graphics. Once loaded, most graphics do not have to be reloaded for repeated display after the initial loading. So, rather than reload graphics or files of any kind, the virtual proxy design first checks to see if your materials are already loaded, and if they are, it uses the extant materials. If not, it simply calls the real loader and loads up what’s required. Any application that has multiple users over the Web (which is most programs) the virtual proxy can significantly improve the performance of the program because it reuses loaded materials.
How the Proxy Design Pattern Works
The Proxy pattern does not have different class diagrams for the different types of proxies. Figure 1 shows the pattern diagram used for all variations of the pattern.
Figure 1: Proxy Class Diagram
The client is not a participant, but it is included to indicate where it sends its request. It is loosely coupled to the participants following the path illustrated in Figure 2.
Figure 2: Proxy Object Diagram
The proxy is used to check and see if a real subject is required, and if so, it sends the request to the real subject. However, if the proxy can handle the request without using the real subject, it will do so. In effect, the proxy acts like a gatekeeper. It inspects all requests, and if it can deal with the requests it does, but no requests go directly to the real subject from the client. Figure 2 shows the intermediary position of the proxy. The only problem with the object diagram is that it looks like the proxy is a stop along the way to the real subject, but depending on the application, the request may never get to the real subject.
Continue reading ‘ActionScript Proxy Design Pattern : The Virtual Proxy (A Minimal Abstract Example)’
Bill Sanders
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