Archive for the 'Proxy Pattern' Category

On June 30, 2010 I met with the Flash Coders New York (FCNY). I walked over to the meeting site from Grand Central Station—it’s a straight shot up Park Ave., dogleg at Broadway and Bob’s your uncle! It’s right there near 4th on Mercer in a joint called Think Coffee. (East Village area near NYU.) I was lugging my Mac PowerBook and it and the case weighted 11 pounds. (After the two mile hike, my computer felt considerably heavier. If you plan to go, you’ll find that the subway— #6 to Brooklyn Bridge—from Grand Central is easier.)

Making Changes to the Original Protection Proxy

FCNY meets in the basement room at Think Coffee that reminded me of a (smoke free) anarchist’s safe house. (I was expecting to see Sacco and Vanzetti there.) They helped me hook up my PowerBook to a projector, and off we went. Having found myself in a perfect storm of doing the author edits of a Flash Catalyst book and starting on another new book, I was pressed for time; so I took the protection proxy from this blog and changed the proxy participant to include an algorithm that would filter out different “levels” of access. The level of sophistication of the algorithm and the added classes for different levels of access were quite simple. However, the point is to focus on the design pattern and not my acumen at writing algorithms. (If you don’t like the algorithms, you can always re-write them and send them in.) You can download the package by clicking the download button—the .zip file includes the PPT/KeyNote pages as well.

What was most exciting for me was the fact that the Protection Proxy was so easy to change. Here was a program with several different classes, and I had to add new functionality in a hurry that did several different things that would not crash my program. Essentially, I was adding three new concrete RealSubject classes that would work with the current program and make changes in the Proxy (SubProxy) to filter through three different “level of access” options. I knew that if I followed the interface rules, no matter what algorithm I used, it’d work. Figure 1 shows the class diagram of the revised Protection Proxy (from the FCNY presentation.)

The three RealSubject participants (International, National, Local) are pretty much the same except that each loads a different text file. However, as long as each maintains the interface, no matter how different the algorithms are, everything happily hums along.

Continue reading ‘ActionScript 3.0 Protection Proxy Design Pattern 2: The FCNY Meeting’

The images that people use on dating sites may not exactly represent who they are. So instead of putting in a real picture, they may use a proxy that is better looking, younger (or older), and generally more likely to attract a date. The Proxy Design Pattern deals with object access. In our book, we employed an unusual proxy, the Symmetric Proxy Pattern that uses a remote proxy. In fact, it uses mirrored proxies to access two different objects simultaneously. Our example showed how to use it with Flash Media Server, and so probably not a lot of developers use it as a model for understanding a Proxy pattern. However, on this blog we provided a virtual proxy that should be a clear and simple example of a Proxy design pattern. So, we covered two of the three Proxy designs. The sole remaining type is the Protection Proxy, and that’s what we’re going to look at now. (You can run it and download the files using the buttons below:)

The Easiest Proxy

Of all of the Proxy designs, the Protection Proxy is very simple both in terms of implementation and concept. The Client makes a request to the Proxy, and the Proxy decides whether to give the Client access to the Subject. Where more than a single Subject is available, the Proxy decides on the level of access. For example, we started off with an example of a dating site. Some users have no access to the dating site until they sign up as a customer. Basically, the Proxy denies them all access to the subject. Once they sign up, they may be given access only to a specific gender. For example, the girls may only be given access to boys if that’s who they’re interested in meeting. The Proxy controls that access. Also, they may be given access only to change their own profile; again controlled by the Proxy. Of course the site administrator needs total access so that he can make sure that no one is posting materials that they shouldn’t be. (When I posted that I was an Astronaut, Rock Star who owns a Silicon Valley software company, the administrator changed it—and without my permission!.)

This design requires only three participants to implement : Subject (interface), Proxy and RealSubject with the Client making a request through the Subject interface to the Proxy. Figure 1 shows the Proxy class diagram shows and the different relationships:

The sequence is unique in that the request, while for the RealSubject, goes through the Proxy as shown in the object diagram in Figure 2:

As you can see, the Proxy design is pretty straightforward, and given the three modes (remote, virtual, and protection), it’s versatile. However, in looking at it, it doesn’t seem to do too much in terms of loosening up code and reusability. According to the GoF,

The Proxy pattern introduces a level of indirection when accessing an object. The additional indirection has many uses, depending on the kind of proxy: p. 210

So the key to understanding the Proxy pattern is indirection. We need to consider indirection a bit more, which we begin on the next section.
Continue reading ‘ActionScript 3.0 Protection Proxy Design Pattern 1: Shielding the Real Subject’