Archive for the 'Design Patterns' Category

ActionScript 3.0 Protection Proxy Design Pattern 2: The FCNY Meeting

Published on July 2, 2010 in Proxy Pattern. by William B. Sanders 8 Comments

FCNY Meeting

FCNY Meeting

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.
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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.)

<em><strong>Figure 1: </strong>Stratified Protection Proxy</em>

Figure 1: Stratified Protection Proxy


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.

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Meet in NYC on June 30, 2010

Published on June 14, 2010 in Design Patterns and Proxy Pattern. by William B. Sanders 0 Comments

fcnyI’ll be at the Flash Coders New York (FCNY) weekly meeting at Think Coffee, 248 Mercer St. (NYC) on Wednesday, June 30 (Time: 7-9pm) to talk about ActionScript 3.0 Design Patterns. The plan is to look at some of the fundamental principles of design patterns and implementations with ActionScript 3.0. The Protection Proxy used in combination with PHP and multiple levels of access is the planned application that will be discussed plus any other ActionScript 3.0 Design Pattern of interest. Bill

ActionScript 3.0 Protection Proxy Design Pattern 1: Shielding the Real Subject

Published on June 13, 2010 in Proxy Pattern. by William B. Sanders 10 Comments
Proxy and Real Subject

Proxy and Real Subject

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:)

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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:

<em><strong>Figure 1:</strong> Proxy class diagram</strong>

Figure 1: Proxy class diagram

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:

<em><strong>Figure 2: </strong> Proxy object diagram</em>

Figure 2: Proxy object diagram

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.
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ActionScript 3.0 Lazy Initialization and the Factory Method Design Pattern

Published on May 24, 2010 in Factory Method and Lazy Initialization. by William B. Sanders 8 Comments
Lazy Initialization

Lazy Initialization

The other day I was re-reading Chandima’s description of key OOP concepts used in the Factory Method design pattern. It is beautifully encapsulated in a little over a half a page (page 84). The Factory Method allows you to separate the creation of objects from their use. It says a bit more, but it is a nice piece in both describing a design pattern principle and explaining what the Factory Method does. This led to re-reading the Factory Method chapter in Design Patterns: Elements of Reusable Object-Oriented Software—the Gang of Four’s book. Normally, I just go over the main points, but I came across a little passage that read:

Just be careful not to call factory methods in the Creator’s constructor—the factory method in the ConcreteCreator won’t be available yet. (page 112)

The comment was directed at some issues in C++ which may or may not have any bearing on ActionScript 3.0, but just in case it did, I continued reading. The point being made was you can avoid these problems by accessing products only through accessor operations that create products on demand. So what are accessor operations? Generally these are public methods that you can call to initiate creation of a product.

Time Out! Before we get too far ahead of ourselves, you can download all of the .as files for the Factory Method example by clicking the following button:
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The example is a simple one that creates three different shapes—ellipse, rectangle and triangle. Figure 1 shows how the Client arranged them to display a house in front a a pond with a cloud overhead:

<em><strong>Figure 1:</strong>Triangle, Ellipse and Rectangle Products</em>

Figure 1:Triangle, Ellipse and Rectangle Products

The program uses concrete creators to fetch shapes. The Client makes requests that size, color, and position the shapes.

Don’t Enslave Your Constructor Functions

This issue of the constructor functions reminded me of a post by Miško Hevery, the guy at Google who helps set Google’s programming standards. One of the key flaws in a program is where the constructor does real work. Besides violating the single responsibility rule, Miško goes on to explain the tell-tale signs that one has used his constructor function with outrageous disregard for both agile programming and loose coupling. As a result of reading Hevery’s work, a year or so ago, I began to seriously gear back on using a class’ constructor function; primarily by not including one in most of my classes. (My Client class is a glaring exception to the rule.) I’d usually have just the properties and operations (methods, functions) that I needed. The properties would initialize themselves—sort of. They’d be given an ID and then typed and that was all. The child classes could then use them in operations. Whenever I needed something, the Client would type a request to the interface (including abstract classes) and then make a request through concrete method in an operation. With the Factory Method, I make requests to the ConcreteCreator through the abstract Creator class.

In returning to GoF, they point out,

Instead of creating the concrete product in the constructor, the constructor merely initializes it to 0. (Page 112)

In this discussion, they note that Factory methods in C++ are always virtual functions. WTF?! (What’s That Function?!) Simply stated, any function that can be overridden in the child classes is a virtual one. Well, we can do that in ActionScript 3.0. In fact, that’s what we do in creating Abstract classes. (Why we can override and yet not have real Abstract classes is still a mystery to me.)

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Design Patterns with Missing Pieces

Published on April 15, 2010 in Design Patterns. by William B. Sanders 4 Comments

Why are Participants Missing? Note: Gentle reader, This article was written for our new PHP Design Pattern blog, but the points apply as well to ActionScript 3.0. As a result, I decided to include it here. The links in this post have been changed to go to the relevant ActionScript 3.0 posts, but otherwise, the article is the same. After writing design patterns in PHP for a while, I’ve come to appreciate the strongly typed structure of ActionScript 3.0. Programming to the interface in PHP is tricky, but possible. However, when you have strong typing as ActionScript 3.0 does, you really appreciate it more once you’ve worked with a dynamic language without strong data typing.

The Case of the Missing Participants

Recently I was working on a Decorator pattern and struggling through PHP’s way of doing things. So naturally I thought why not take a look at how others have solved these same problems. The Gang of Four refer to the different classes and interfaces that make up a design pattern, as participants, and when I look at a design pattern, the first thing I do is to see how the developer handles the different participants. What I found was surprising—they left out chunks of a design pattern in their examples. Alternatively, they just did some kind of workaround that effectively made it work (run), but it was no longer a design pattern or had the advantages of a design pattern.

The Strategy with the Missing Context

The Strategy pattern decouples the algorithms from the Client by using a Context class. All of the requests for the algorithms (found in concrete Strategy classes) go through the Context to the Strategy interface and on the the implemented strategies in the form of concrete classes. However, in many of the PHP examples, they leave out the Context class and Strategy interface and go directly from an Object to concrete Strategies. In several examples, the code authors chose to ignore the interfaces and even the abstract classes and have fairly direct connections between concrete strategies and some kind of object. Figure 1 shows the kinds of UMLs that accompany these kinds of PHP design pattern implementations—if indeed they are implementations.

Figure 1: Minimalist UML

You can see clearly that the UML in Figure 1 has little in common with the class diagram we used that was an exact replica of the Strategy pattern that the Gang of Four uses.

The authors of this kind of “explanation” of the Strategy pattern seem to think that the separation of the algorithms from the object is a matter of nothing more than taking the algorithms out of some object and using them independently of the main object. However, while that is an improvement on cramming everything into a single object (class), it is not a design pattern. Yes, it does add flexibility because more than one class can use the same concrete strategies. So, it’s a step away from single-class programming and even procedural programming. However, it is not a design pattern and it does not have the design pattern’s flexibility. The Context class uncouples the request from the strategy and without it, that concrete strategies are tightly linked to the “object”—whatever that may be.
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