How to Pick the Right Design Pattern: Fortune Favors the Prepared Mind

Which Design Pattern Should I Use?
One of the most difficult questions to answer is also the most often asked: How do you know what design pattern to use? My standard response to that query is,

Ask, “What varies?”

Of course, it seems that everything varies just when you need a nice clear answer to that question. The Gang of Four provides a list of design patterns and what varies in each pattern—such as states of an object, an algorithm, responsibilities of an object without subclassing and so on. However, determining the nature of the variation can be as difficult as deciding the design pattern to use as looking at class diagrams. (Download the AIR version of the Variation Table—also known as The Magic Table.)

Go To The Original Source

Chapter 1 in the Gang of Four’s book is the gateway to design pattern knowledge. I must have re-read it 50 times. Further, every time I re-read it, I get something new. You can download Chapter 1 of Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides—just click the download button to bring up the PDF file and then save it:

Find the section, How to Select a Design Pattern and read it. Even if you don’t get it all the first time through, you’ll have a better idea of design pattern selection. Now you’re all set to continue with the rest of this post. Read on!

How Do You Get to Carnegie Hall?

There’s an old joke about a young violinist trying to get directions to Carnegie Hall. He stops and asks an old man, and the old man responds,

Practice, young man, practice!

That may well be the same answer to the question, Which design pattern should I use? Also, I think that’s what Louis Pasteur meant in his oft-repeated quote,

Fortune favors the prepared mind.

(For you purists, his exact quote is, In the fields of observation chance favors only the prepared mind and our French-speaking readers know he actually said, Dans les champs de l’observation le hasard ne favorise que les esprits préparés). The extent to which you play around with design patterns, the better you will understand how to use them. Which one to use is not going to jump out at you unless you’re very lucky–keeping in mind that luck favors those who are prepared. Everyone who has programmed for any amount of time knows his or her algorithms. However, they would be hard-pressed to answer the question,

Which algorithm is the best one to use?

Besides setting off arguments about which is the best, most programmers would point to very specific ones, like a sort algorithm. The more time you spend with algorithms, you begin to know which to use without really thinking about it. The same is true with design patterns, but design patterns are a bit more difficult. As you work with them, the sheer experience will nudge you toward a better understanding. However, they’re not going to clarify themselves.

So how are we supposed to know which design pattern is appropriate? (Don’t even think about which one is best—to get started, start thinking which is appropriate.) Begin with the examples. Concrete examples are good, but you need to be able to see them in a broader scope. In other words, you have to think in terms of applications like this or processes like this. The more you play around with them, the more likely you are to come to the right one.

Begin in Your Own Back Yard

The best place to start thinking about which design pattern is the most appropriate is with the kinds of applications you’re most likely to be called upon to produce. For example, if you’re into streaming media like I am, you want to think about design patterns and the processes involved in video applications. Other areas where Flash/Flex are involved include animation, games, RIAs, education, photography and graphics, communication and e-business of all kinds.

If you begin experimenting with different design patterns for the kind of work you do (or would like to do), then you can see what “fits” in the sense that the design provides a workable solution that allows for easy reuse and update.

Practice, Practice, Practice

To get started, let me throw another Louis Pasteur gem your way and help you understand the mysteries of design patterns.

I am on the edge of mysteries and the veil is getting thinner and thinner.

With that encouragement, I’d like to look at a typical application that might show up on our doorsteps. Further, I’ll keep streaming video out of it because I’ve done so many examples using streaming video already. (Okay, I’ll even omit progressive download videos.) So here it is:

Corporate Trainer Application: This application will be used to train corporate personnel how to adopt and implement new policies including use of new technologies, procedures, and operations. Included features are:

• Text
• Graphics
• Interactive learning
• Ability to be used for wide variety of cases

Those features are off the top of my head, and if you want to tweak it, that’s fine with me. (You can even add video.) Next, look at a summary Gang of Four’s steps (you can find the full version in Chapter 1 of their book that you can download—see download button above.)

1. Consider how design patterns solve design problems.
2. Scan Intent sections
3. Study how patterns interrelate
4. Study patterns of like purpose.
5. Examine a cause of redesign.
6. Consider what should be variable in your design

There ought to be enough examples using ActionScript 3.0 between this blog and our book that you can compare the different solutions and work through GoF’s 6-Step process. Also, remember to download the handy variation table for Step 6.

We’ll carry on the discussion in the Comments section. The purpose is not to get the one correct answer but instead to learn how to use the process. We’re really interested in seeing what you come up with and put the Comments to work.